National Renewable Energy Laboratory

Innovation for Our Energy Future 

A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

Development of Automated 

Production Line Processes for 

Solar Brightfield Modules 

Annual Technical Progress Report 

1 January 2003 – 30 June 2004 

M.J. Nowlan, J.M. Murach, S.F. Sutherland, 

D.C. Miller, S.B. Moore, and S.J. Hogan

Spire Corporation 

Bedford, Massachusetts 

Subcontract Report 

NREL/SR-520-36608 

June 2005 
Subcontract Report 

NREL/SR-520-36608 

June 2005 

Development of Automated 

Production Line Processes for 

Solar Brightfield Modules 

Annual Technical Progress Report 

1 January 2003 – 30 June 2004 

M.J. Nowlan, J.M. Murach, S.F. Sutherland, 

D.C. Miller, S.B. Moore, and S.J. Hogan

Spire Corporation 

Bedford, Massachusetts 

NREL Technical Monitor: K. Brown 

Prepared under Subcontract No. ZDO-3-30628-12 

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

Operated for the U.S. Department of Energy

Office of Energy Efficiency and Renewable Energy

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337
This publication was reproduced from the best available copy

Submitted by the subcontractor and received no editorial review at NREL

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TABLE OF CONTENTS 

Page

1 INTRODUCTION ................................................................................................................................ 1

1.1 Objective ....................................................................................................................................... 1

1.2 Approach....................................................................................................................................... 1

2 TECHNICAL DISCUSSION ............................................................................................................... 2

2.1 Task 1 – Design Brightfield Module and Production Line ........................................................... 2

2.1.1 Brightfield Photovoltaic Module Design .............................................................................. 2

2.1.2 BOS Electrical Design Analysis ........................................................................................... 7

2.1.3 Production Line Definition ................................................................................................... 8

2.2 Task 2 – Develop Cell String Inspection Process....................................................................... 10

2.2.1 Visible Defect Inspections.................................................................................................. 10

2.2.2 Microcrack Inspection......................................................................................................... 13

2.2.3 String Inspection System Integration.................................................................................. 19

2.3 Task 3 – Develop Advanced Lamination Process....................................................................... 21

2.3.1 Faster Curing Encapsulant .................................................................................................. 21

2.3.2 Laminator Cold Trap........................................................................................................... 22

2.3.3 Laminator Heater Thermal Compound ............................................................................... 24

2.4 Task 4 – Design Module Lay-up System.................................................................................... 25

2.4.1 Stacking on a Tray .............................................................................................................. 25

2.4.2 Winding onto a Tube .......................................................................................................... 27

2.4.3 Direct Dispensing onto the Module .................................................................................... 27

2.5 Task 5 – Design String Busing System....................................................................................... 29

2.5.1 Bus Ribbon Fabrication ...................................................................................................... 29

2.5.2 Bus Ribbon and Diode Installation – SCARA Robot Approach ........................................ 31

2.5.3 Bus Ribbon and Diode Installation – Cartesian Robot Approach....................................... 32

2.5.4 Automated Soldering Process Development....................................................................... 35

3 CONCLUSIONS................................................................................................................................. 38

iii
LIST OF ILLUSTRATIONS 

Page

1 PV sub-arrays at Tucson Electric Power in Springerville, AZ............................................................... 2

2 Diode assembly temperature rise vs. forward current. ........................................................................... 3

3 Module circuit with laminated diodes and center outputs...................................................................... 4

4 Module circuit with laminated by-pass diodes and end outputs............................................................. 4

5 Isometric views of the Brightfield module design.................................................................................. 5

6 Bending stress profile under maximum wind and snow load................................................................. 6

7 Steel frame deflection under 113 psf wind and snow load..................................................................... 6

8 Module glass and steel deflection under 113 psf wind and snow load................................................... 7

9 Brightfield module line process flow chart. ........................................................................................... 9

10 Preliminary module production line floor plan for the large area Brightfield module........................... 9

11 Automated cell string inspection system with a string of 125 mm cells. ............................................. 10

12 Vision sensor images of a 125 mm polycrystalline silicon cell............................................................ 11

13 Cell front side vision sensor with filter, lamps with reflectors, and backlight ..................................... 11

14 Cell backside vision sensor, LED array, backlight, and mirror............................................................ 12

15 String front side vision inspection tools on a 125 mm polycrystalline Si cell ..................................... 12

16 String back side vision inspection tools with front lighting ................................................................. 13

17 String back side vision inspection tools without front lighting............................................................ 13

18 IR images of the back surface of a 125 mm square single-crystal Si solar cell.................................... 14

19 Cell flexer on test stand. ....................................................................................................................... 15

20 Prototype cell flexer on test stand with a string of Shell Solar 103 mm cells. ..................................... 16

21 IR images of a 125 mm monocrystalline Si cell with a microcrack ..................................................... 17

22 IR images of 103 mm monocrystalline Si cell with a partially soldered interconnect ribbon.............. 17

23 Thermal image and software detection of a microcrack in a 125 mm monocrystalline silicon cell..... 18

24 Thermal image and soldering inspection results for a 125 mm mono-crystalline silicon cell. ............ 18

25 String inspection system integrated with string and module assembly. ............................................... 19

26 Cell string inspection system with a string of 125 mm cells. ............................................................... 19

27 Inspection system main window with cell front side camera view selected. ....................................... 20

28 Inspection results window. ................................................................................................................... 21

29 Module laminate made with faster-curing EVA................................................................................... 22

30 Cold trap and chiller installed on laminator vacuum system................................................................ 23

31 Laminator process chamber pump down with and without a cold trap................................................ 24

32 Laminator heater test fixture control program GUI.............................................................................. 25

33 Pick-and-place option for material lay-up in module production......................................................... 26

34 Tedlar sheet (5′ x 12′) lifted by a vacuum end effector........................................................................ 26

35 End effector with cylinder actuated, cup extended, and bus lead fed through slit. .............................. 27

iv
36 Prototype sheet unroller........................................................................................................................ 27

37 Cover Sheet Applicator for feeding, cutting and dispensing sheet materials. ...................................... 28

38 Module conveyor with CSA sheeter configured for EVA dispensing on glass. .................................. 29

39 Mechanical design for the bus ribbon fabrication machine.................................................................. 30

40 SCARA robot approach to string busing.............................................................................................. 31

41 Vacuum pick-and-place end effector design for robot arm.................................................................. 32

42 Cartesian robot approach to string busing............................................................................................ 33

43 Bus ribbon tray assembly ..................................................................................................................... 33

44 Robot end effector with pick-and-place and soldering tools................................................................ 34

45 A robot inserts a bus bar in the bending tool, an air cylinder extends and retracts.............................. 34

46 Ribbon bender, Spire design. ............................................................................................................... 35

47 Flux applicator and diode assembly in the module .............................................................................. 36

48 Preliminary thermode design for string-bus and bus-diode-bus joints................................................. 36

49 Spire soldering tip assembly................................................................................................................. 37

v
LIST OF TABLES 

Page

1 Program tasks. ........................................................................................................................................ 1

2 Nominal specifications for the Brightfield module. ............................................................................... 3

3 Electrical BOS equipment cost............................................................................................................... 8

4 Laminate electrical performance at STC. ............................................................................................. 22

vi
1 INTRODUCTION 

This is Spire Corporation’s Annual Technical Progress Report for Phase 1 of a program entitled 

“Development of Automated Production Line Processes for Solar Brightfield Modules.” This program is 

supported by the U. S. Department of Energy under the National Renewable Energy Laboratory’s 

Photovoltaic Manufacturing R&D (PVMRD) project. Phase 1 efforts extended from January 1, 2003 to 

June 30, 2004. 

1.1 Objective 

Spire is addressing the PVMRD project goals of improving PV manufacturing processes and products 

while reducing costs and providing a technology foundation that supports significant manufacturing 

scale-up. To accomplish this, we are focusing our efforts on the design of a large area utility-scale 

module and the development of the necessary manufacturing techniques and equipment to manufacture 

such a module in a high-volume production environment. 

1.2 Approach 

A three-phase program is underway for developing and demonstrating new automated systems for 

fabricating very large photovoltaic (PV) modules ideal for use in multi-megawatt grid-connected 

applications. We designed a large area 800 W module and we are developing associated module 

production equipment that will minimize the total installed system cost for utility-scale PV arrays. 

Unique features of the module design include a cantilevered glass superstrate to reduce the glass thickness 

and internally laminated by-pass diodes that simplify internal busing and junction box designs. Other 

program activities include the development of automation for solar cell string inspections, string busing, 

materials lay-up, and lamination; enhancements to the lamination process; and performance testing of 

large area modules. Program tasks are listed in Table 1. 

Table 1 Program tasks. 

Phase Task 

1 – January 03 to June 

04 

1 Design Brightfield Module and Production Line 

2 Develop Cell String Inspection Process 

3 Develop Advanced Lamination Process 

4 Design Module Lay-up System 

5 Design String Busing System 

2 - July 04 to June 05 6 Fabricate & Test Module Lay-up System 

7 Fabricate & Test String Busing System 

8 Design Advanced Lamination Process Tool 

9 Design Lamination Automation 

10 Design Large Area Solar Simulator 

3 - July 05 to June 06 11 Fabricate & Test Advanced Laminator 

12 Fabricate & Test Laminator Automation 

13 Fabricate & Test Large Area Solar Simulator 

14 Develop Computer Integrated Manufacturing System 

Spire partnered with two lower-tier subcontractors in Phase 1. Endecon Engineering assisted with PV 

balance-of-systems (BOS) issues for module design in Task 1. The Automation & Robotics Research 

Institute (ARRI) at the University of Texas at Arlington (UTA) provided process automation support for 

the module lay-up system (Task 4) and the string busing system (Task 5). 

1
2 TECHNICAL DISCUSSION 

2.1 Task 1 – Design Brightfield Module and Production Line 

The objective of Task 1 is to design a large area PV module and associated module production line that 

will minimize the total installed system cost for multi-megawatt utility-connected Brightfield PV arrays. 

Balance-of-system (BOS) requirements were considered early in the module design process, through a 

site visit to Tucson Electric Power’s (TEP) PV array in Springerville, AZ, and through system analyses 

done by Endecon Engineering. 

 

2.1.1 Brightfield Photovoltaic Module Design 

Spire Solar and Endecon engineers visited TEP’s 3.5 MW PV array in August 2003. The array uses three 

types of modules: 45 W amorphous Si, 50 W CdTe, and 300 W EFG Si (Figure 1). TEP found that large 

modules minimize installation labor for mounting and making DC electrical connections. BOS costs 

reportedly ranged from $3.35/W for small modules to $0.90/W for large modules (at STC, DC). In 

addition, TEP installed the modules low to the ground to minimize wind loading and resulting array

structure and anchoring costs. Thus a large module with a high aspect ratio (e.g., two or more) is 

preferred. 

Figure 1 PV sub-arrays at Tucson Electric Power in Springerville, AZ, made with 300 W (left) 

and 50 W (right) modules. 

Other module design factors we considered include glass size, thickness, deflection and stress under wind 

and snow loads, module weight, cell string busing arrangement, bus ribbon sizing, and junction box and 

diode designs. A survey of module materials was done to determine the maximum sizes available for 

glass, EVA and back sheet. Both single- and double-glass laminates were considered. 

Our final design is a single-glass module, nominally 5 ft by 12 ft, made with 240 silicon solar cells, 

150 mm square, connected in 10-cell strings. The strings are wired twelve in series by two in parallel. 

Six by-pass diodes are provided, with each diode protecting four strings, two (20 cells) in series by two in 

parallel. A nominal output of 3.38 W/cell (15% cell efficiency) results in a module power of 811 W. 

Dimensions and nominal electrical characteristics at Standard Test Conditions (25°C, 100 mW/cm2

, 

AM1.5 Global spectrum) are provided in Table 2. 

2
Table 2 Nominal specifications for the Brightfield module. 

Parameter Value 

Cells in series 

Cells in parallel 

Glass width mm (inch) 

Glass length mm (inch) 

Glass weight kg (lbs) 

Module weight kg (lbs)* 

Pmax W 

Imp A 

Vmp V 

Isc A 

Voc V 

120

2

1575 (62.00)

3683 (145.00)

92 (203)

140 (307)

811

14.5

60.0

14.5

74.0

*Includes steel support frame 

The conventional method of mounting diodes in a junction box presented some serious design issues, 

including a complex busing arrangement in the laminate and substantial diode heat generation in the 

junction box, should the diodes turn on. We eliminated these problems with an alternative approach, in 

which the diodes are laminated with the solar cells. A Schottky chip diode was selected for its low power 

dissipation under forward bias and its thin profile for lamination. Copper bus ribbon with a large crosssection was selected to conduct heat away from the diodes. Diode assemblies were fabricated with 

different size bus ribbons and tested for temperature rise vs. forward current. The results are shown in 

Figure 2. A diode laminated as it would be in a module (plotted as triangles in Figure 2) operated a few 

degrees cooler than a diode tested in air. The slopes of the lines fit to the data are inversely proportional 

to the ribbon cross-section area. We used this relationship to select a bus ribbon size that limits the diode 

temperature to prevent damage to the diode, the solder joint, or the encapsulant. 

slope = 9.3756

slope = 4.7065

slope = 4.6821

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16 18

Forward Current (A)

∆T (°C)

0.320 in. ribbon, bare

0.625 in. ribbon, bare

0.625 in. ribbon, laminated

Linear (0.320 in. ribbon, bare)

Linear (0.625 in. ribbon, bare)

Linear (0.625 in. ribbon, laminated)

Figure 2 Diode assembly temperature rise vs. forward current. 

3
The use of laminated diodes allows us to simplify the junction box, since the box is only the termination 

for the positive and negative outputs. We considered a central box with both outputs, as shown 

schematically in Figure 3. However, due to the 3.68 m length of the module, we were able to 

significantly reduce series resistance losses in the laminate busing and in the external cabling by using 

separate terminal boxes at opposite ends of the module. Only short cables and a connector are needed to 

connect two modules in series. The module circuit for this design is shown in Figure 4. 

+ -

- +

+ +

+ + +

- +

-

- -

- -

B1 A1 G2 F2 D2

2 parallel x 10 series cells, 12 places Outputs

A1 = Positive

G2 = Negative

+

+

-

+ -

- +

-

D1 C2 C1 B2 A2 F1 E2 E1

03-087WA

Figure 3 Module circuit with laminated diodes and center outputs. 

+ -

- +

+ +

+ + +

- +

-

- -

- -

A1 B1 D2 F2 G2

2 parallel x 10 series cells, 12 places Outputs

A1 = Positive

G2 = Negative

+

+

-

- +

+ -

-

A2 B2 C1 C2 D1 E1 E2 F1

+

-

G1

03-088WA

Figure 4 Module circuit with laminated by-pass diodes and end outputs. 

Glass deflection and stress under wind and snow loads were calculated for various glass sizes and 

thicknesses, and for single and double glass module constructions. In place of a conventional extruded 

aluminum edge frame, we designed a frame from welded steel angle for mounting and mechanical 

support. The frame is powder coated for corrosion protection and attached to the module with a structural 

silicone adhesive. The steel angles are arranged so that the glass is supported at approximately 1/5th and 

4/5th of the module’s width. This reduces the glass span by 40% in the width direction, allowing the use 

of a thinner (lighter and less expensive) glass superstrate. A center support is provided at the midway

point of the module’s length for attaching a pair of support legs. This reduces the length of the supported 

span by half to allow the use of smaller steel angles. The design is shown in Figure 5. 

4
Figure 5 Isometric views of the front (top) and back (bottom) of the 1.58 m x 3.68 m Brightfield 

module design, showing the cantilevered laminate and frame. 

With this geometry, we are able to support the module with a single layer of 6.3 mm thick glass. Modules 

must be able to withstand wind and snow loads up to 5.41 kPa (113 psf), per IEC 61215 and IEEE 1262 

standards. We calculated the maximum stress on a 6.3 mm thick, 1575 mm wide glass to be 44.0 MPa 

(6380 psi), as shown in Figure 6. This provides a comfortable factor of safety of 3.8, since the tensile 

strength of tempered glass is 165 MPa (24,000 psi). Note that our choice of support frame spacing 

balances the maximum bending stress in the center with the maximum stresses at the support angles. 

A solid CAD model was created to define the steel and glass structural geometry for the module. The 

frame is fabricated from welded 2” x 2” x 0.19”steel angles, and includes six bolt holes for attaching legs, 

two on each short frame member. The model was then exported to the ANSYS® engineering software 

program for structural finite element analysis. The worst-case 113-psf wind and snow load was applied. 

The deflection of the steel frame is shown in Figure 7, with the z-scale exaggerated to show the shape of 

the deflection. The maximum deflection is 0.23”. The total module deformation (glass and steel) is 

shown in Figure 8, with a maximum deflection of 0.50”. 

5
Supports spaced 36.32" apart (optimal 58.6% of module width)

0

1000

2000

3000

4000

5000

6000

7000

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Fractional distance along width of module

Bending stress (psi)

03-098WA

Figure 6 Bending stress profile under maximum wind and snow load (113 psf) for a 1575 mm 

wide module with a 6.3 mm thick cantilevered single glass sheet.

04-005WA 

Figure 7 Steel frame deflection under 113 psf wind and snow load. 

6
04-006WA 

Figure 8 Module glass and steel deflection under 113 psf wind and snow load. 

2.1.2 BOS Electrical Design Analysis 

Behnke, Erdman & Whitaker Engineering, Inc., completed a balance of system (BOS) study for 

megawatt-size PV systems under Spire’s subcontract to Endecon Engineering. The study considered the 

elements of the electrical system from the module output to the medium voltage terminals of the step-up 

transformer that feeds the power line. The PV modules, AC medium voltage distribution, civil work, and 

construction labor costs were excluded. 

The study assumed a system would be built in 1 MW blocks, each with its own medium voltage (15 to 

35 kV) AC interconnection. The PV array uses our nominal 5’ x 12’ 800 W modules. Wires were sized 

to limit the overall voltage drop to 1%. Four cases were considered: 600 VDC monopolar (base case), 

±600 VDC bipolar, 1000 VDC monopolar, and ±1000 VDC bipolar. Circuit diagrams, plant layouts, and 

bills of materials for 1 MW systems were designed and costs were determined for each of the four cases. 

600 VDC monopolar – This is the base case for the study and the state-of-the-art for commercial, 

industrial, and utility scale systems. UL-listed inverters and other components (e.g., disconnects, fuses, 

cabling, and combiners) with third party listings are readily available. The study assumes there are ten 

parallel source circuits of seven series-connected modules forming a 50 kW sub-array, four sub-arrays

feeding a 200-kW inverter and transformer, and five inverters and transformers per 1 MW array.

±600 VDC bipolar – This design takes advantage of the 2002 NEC revision to Article 690.7, which 

allows bipolar circuits to be considered as two 2-wire circuits when determining maximum system

voltage, so long as one wire is grounded. Thus readily available 600 V DC rated components can be used 

for systems with inverter inputs up to 1200 VDC. No UL-listed inverters are currently available, 

although suitable inverter technology exists. Non-listed inverters are suitable for “behind the fence” 

7
utility applications. The design study assumes the use of a high power (1 MW) inverter. Listed high 

power inverters for non-utility applications could be available in twelve months time, with a commitment 

to purchase 10 MW or more per year. In this case, there are ten parallel source circuits of seven seriesconnected modules forming a 50 kW sub-array and twenty sub-arrays feeding a single 1000-kW inverter 

and transformer for each 1 MW array. Significant cost reductions were seen for this design compared to 

the base case due to the reduced number of inverters and transformers. 

1000 VDC monopolar – Modules can be rated for PV systems up to 1000 VDC under UL 1703. 1000 V 

BOS components (except for inverters) are available for traction, mining, and crane applications, but 

these components are generally not UL-listed. Listed inverters are not currently available, but suitable 

inverter technology exists. Load-break DC disconnects with low current ratings are not readily available, 

making these components more expensive than the 600 V versions. Twelve parallel source circuits of 

twelve series-connected modules form a 100 kW sub-array. Ten sub-arrays feed a single 1000-kW 

inverter and transformer for each 1 MW array.

±1000 VDC bipolar – This design has the same advantages as the 600 V bipolar design with respect to 2-

wire circuits when determining maximum system voltage ratings for components. It also has the 

potential, as does the 600 V bipolar design, to eliminate the inverter isolation transformer for low voltage 

(480 VAC) interconnections. However, it has the same drawbacks as the 1000 V monopolar design 

regarding the lack of available listed components. This system has the highest inverter input voltage, 

resulting in the lowest inverter cost. Like the 1000 V monopolar case, twelve parallel source circuits of 

twelve series-connected modules form a 100 kW sub-array, and ten sub-arrays feed a single 1000-kW 

inverter and transformer for each 1 MW array.

A cost comparison of the four BOS designs is provided in Table 3. The 600 V bipolar system has the 

lowest electrical BOS equipment cost, due to the availability of commercial listed components and a 

higher input voltage, lower cost inverter than present monopolar designs. The 1000 V bipolar system

could be lower cost in the future, but currently suffers from high DC switchgear cost and availability. 

Table 3 Electrical BOS equipment cost. 

Design Case Cost ($/W) 

 600 VDC monopolar (base) 0.375 

 ±600 VDC bipolar 0.266 

 1000 VDC monopolar 0.385 

±1000 VDC bipolar 0.325 

2.1.3 Production Line Definition 

At 92 kg (203 lbs), the module glass is too heavy to be handled by two operators. As a result, the glass 

and laminates will be transported through the module production process on powered roller conveyors. A 

conveyor width of 68 inches was selected to handle the 62 inch wide glass. All of the assembly and test 

procedures are done with the module in a facedown orientation, so there is no need to turn the module 

over. 

The module line process flow is shown in Figure 9. A preliminary production line floor plan was also 

designed, as shown in Figure 10. 

8
Cells In Test & Sort

Cells

Fabricate Cell

Strings

Attach Buses

& Diodes

Feed, Cut &

Place EVA on

Glass

Wash, Rinse,

Dry Glass Glass In

QC

Dark I-V &

Visual

Inspection

QC

Feed, Cut &

Place EVA &

Back Sheet on

Strings

Laminate Trim Edges

High Voltage

Isolation Test

QC

Performance

(I-V) Test

QC

Install J-boxes Modules Out

Load Glass on

Conveyor Buffer Glass

Inspect Cell

Strings

QC

Align Strings &

Trim End Tabs

Place Strings

on EVA/Glass

Attach Frame

04-017WA

Figure 9 Brightfield module line process flow chart. 

Figure 10 Preliminary module production line floor plan for the large area Brightfield module. 

9
2.2 Task 2 – Develop Cell String Inspection Process 

Given the large size of the Brightfield modules, each string must be carefully inspected before lamination 

to ensure high yields. Our efforts in this task focused on two main activities: developing machine vision 

techniques for identifying visible defects in cell strings, and investigating methods for detecting 

microcracks in solar cells that are too small to see with a vision system. This work culminated in the 

design, fabrication, and testing of a full-scale prototype automated string inspection system, shown in 

Figure 11. 

04-062WA

Figure 11 Automated cell string inspection system with a string of 125 mm cells. 

2.2.1 Visible Defect Inspections 

We applied machine vision techniques to find visible defects in strings, including cell chips, separated 

cell cracks, cell alignment and orientation in the string, and ribbon alignment to the cell. This work began 

with a vision system product survey, aimed at selecting the optimum system based on capabilities, speed, 

cost, and ease of use. Both traditional vision systems and recently developed vision sensors were 

considered. After reviewing and comparing the capabilities of these systems, we selected a highresolution DVT unit for string front and back side inspections. 

Camera lenses and spacers were selected to obtain the correct field of view for front and back side 

inspections. Calibration plate spot patterns were designed for pixel to real world coordinate (mm or inch) 

translations for both front and back views. The patterns also allow corrections for lens distortion. 

Calibration software projects were developed for use by the operator. A Visual Basic user interface 

project was coded, incorporating the DVT to host PC communications protocol and display capabilities. 

Polycrystalline cells present a special challenge for vision systems because they have randomly located, 

randomly oriented crystal grains, some of which reflect strongly under normal lighting conditions. The 

result is an image containing a number of bright objects of random size and location, as shown in the left 

image in Figure 12. These bright grains prevent the vision tools from reliably determining the locations 

of the cell edges and contact buses. 

10
04-063WA

Figure 12 Vision sensor images of a 125 mm polycrystalline silicon cell using unfiltered direct 

lighting (left) and using two diffuse reflectors and a spectral filter (right). 

Initial lighting experiments showed that bright silicon crystal grain reflections could be eliminated by

shining red LED arrays at a shallow angle onto the cell surface. However, two large, expensive LED 

arrays were needed to fully illuminate the cell area with sufficient brightness. After testing several 

alternative lighting methods, we found a lower cost approach that achieves good results. In this approach, 

linear fluorescent lamps shine onto diffuse reflectors on opposite sides of the cell, while a filter on the 

camera lens cuts out the short wavelengths responsible for most of the crystal grain reflections. The 

results are shown in the right image in Figure 12. The vision sensor, lamp and reflector arrangement is 

shown in Figure 13. 

04-064WA 

Figure 13 Cell front side vision sensor with filter (bottom), lamps with white reflectors (left and 

right), and backlight (top) with a string of 125 mm cells. 

11
We cannot use this same lighting technique for the cell backside inspections because the lamp must be 

cycled on and off for each cell inspection. The lamp is turned on for cell presence and tab alignment 

measurements, then turned off for chip and crack detection and cell alignment measurements, which use 

only backlighting for maximum contrast. Fluorescent lamps are not suited for cycling, due to their long 

warm-up time, so we chose the red LED array approach. Since the LED array illuminates the cell surface 

at a shallow angle, we were able to obtain good images with only one array by installing a mirror on the 

opposite side of the string, as shown in Figure 14. 

Figure 14 Cell backside vision sensor (top), LED array (right), backlight (bottom), and mirror (left). 

04-065WA

Once the lighting methods were determined, we developed software for visual inspections of the front and 

back sides of cell strings by applying image-processing tools to the vision sensor images. The tools used 

to inspect the string front side, shown in Figure 15, check for cell presence, cell orientation (0°, ±90°, or 

180°), cell upside down, tab presence, tab alignment, and end of tab location. 

04-066WA

Figure 15 String front side vision inspection tools on a 125 mm polycrystalline Si cell. The right 

image is a close-up of the tools at the top right of the cell. 

12
The tools used to inspect the string backside, with front lighting on, are shown in Figure 16. These tools 

inspect for cell presence, tab presence, and tab alignment. Another set of inspections done with the front 

light turned off is shown in Figure 17. These tools inspect for cell chips and cracks, cell-to-cell distance, 

and alignment of the cell’s top edge to the adjacent cell’s top edge. 

04-067WA

Figure 16 String back side vision inspection tools with front lighting on a 125 mm polycrystalline 

Si cell. The right image is a close-up of the tools at the top right of the cell. 

04-068WA

Figure 17 String back side vision inspection tools without front lighting on a 125 mm polycrystalline Si cell. The right image is a close-up of the tools at the top right of the cell. 

2.2.2 Microcrack Inspection 

Both mono- and poly-crystalline silicon cells occasionally contain microcracks that are difficult or 

impossible to see with the human eye or a vision system. Module manufacturers typically use manual 

visual inspection to find cracked cells after cell stringing and eliminate them before module lamination. 

This process is labor intensive and not completely effective in finding microcracks. 

13
Several potential methods for detecting microcracks in silicon cells were identified and considered: 

Ultrasonics – Lassen Technologies developed a technique using a piezoelectric transducer to generate 

ultrasonic vibrations in a cell and detect the sonic signature of cracks. We contacted Lassen and they

informed us that they are not interested in commercializing this technique. 

Radiant Heating – A system using pulsed radiant heating (generated by a 3 kW flash tube) for crack 

detection is available from Thermosensorik GmbH in Germany. Cell cracks cause discontinuities in heat 

flow, and an infrared camera detects the temperature difference across the crack. This system was not 

selected because the cost is very high, and false crack detection is specified at <3%, which is too high for 

production. 

Thermosonics – In this technique, ultrasonic energy causes friction heating of cracks, which an IR 

camera can detect as a temperature rise near the crack. Indigo Systems demonstrated their 

ThermoSoniXTM system at Spire on cells made from single crystal Si, cast polycrystalline Si and EFG 

ribbon Si. An ultrasonic horn with a blunt tip contacts the surface of a cell with a force applied by an air 

cylinder. A tip force of 8 to 10 lbs was applied to the cell, and 10% of full ultrasonic power was applied 

for 0.2 seconds. No damage was done to any of the cells during the tests. 

The IR camera clearly detected microcracks approximately one second after the ultrasonic energy was 

applied. The system worked equally well whether the cells were face up or face down, and for gridded or 

solid metal back contacts. Two frames from an IR video of a test with a single crystal Si cell are shown 

in Figure 18. The images show the cell before and after the application of ultrasonic energy. Additional 

tests showed that the thermosonic system is effective at finding cracks in all three types of silicon cells. 

 

IR heating at 

hairline crack Ultrasonic probe

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Figure 18 IR images of the back surface of a 125 mm square single-crystal Si solar cell, before 

the test (left) and 1 s after the application of ultrasonic energy (right). 

14
The ThermoSoniX system includes the software for controlling the ultrasonics and recording IR images, 

but it relies on a human operator to view the images and decide if a crack is present. Vision tools and 

analysis software are needed to automate crack detection. This approach was not pursued further because 

of its expense: $95,000 for the camera, lens, ultrasonics, software, and host PC. There would be 

additional costs for the vision tools and crack detection software needed for full automation. 

Mechanical Flexing – We developed an approach based on a manual cell flexing technique found to be 

effective for screening cells for microcracks prior to soldering. Cells are held at diagonally opposed 

corners and gently flexed several times. Cells with microcracks break apart easily, while good cells are 

not damaged. A vision sensor used to do visual inspections could then easily detect a broken cell. Since 

this is a low cost technique for microcrack detection, we designed, built, and evaluated a prototype flexer. 

The flexing mechanism design is shown in Figure 19. The flexer has two arms mounted at 90° to each 

other that move up and down independently on ball slides. The arms are driven by air cylinders with low 

friction metal seals for smooth operation. Each arm has two rubber vacuum cups that hold a solar cell at 

opposite corners. The cup mount locations are adjustable for different cell sizes. A load cell on each arm

measures the applied force in tension and compression. 

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Figure 19 Cell flexer on test stand. 

A PC with an analog and digital input/output (I/O) board controls the system automatically and acquires 

data from the load cells. The completed system is shown in Figure 20. 

Visual Basic software was written for flexer control, data acquisition, and user interface. The software 

was originally written for a time-controlled cycle, in which two diagonally opposite cell corners are 

pulled for a fixed time and released, and then the other two corners are pulled for a fixed time and 

released. The user can set the pull time and the number of cycles per test. 

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Figure 20 Prototype cell flexer on test stand with a string of Shell Solar 103 mm cells. 

A force-controlled cycle was added, in which the cell corners are pulled to a selected maximum force and 

released. Flexing tests were done with RWE Schott Solar EFG ribbon Si cells, 300 μm by 100 mm

square, Shell Solar monocrystalline Si cells, 300 μm by 103 mm square, and AstroPower monocrystalline 

Si cells, 630 μm by 155 mm square. Flex tests were done at two force set points, 0.5 and 1.0 lb. The 

software stores the peak force recorded from each load cell during each cycle in a data file. The peak 

force repeatability (standard deviation/average) was ±0.3% or less in these tests, an excellent result given 

that the force is applied by air cylinders. The 10 lb load cell range and the 12 bit analog to digital 

converters provide a force resolution of 10 lbs/212 = 0.0024 lbs. 

Thermal Imaging with Pulse Heating – We developed an approach that uses pulse heating, a thermal 

camera, and Spire-developed analysis software to identify microcracks in interconnected cells. Tests 

showed that this method can also identify poor soldering of interconnect leads to cells. Spire applied for a 

U.S. patent for this method and apparatus. 

We began this development by recording infrared (IR) images while heating various types of cell strings. 

Microcracks appeared clearly on the IR images as thermal discontinuities across the surface of the cell, as 

shown in Figure 21. 

The left image in Figure 21, taken before cell heating, shows a large chip missing at the upper right corner 

of the cell, but no cracks are evident. After three seconds of heating, the right image shows a long 

terminated microcrack, originating at the chip and extending diagonally down to the left. This technique 

has several advantages over the thermosonic method described previously. It is less expensive (no 

ultrasonic equipment), gentler on the cell (no ultrasonic vibrations), and the entire cell can be viewed 

during the test, since no probe is placed on the cell. 

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Figure 21 IR images of a 125 mm monocrystalline Si cell with a microcrack, before (left) and after 

(right) 3 seconds of heating. 

Our tests showed that this technique is also capable of finding sections of unsoldered interconnect ribbon 

on either the top or bottom cell contacts, which the thermosonic and the flexing methods cannot do. 

Figure 22 shows a monocrystalline Si cell with an unsoldered section of interconnect ribbon at the top left 

corner of the cell. Both ribbons on the near side of the cell were properly soldered, but a 25 to 30 mm

length of ribbon on the far side of the cell was not soldered. 

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Figure 22 IR images of 103 mm monocrystalline Si cell with a partially soldered interconnect 

ribbon, before (left) and after (right) 3 s of heating. 

The IR camera software displays and records thermal images for viewing, but inspecting the images and 

deciding if there are defects were still manual processes. Therefore, we developed software that 

automates the inspections by applying image-processing tools to the thermal images to identify

microcracks and poor ribbon soldering. 

The microcrack detection method uses a blob tool configured with a gradient threshold to find thermal 

discontinuities when cracks are present. The left image in Figure 23 is a thermal image of a cracked 

monocrystalline Si cell; temperature increases as the pixel intensity varies from black to white. The right 

image in Figure 23 shows the blob tool inspection results for the same image. Three inspection areas 

17
were defined on the left, center and right portions of the cell, to avoid misidentifying the contacts or 

interconnect ribbons as cracks. Within the inspection areas, any large change in intensity between 

neighboring pixels is identified and displayed in white. 

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Figure 23 Thermal image (left) and software detection (right) of a microcrack in a 125 mm 

monocrystalline silicon cell. 

A method of detecting poor ribbon soldering was also developed. The method uses a series of intensity

tools and Spire-developed scripts to measure the intensity variance (σ2

) of twelve sectors of the cell. If 

the variance is above a selected threshold, the inspection software reports a failure. Figure 24 shows a 

thermal image of a cell with poor soldering in the top left quadrant and the results of the intensity tools 

placed on it. The sector in the top left corner and the adjacent sector to the right have low intensities that 

increase the variance above the failure threshold. An alternative method was also developed, where the 

intensity of each sector is compared to its counterpart from a trained good cell. A failure is reported if 

any of these comparisons exceeds a selected threshold. 

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Figure 24 Thermal image (left) and soldering inspection results (right) for a 125 mm monocrystalline silicon cell. 

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2.2.3 String Inspection System Integration 

A prototype automated string inspection system was designed, fabricated, and tested. The system can be 

operated as a stand-alone unit or integrated with a cell string fabrication tool, such as the SPIASSEMBLER 5000, as shown in Figure 25. 

Figure 25 String inspection system integrated with string and module assembly (plan view). 

An o-ring conveyor provides string transport with maximum visibility for inspecting both sides of the 

string. The conveyor and the component mounts for cameras, front lighting, backlighting, and probes are 

adjustable for inspecting cell sizes up to 200 mm square. A PC running Microsoft Windows 2000 

controls the inspection system and acquires data from the vision sensors and thermal camera through 

Ethernet and USB connections. The completed system is shown in Figure 26. 

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Figure 26 Cell string inspection system with a string of 125 mm cells. 

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Visual Basic software was developed for system operation, diagnostics, calibration, and cell string 

inspections. A complete graphical user interface (GUI) was also developed. The main operating window, 

shown in Figure 27, includes operator controls, tab access to all three camera views, string and defect 

counters, an animated view of string location on the conveyor, and drop-down menus for all software 

functions. 

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Figure 27 Inspection system main window with cell front side camera view selected. 

Figure 28 shows the inspection results window, which displays the pass/fail results of the six front side 

visual inspections, six back side visual inspections, and three thermal inspections. Measured inspection 

data is also displayed, including average pixel intensity, cell angle, tab locations, number of chips, cell-tocell gap, cell top edge alignment, number of cracks, and thermal intensity variance. 

The results of all inspections are logged in a database file. String inspection failure reports can be 

generated for each batch of strings. The number of failures can be plotted by inspection category in a bar 

chart. Trend analyses can be done on the inspection data to monitor the measured values over time. 

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Figure 28 Inspection results window. 

2.3 Task 3 – Develop Advanced Lamination Process 

2.3.1 Faster Curing Encapsulant 

We evaluated a new faster-curing ethylene vinyl acetate (EVA) based encapsulant, type 15420P from

Specialized Technology Resources (STR), to see if we could improve the productivity of our lamination 

process. A lamination test matrix was completed under a range of process conditions (temperature, pump 

time, press time) with a SPI-LAMINATORTM 480. The laminates were evaluated with gel tests, peel 

strength tests, and visual inspections for defects such as bubbles or wrinkles in the back sheet. The total 

process time, including time for loading and unloading, was reduced from 14.0 to 8.5 minutes, while 

maintaining good adhesion and high EVA gel contents (>80%). 

Two laminates produced using the new process were shipped to NREL in April 2004. Each laminate is 

52.5 cm x 118.2 cm and contains thirty-six 125 mm pseudo-square monocrystalline silicon solar cells 

from BP Solar. The laminates were fabricated in the module production facility at Spire Solar Chicago. 

One of these laminates (serial no. 2813, shown in Figure 29) was made using a process we developed 

earlier on test laminates made with reject cell strings. This process has a 2.5-minute pump time and a 4-

minute press time at 175 °C. Attempts to make an additional laminate with this process failed, due to the 

presence of bubbles in the encapsulant after lamination. While these laminates are the same size as the 

test laminates made previously, they contain more cells and bus ribbons, which are soldered at the ends of 

the strings. We solved the bubble problem by increasing the pump time by 30 s, which indicates that the 

additional components, or perhaps added volatiles from the flux residue in the additional solder joints, 

require more time for complete outgassing. Therefore, the second laminate we delivered (serial no. 2832) 

was made with a 3-minute pump time and a 4-minute press time. 

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 04-076WA

Figure 29 Module laminate made with faster-curing EVA (serial no. 2813). 

The electrical performance of the two laminates was measured at Standard Test Conditions (STC, 

100 mW/cm2

, 25 °C, Air Mass 1.5 Global spectrum) with a SPI-SUN SIMULATORTM 350i. The results 

are provided in Table 4. 

Table 4 Laminate electrical performance at STC. Laminate area = 6205.5 cm2. 

Serial No. VOC (V) ISC (A) PMAX (W) FF (%) VMP (V) IMP (A) ηMOD (%) 

2813 21.83 4.811 77.52 73.8 17.71 4.378 12.5

2832 21.68 4.771 76.53 74.0 17.53 4.366 12.3

Our standard lamination cycle with STR formulation 15295P/UF EVA is processed at 155 °C with a 5.5-

minute pump time and a 7-minute press time, for a total of 12.5 minutes. The new faster-curing 

15420P/UF EVA, with a 3-minute pump and a 4-minute press, saves 5.5 minutes per run. Since both 

processes require roughly 1.5 minutes for cover opening, loading, and cover closing, the total cycle time 

has been reduced from 14 minutes to 8.5 minutes. As a result, production through the laminator increases 

from 4.3 to 7.1 runs/hour. 

2.3.2 Laminator Cold Trap 

We investigated methods for reducing laminator maintenance costs and equipment downtime caused by

contamination of the vacuum pump oil by volatiles released from the EVA encapsulant. The volatiles 

increase the oil viscosity to the point where the oil must be changed frequently (typically after 40 hours 

22
of operation) to prevent pump damage. We considered replacing the standard oil-filled rotary vane pump 

with a dry pump, but we found that the costs of dry vacuum pumps with suitable flow rates are extremely

high. We then decided to investigate the use of a cold trap for condensing volatiles from the EVA before 

they reach the vacuum pump and contaminate the oil. 

A cold trap assembly was designed, fabricated, and installed in the vacuum line between the laminator 

process chamber and the vacuum pump, as shown in Figure 30. The main components are a water-cooled 

stainless steel cold trap and a chiller for circulating coolant in a closed loop. The chiller is rated for 

temperatures down to -25 °C, so a 50/50 solution of ethylene glycol and water is used to prevent the 

coolant from freezing. The assembly was installed and is under evaluation on a laminator in the module 

production facility at Spire Solar Chicago (SSC). 

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Figure 30 Cold trap and chiller installed on laminator vacuum system. 

Vacuum system pump down tests were done with and without the cold trap to see if the trap reduces the 

laminator pumping speed. The trap was at room temperature during these tests. Figure 31 shows the 

pressure in the laminator process chamber during pump down, starting from a fully vented (760 Torr) 

condition. The trap introduced a slight restriction during the initial high-flow portion of the pump down, 

resulting in a higher chamber pressure (191 vs. 85 Torr) after 10 s of pumping. However, the chamber 

pressure was nearly identical (1.01 vs. 0.96 Torr) after 30 s of pumping and beyond. Since typical 

lamination process pump times range from 3 to 5 minutes, the cold trap is not expected to have a negative 

impact on the lamination process. 

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0.1

1

10

100

1000

0 10 20 30 40 50 60

Time (s)

No cold trap

With cold trap

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Figure 31 Laminator process chamber pump down with and without a cold trap. 

2.3.3 Laminator Heater Thermal Compound 

Module laminators use heated platens to melt and cure the EVA encapsulant. Since the process is done in 

a vacuum, Spire laminators use a thermal compound to ensure good thermal conduction between the 

heaters and the platen. The compound is a paste that, after some years of use, can migrate away from the 

heater-platen interface, due to repetitive thermal cycling. If migration occurs, the temperature uniformity

of the platen can degrade with time, and the platen must be rebuilt, which involves disassembly, cleaning, 

application of new compound, and reassembly.

We are investigating a solid thermally conductive material that may eliminate the labor and downtime 

required to rebuild the platen. We compared several different materials and selected a graphite composite 

sheet, 0.13 mm (0.005 inch) thick, with a thermal conductivity of 5 W/mK and a temperature rating of 

300 °C. 

We designed and fabricated a single-heater section of a laminator platen for bench tests. Our objective is 

to evaluate the durability of the conductive material to withstand thermal stress from the heater and 

mechanical stress from thermal cycling. A heater and a water cooling line were clamped to the platen 

with the solid conductive material in place of thermal compound. The platen is instrumented with a 

control thermocouple and an over-temperature protection thermocouple on the bottom surface of the 

platen, and six monitor thermocouples on the top surface. Controls were designed and assembled to allow 

automatic thermal cycling. Software was written for operation and data acquisition. The graphical user 

interface (GUI) is shown in Figure 32. 

The test platen was disassembled to inspect the condition of the graphite composite material on May 13, 

2004, after the platen assembly had completed 823 thermal cycles. Most of the cycles were between 

100 °C and 160 °C. No damage or other visible change was apparent in the graphite material at the 

heater-platen interface or at the cooling line-platen interface. The platen was reassembled and the 

temperature cycling tests are continuing at temperatures up to 180 °C. As of June 2, an additional 243 

cycles were completed. 

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Figure 32 Laminator heater test fixture control program GUI. 

2.4 Task 4 – Design Module Lay-up System 

Concepts were developed for automating the placement of large sheets of EVA and back sheet material 

for module assembly prior to lamination. The Automation & Robotics Research Institute (ARRI) 

conducted a design for automation (DFA) analysis of the busing lay-up and back sheet feedthrough for 

connecting the output buses to the junction box. Most current production techniques are designed for 

manual assembly, and require multiple folds or splices of bus bar ribbons and the placement of insulating 

material to prevent shorting the buses to the cells. We devised an approach that uses a single vertical 

bend in the bus bar at each end of the module, a step that is easily automated and allows vertical 

placement of pre-slit sheet materials. 

Due to the size of our large area module, careful consideration was given to automating the placement of 

the sheet materials. Commercial sheeters are available for dispensing materials from rolls and cutting 

them to length. Three different methods were considered for transferring the cut sheets onto the module: 

stacking on a tray followed by pick-and-place operations; winding on a tube followed by unrolling onto 

the module; and direct dispensing from the sheeter onto the module. 

2.4.1 Stacking on a Tray 

In this process, a sheeter makes an aligned stack on a tray for each material lay-up step. The trays are 

transported on conveyors or carts to two large pick-and-place machines that transfer the materials to the 

module. The first pick-and-place transfers EVA onto glass, while the second transfers EVA and back 

sheet over the cell strings, as shown in Figure 33. 

25
Figure 33 Pick-and-place option for material lay-up in module production. 

ARRI designed and fabricated a prototype full-scale (5 ft x 12 ft) vacuum end effector to demonstrate a 

pick-and-place stack transfer process for large area sheets. Figure 34 shows a 5 ft x 12 ft sheet of Tedlar 

being lifted by the vacuum end effector. Miniature needle grippers and non-contact venturi air grippers 

were investigated, but found to offer no cost advantage relative to the standard vacuum cup method. 

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Figure 34 Tedlar sheet (5′ x 12′) lifted by a vacuum end effector. 

A prototype mechanism was developed for opening a slit in the EVA and the Tedlar to allow the output 

bus leads to feed through. This allows the sheet materials to be lowered onto the module in a single 

vertical motion. The mechanism, mounted at the corner of the sheet end effector, consists of a cleated 1-

inch diameter vacuum cup attached to the end of a small pneumatic cylinder. The cylinder is positioned 

so that when it is retracted, the bus lead slit is almost tangent (2 mm offset) to the attached vacuum cup. 

One of the stationary vacuum cups is located on the opposite side of the slit. As the sheet is lowered 

toward the vertical part of the bus lead, the pneumatic cylinder extends and the vacuum cup opens the slit 

to allow clearance for the lead (Figure 35). 

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Figure 35 End effector with cylinder actuated, cup extended, and bus lead fed through slit. 

2.4.2 Winding onto a Tube 

In this approach, the sheeter has a material rewind system for winding cut sheets on tubes. The tubes are 

transported to two simple lay-up stations where the material is unrolled onto the module. Tracks on both 

sides of a module conveyor guide the tube wound with sheet material across the module, powered by

gravity. This approach replaces the large pick-and-place mechanisms with simpler and much smaller 

track structures. ARRI successfully prototyped this unrolling process for large sheets of material, as 

shown in Figure 36. While this method is the most space efficient and least costly of the three methods 

we considered, only one sheet of material is wound on each roll, and the roll handling is manual, so this 

approach makes sense when complete automation is not required. 

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Figure 36 Prototype sheet unroller. 

2.4.3 Direct Dispensing onto the Module 

This approach provides full automation using a sheeter with an output conveyor belt for cut material, 

mounted over the module conveyor. The sheeter dispenses the material directly onto the module, as the 

module passes below. Sensors and servomotors synchronize the material with the module. Two sheeters 

are required, one for dispensing EVA onto glass, and one for dispensing EVA and Tedlar back sheet over 

the cells strings. 

27
Rosenthal Manufacturing produces sheeters of this type, which they call Cover Sheet Applicators (CSA). 

A standard CSA is shown in Figure 37. The CSA can dispense a single layer of dimensionally stable 

material, such as Tedlar, with a tolerance of ±0.8 mm (±1/32 inch), while EVA sheet, due to its 

elastomeric nature, can be dispensed with a tolerance of ±4.8 mm (±3/16 inch). 

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Figure 37 Cover Sheet Applicator for feeding, cutting and dispensing sheet materials. 

We are working with Rosenthal to define a customized CSA that can handle the material requirements of 

our Brightfield module. Tests are underway to determine if the CSA will be capable of feeding and 

cutting to length two different sheet materials (EVA and Tedlar back sheet) simultaneously, without 

excessive slipping of the top layer. ARRI sent rolls of Tedlar (50 ft × 60 inch) and EVA (50 ft × 52 inch) 

to Rosenthal for these tests. If the tests are not successful, a second output conveyor belt will be added 

for dispensing the EVA and Tedlar separately.

An in-line die set will be installed to punch a rectangular cutout at two corners of the sheet for feeding 

bus leads through to the junction boxes. The CSA will also be able to handle a single layer of material, 

with the die set disabled, to demonstrate the lay-up of the first layer of EVA onto the glass sheet. A CSA 

sheeter configured for EVA dispensing is shown in Figure 38. 

The Cover Sheet Applicator is preferred over the sheeter-stacker for the sheeting and lay-up tasks for 

several reasons. It eliminates the need for two 5 ft x 12 ft buffer stacks and two 5 ft x 12 ft pick-andplace units, greatly reducing the floor space requirements. Also, the footprint of the CSA machine is 

smaller than that of the sheeter-stacker system. The CSA eliminates the need to transport stacks of sheet 

material from the sheeter-stacker to the two pick-and-place machines. The CSA has no drawbacks in 

terms of cycle time or placement precision relative to the stacking system. 

28
Figure 38 Module conveyor with CSA sheeter configured for EVA dispensing on glass. 

2.5 Task 5 – Design String Busing System 

The design of a production prototype string busing system is underway at Spire, based on process 

evaluations done at ARRI. The system includes automation for transporting and aligning large area 

modules; feeding, cutting, punching, and placing bus ribbons; applying flux, placing diodes, and 

soldering bus ribbons to the diodes and cell strings. The string busing tasks are divided into two 

independent systems, one for prefabricating bus ribbons off-line, and one for installing bus ribbons and 

diodes in a module on an assembly line. 

2.5.1 Bus Ribbon Fabrication 

Spire procured two types of copper bus ribbon with the proper dimensions and solder coating required for 

the Brightfield module design. Due to the high currents generated by the 800 W module, both ribbons are 

substantially wider (0.320” and 0.500”) than the widest ribbon we normally use in our 75 W modules 

(0.200”). The ribbons are wound on cardboard reels that contain roughly 5 kg (11 lbs) of ribbon per reel. 

We set up a commercial ribbon feed-and-cut machine and a ribbon straightener to evaluate the bus ribbon 

feed, straighten, and cut processes. The machine uses a stepper motor with rubber rollers to dispense a 

measured amount of ribbon and a shear to cut it to length. Tests showed that the ribbon lengths varied 

and were shorter than selected. This indicates that the ribbon is slipping in the drive rollers, due to the 

drag from the heavy ribbon reel and the straightener. We designed larger drive rollers that apply more 

pressure on the ribbon to eliminate slipping. 

The shear method is appropriate for the 0.320” wide ribbon, which is simply cut straight at both ends. 

However, the 0.500” wide ribbon needs a tab punched at one end and a straight cut at the other for 

connecting to the top and bottom surfaces of a diode. In this case, a punch and die are required to cut the 

ribbon instead of the shear. We considered modifying the existing machine to add a punch to the ribbon 

path in addition to the sheer, but it was not possible to fit it in the space available, so we proceeded with a 

custom machine design. 

29
Once the bus ribbon is cut or punched to length, it must be loaded into a tray that will hold a stack of 

ribbons. Most of the ribbon length will be pushed into the tray by the drive rollers, but the last several 

inches will remain in the shear or die set. We designed a two-axis pneumatic pick-and-place mechanism

with a vacuum cup to pick up the trailing end of the ribbon, advance it so it is completely over the tray, 

and drop it into the tray. 

It is important for the ribbon to remain relatively flat for stacking in the tray and for accurate assembly in 

the module. We were concerned that as the ribbon is pushed into the tray, the edges of the ribbon might 

rub on the tray sidewalls and bend, so we fabricated a prototype tray for 0.500” wide ribbon. Tests 

showed that the drive rollers reliably pushed the ribbon into the tray without distortion. Tests simulating 

the final pick-and-place operation were also successful in maintaining flat ribbon. 

We designed a bus ribbon fabrication machine, shown in Figure 39, that feeds ribbon from reels, 

straightens it to remove coil set, cuts it to length, punches one end to form a tab for connecting to a diode, 

and fills trays with stacks of ribbon. Our module design requires three different lengths of ribbon, two 

with tabs for the diodes, all of which can be made automatically on this system. Mechanical assemblies 

and electrical controls were designed and are ready to release for fabrication in Phase 2 of the program. 

RIBBON REEL 

HOLDER 

PUNCH 

CUTTER 

RIBBON 

DRIVE PICK & PLACE 

RIBBON TRAY

STRAIGHTENER 

ENCODER 

SERVOMOTOR 04-088WA

Figure 39 Mechanical design for the bus ribbon fabrication machine. 

The ribbon reel holder has an adjustable magnetic clutch to prevent over-spin when the drive wheel pulls 

ribbon. The ribbon first passes through a straightener to remove coil set. The straightener consists of 

three unpowered rollers. The position of the center roller can be adjusted to press on the ribbon as 

required for straightening. The ribbon then enters a guide for alignment and passes under a spring-loaded 

wheel attached to an encoder, which monitors the ribbon for slip in the drive rollers. A lever and a limit 

switch are provided to sense when the reel runs out of ribbon. 

30
The ribbon drive assembly consists of a spring-loaded pinch roller and a drive roller powered by a 

servomotor. A lever and cam lift the pinch roller up above the drive roller to simplify ribbon threading 

when a new ribbon reel is loaded. 

The ribbon punch consists of a stationary die and an air-driven clamp and punch assembly. When the air 

cylinder is extended, the clamp holds the ribbon in place on both sides of the die while the punch cuts a 

tab shape on one end of the ribbon and a straight cut on the other end. The tab shape allows a connection 

to be made to the top surface of the diode without shorting to the bus ribbon connected to the bottom

surface of the diode. The ribbon cutter assembly has a stationary blade and an air-driven blade that cuts 

across the ribbon as it moves down, like a paper cutter. The moving blade is spring-loaded against the 

stationary blade for self-alignment. 

Once the bus ribbon is cut or punched to length, it is loaded into a tray that holds a stack of ribbons. Most 

of the ribbon length is pushed into the tray by the drive rollers, but the last several inches remains in the 

shear or die set. A two-axis pneumatic pick-and-place mechanism with a vacuum cup picks up the 

trailing end of the ribbon, advances it so it is completely over the tray, and drops it into the tray. 

2.5.2 Bus Ribbon and Diode Installation – SCARA Robot Approach 

The automation for bus ribbon and diode installation has gone through two design iterations. The first 

approach is based on a SCARA robot work cell similar to the one that ARRI used to develop the 

installation processes. In this approach, two SCARA robots, one on each side of the module, install bus 

ribbons and diodes, as shown in Figure 40. One third of the module is processed at a time, then the 

module is indexed forward on the conveyor to do the next third. This allows us to use moderate size 

robots with an 850 mm reach. 

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Figure 40 SCARA robot approach to string busing (plan view). Inset shows a robot with a 

vacuum end effector for bus ribbon and diode handling. 

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Each robot has a tool changer and two end effectors, one to pick and place components and another to 

solder connections between them. The pick and place end effector, shown in Figure 41, handles an 

insulating bar, bus ribbons of different lengths, and chip diodes, and assists with ancillary processing 

steps, such as bus ribbon bending and flux application. The insulating bar is temporarily placed on the 

module to allow the buses and diodes to be soldered without melting the EVA. 

TOOL CHANGER 

SPRING LOADED 

VACUUM CUP 

ARM DOWN SENSOR 04-083WA

Figure 41 Vacuum pick-and-place end effector design for robot arm. 

2.5.3 Bus Ribbon and Diode Installation – Cartesian Robot Approach 

An alternate approach for bus ribbon and diode installation uses a single Cartesian robot in place of the 

two SCARA robots. This approach is preferred if it has sufficient processing speed, because we can 

eliminate one robot and its tools, thereby reducing the system cost. 

ARRI ran a computer simulation of the bus ribbon and diode installation process to determine the total 

cycle time for both methods. The simulation results show that the SCARA method, which uses two 

robots working simultaneously on opposite sides of the module, can complete a module in 5 minutes and 

13 seconds. The Cartesian robot method, which uses a single robot for processing both sides of the 

module, can complete a module in 8 minutes and 49 seconds. A solar cell string assembly machine, 

producing at a rate of 600 cells per hour, will fabricate one Brightfield module (24 10-cell strings) in 

24 minutes. Therefore, the Cartesian robot is more than fast enough to handle the output from two string 

assembly machines. 

Since it is relatively easy to extend the axis lengths of a Cartesian robot, we designed the robot with 

sufficient reach to process the entire module in one position (i.e., without indexing). The robot has four 

axes: x, y, z, and θ (rotation). The system design is shown in Figure 42. 

32
4-AXIS CARTESIAN 

ROBOT

END EFFECTOR

BUS RIBBON TRAY

INSULATOR BAR 

MODULE 

BUS RIBBON TRAY

FLUX APPLICATOR

DIODE PACK 

RIBBON BENDER 

ROLLER CONVEYOR 

WITH MODULE ALIGNER

Figure 42 Cartesian robot approach to string busing (perspective view). 

The ribbon tray assembly design is shown in Figure 43. The trays are filled by the ribbon fabrication 

machine (Figure 39), after which an operator transfers them to the string busing system (Figure 42). The 

assembly has an optical sensor located under a hole in each tray to detect when the tray is empty. Two 

proximity sensors detect steel pins inserted in the wall of the trays to determine which tray type is loaded. 

There are three tray types, one for each type of ribbon used in our module design. 

RIBBON TRAY, 

1 OF 3 TYPES 

TRAY EMPTY

SENSOR TRAY TYPE SENSORS 04-090WA

Figure 43 Bus ribbon tray assembly (table not shown). 

A bus ribbon location tolerance study was done as part of the tray design. Given the large camber (inplane curvature) specification provided by the ribbon supplier, the ribbon location tolerance is as large as 

±1.19 mm (±0.047 inch) in the ribbon width direction for the longest ribbon in our module. This 

tolerance is too large for aligning the bus ribbons to the diodes in the module, so the location of the 

ribbon ends must be determined after they are picked up from the tray by the robot. While a vision 

system could find the ribbon ends, we will use a fiber optic sensor, since the hardware is a significantly

less expensive. 

33
The robot end-effector design was reviewed and we were able to eliminate the tool changer by mounting 

the pick-and-place and the soldering tools on the same end effector, as shown in Figure 44. Eliminating 

the tool changer saves process time, end-effector complexity, and cost. 

GUIDED AIR CYLINDER

SOLDERING TIP 

SPRING LOADED 

VACUUM CUP ARM DOWN SENSOR 

Figure 44 Robot end effector with pick-and-place and soldering tools. 

The output bus ribbons at opposite ends of the module are bent up at a 90° angle to extend through the 

EVA and back sheet. A simple bus ribbon bending tool was prototyped at ARRI. The process sequence 

with ARRI’s tool is illustrated in Figure 45. When the robot picks up an output bus, it places the end of 

the bus in a slot in the bending tool. A guided air cylinder pushes a plate over the ribbon to form a 90°

bend. After the air cylinder retracts, the robot removes the ribbon from the tool and places it on the 

module. 

04-057WA

Figure 45 A robot inserts a bus bar in the bending tool (left), an air cylinder extends to bend the 

ribbon (center), and the cylinder retracts (right). 

34
Spire designed a ribbon bending tool based on this concept. The tool, shown in Figure 46, uses a springloaded roller in place of the plate to form the bend in the ribbon. The roller eliminates scuffing or other 

damage that a plate might do to the ribbon, while the spring load allows the roller to follow the surface of 

the vertical plate without the need for precise alignment. 

BEVELLED 

RIBBON 

SLOT

SPRING LOADED 

ROLLER 

GUIDED AIR 

CYLINDER 

Figure 46 Ribbon bender, Spire design. 

The by-pass diodes are solderable chips with silver contacts covering their top and bottom surfaces. A 

thin coating of flux must be applied at both (top and bottom) diode-bus ribbon interfaces for reliable 

soldering. ARRI demonstrated a robotic flux application method as follows. A robot with a vacuum end 

effector picks up a diode and places it momentarily on a sponge soaked in flux to coat its bottom surface. 

The diode is then placed on the appropriate bus ribbon in the module. Next, the robot picks up the bus 

ribbon that will contact the top of the diode and the end of the ribbon is momentarily placed on the 

sponge to coat its bottom surface. The ribbon is then placed with its fluxed end on the diode. 

Spire then designed a flux applicator to implement this process. The applicator, shown in Figure 47, has a 

small flux reservoir, a float switch, a peristaltic pump, and a sponge. The float switch and the pump 

maintain a constant level of flux in the reservoir, roughly at the mid-point of the sponge thickness. 

2.5.4 Automated Soldering Process Development 

ARRI investigated reflow soldering techniques for soldering the bus ribbons to the diodes and cell strings. 

ARRI worked with Unitek Miyachi on pulsed hot bar reflow soldering equipment selection and design of 

a customized thermode for the soldering requirements specific to the Brightfield module, specifically the 

string-bus and bus-diode-bus joints. To simplify the soldering process both in terms of cycle time and 

cost, an effort was made to design a thermode head that is suitable for both joint types. 

35
FLOAT SWITCH 

SPONGE 

Figure 47 Flux applicator (left) and diode assembly in the module (right). 

String-bus soldering: Due to the large number (96) of string-to-bus joints on the Brightfield module, a 

two-pronged thermode is recommended so that the two ribbons at the end of each cell string can be 

soldered in a single step. According to Unitek’s recommendation, a thermode head can be up to 130 mm

wide and still maintain a constant temperature across the length of the head. The string lay-out on the 

module requires an 81.6 mm wide thermode head, well under the 130 mm limit. Testing at ARRI on 

string-to-bus joints showed that sufficiently sound joints can be formed with a thermode contact area of 

2.2 mm × 3.0 mm. Each prong of the Unitek solder head would have dimensions of 7.2 mm × 3.0 mm,

allowing for a variation in the position of the string on the bus of ±2.5 mm. A suggested thermode design 

is shown in Figure 48. 

04-091WA

Figure 48 Preliminary thermode design for string-bus and bus-diode-bus joints (dimensions in 

mm). Thickness is 3 mm. 

Bus-diode-bus soldering: ARRI sent tabbed, fluxed bus bars and diodes to Unitek Miyachi along with 

detailed drawings, and requested that they perform tests to determine what would be required (thermode 

design, power supply model, and reflow process) to form the bus-diode-bus joint in a single step. Unitek 

was able to solder the diode between the two bus bars, but did so using a two-step soldering process. 

Based on their testing, Unitek concluded that in order to form the bus-diode-bus joint in a single step, 

heating from both the top and bottom would be required. Unfortunately, this is not compatible with our 

36
module automation approach, and it would almost double the cost of the soldering system due to the need 

for a second high current power supply.

ARRI continued feasibility testing on the bus-diode-bus joints using a temperature controlled industrial 

soldering iron as the heating source. Using a reflow temperature of 300 oC and a reflow time of 7 s, it 

was possible to obtain good solder bonds to the bus bars on both sides of the diode. With this method, the 

heat was applied only from the top. 

The main drawback with a soldering iron is that it has no means for holding the solder joint in contact 

while the solder cools. This problem was recognized by Spire when we first started developing 

automated soldering equipment for solar cells twenty years ago. At that time, we invented a unique 

soldering tip that operates at a constant temperature and has a built-in pin that holds the joint in contact 

until the solder freezes. The soldering tip, shown in Figure 49, is heated by a small cartridge heater and 

its temperature is monitored by a thermocouple. The heated tip and the cooling pin are spring loaded for 

compliance with the soldered parts. 

HEATER PLUNGER BODY

THERMOCOUPLE 

SOLDERING TIP HOLD DOWN PIN 

Figure 49 Spire soldering tip assembly.

A low-cost PID temperature controller is all that is needed for controlling the Spire solder tip. Since the 

heater operates at low current, it eliminates the massive cables needed by the pulsed hot bar soldering 

system. This is a significant advantage, since the solder tips are mounted on a robot, where the mass must 

be minimized. Given the substantial cost savings and reduced mass of the controls, the Spire designed 

solder tip was selected for this application. 

Two solder tips are shown mounted on the robot end effector in Figure 44. This allows the two ribbons at 

each end of each string to be soldered to the bus ribbon at one time. Each tip is mounted on a guided air 

cylinder to keep them retracted when they are not in use, i.e., when the robot is performing pick-and-place 

operations. A single tip will be extended for diode soldering. 

37
3 CONCLUSIONS 

Spire has completed the first phase of a three-phase program for developing automated production line 

processes for solar Brightfield modules. A large area 800 W module for utility-scale PV arrays was 

designed, with emphasis on minimizing BOS costs. Unique module features include internally laminated 

by-pass diodes, which greatly simplified the internal busing and the output terminals, and a cantilevered 

superstrate that allowed for a reduced glass thickness and a simpler frame design. 

The design of automated tools is underway for producing such modules in multi-megawatt per year 

quantities. A cell string inspection system with machine vision and microcrack detection capabilities was 

developed and tested. Spire applied for a patent on the infrared crack detection technique. An advanced 

lamination process was developed with a faster-curing EVA for increased laminator throughput, and 

efforts are underway to reduce laminator maintenance and downtime. Automated systems are being 

designed for dispensing and laying up module materials (EVA and back sheet) and for installing and 

soldering bus ribbons and diodes prior to lamination. 

In the remaining two phases of our program, we plan to fabricate and test the automated module lay-up 

and string busing systems; design, fabricate and test an improved laminator and a large area sun 

simulator; and develop a computer integrated manufacturing system for module line supervisory control 

and data acquisition. 

38
F1146-E(05/2004) 

REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

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PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

June 2005 

2. REPORT TYPE

Subcontract Report 

3. DATES COVERED (From - To)

1 January 2003–30 June 2004 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Development of Automated Production Line Processes for Solar 

Brightfield Modules: Annual Technical Progress Report, 

1 January 2003–30 June 2004 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/SR-520-36608 

5e. TASK NUMBER

PVB56101 

6. AUTHOR(S)

M.J. Nowlan, J.M. Murach, S.F. Sutherland, D.C. Miller, S.B. Moore, 

and S.J. Hogan 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

 Spire Corporation 

 One Patriots Park 

Bedford, Massachusetts 01730-2396 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

ZDO-2-30628-12 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

NREL/SR-520-36608 

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

NREL Technical Monitor: K. Brown 

14. ABSTRACT (Maximum 200 Words)

This report describes how Spire Corporation is addressing the PV Manufacturing R&D project goals of improving 

photovoltaic (PV) manufacturing processes and products while reducing costs and providing a technology foundation 

that supports significant manufacturing scale-up. To accomplish this, we are focusing our efforts on the design of a 

large-area utility-scale module and the development of the necessary manufacturing techniques and equipment to 

manufacture such a module in a high-volume production environment. A three-phase program is under way for 

developing and demonstrating new automated systems for fabricating very large PV modules ideal for use in multimegawatt grid-connected applications. We designed a large-area 800 W module and we are developing associated 

module production equipment that will minimize the total installed system cost for utility-scale PV arrays. Unique 

features of the module design include a cantilevered glass superstrate to reduce the glass thickness and internally 

laminated bypass diodes that simplify internal busing and junction-box designs. Other program activities include the 

development of automation for solar cell string inspections, string busing, materials lay-up, and lamination; 

enhancements to the lamination process; and performance testing of large-area modules. 

15. SUBJECT TERMS

PV; manufacturing; solar cell; utility-scale module; string inspections; string busing; materials lay-up; lamination; 

manufacturing scale-up; high-volume production; automated systems; large-area module; 

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Unclassified

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