Development of a New Solder Paste Printerby Nobuyuki Yamamoto * , Hideaki Fujii * , Toshiya Hikami *2 and Yoshinori Nakamura *2

Recent years have seen an explosive dissemination of cellular telephones, personal digital assistants (PDAs) and other types of hand-held information terminals. This has led to increased use of surface mounted components--ball grid arrays (BGAs),
chip scale packages (CSPs) and microcomponents like the 0603 chip, which are difficult to
inspect or repair once they have been mounted, and this in turn means greater demand for solder
paste printers that offer greater accuracy and higher speed. Accordingly the authors have developed a new type of solder paste printer, offering the same high rigidity and higher alignment
accuracy as Furukawa Electric's currently used model, together with a new squeegee unit thatoffers stable printing irrespective of printing speed and an alignment system capable of simultaneous detection of stencil and board.

ABSTRACT1.

INTRODUCTION

The performance requirements for the solder paste printers used in the surface mounting of electronic components on printed circuit boards are becoming ever more
stringent. This is driven by the increased mounting densities that have accompanied the rapid diffusion of cellular
telephones, PDAs, and ultrathin notebook computers. The
quad flat package (QFP) will also be replaced more andmore by BGAs and CSPs, which are difficult to inspect
and repair after mounting, and by chips such as the1005/0603, so that the need for high alignment accuracy
and finer printing pitch becomes even more pressing. In
addition, a hard look must be taken at printer throughput
to drive down costs through improved productivity. This isbecause unlike the chip mounting process, which can bedivided up among a number of mounter units run in parallel, the printing process is indivisible.
The new type of squeegee unit was developed taking
account of the the behavior of solder paste on the stencil
during printing in order to reduce the number of parameters that must be monitored during printing while assuring
printing performance equal to or better than that of thesqueegee unit currently in use. The new unit alsoachieves higher speed through adoption of an alignment
system that gives simultaneous detection of stencil andboard. The following paper provides a detailed report.

* Technology Development Sec., Industrial Equipment Dept.,
Electronics Components Div.
*2 Components and Mounting Technology Development Dept.,
Ecology and Energy Lab.

2.

DEVELOPING A NEW SQUEEGEE UNIT

2.1 Mechanism of Solder Paste Printing
In solder paste printing, rolling of the paste occurs when it
is removed from the stencil using the squeegee. This
results in an increase in the internal pressure of the paste,causing it to fill the apertures in the stencil so that it istransferred to the board by detaching the stencil to accomplish printing.
The distribution of the internal pressure of the paste acting on the stencil may be determined from Equation (1)psqueegee = ∫dpsqueegee= [2α sinα/(α2 - sin2α)]ηV(1/r)(1)= f(α)ηV(1/r)
where:
r is the distance from the point of contact between
the squeegee and the stencil,
α is the squeegee angle,
V is the printing speed, andη is the solder paste viscosity.
An investigation was then made of relationship inEquation (1) between f(α) as a pressure coefficient affecting the pressure distribution on the stencil surface andsqueegee angle α. Figure 1 shows the result. It can beseen that the internal pressure of the paste undergoes amajor change in accordance with squeegee angle. To
maintain constant internal pressure in the paste for stable
printing it is necessary to control not only squeegee angle
but also printing speed and the physical properties of thesolder paste. Figure 2 summarizes the factors giving riseto internal pressure in solder paste.
When solder paste pressure rises in the printing
process a force is exerted on the squeegee surface thatacts to raise the squeegee unit. This paste reaction force

75

2.0

Pressure coefficient f( α )

1.8
1.6

Pressing force generator

1.4

Paste wiping section

1.2
1.0
0.8
0.6
0.4
0.2

Figure 3

Conceptual diagram of the new-type squeegee.

0
45

50

55

60

65

70

75

Squeegee angle α (deg)Figure 1

Relationship between squeegee angle α and internalpressure coefficient f(α).

Internal pressure(2) Printing speed

apertures are wide, the squeegee may penetrate it,
scooping up the solder paste in the apertures so that notenough is available. Furthermore the increase in friction
between the stencil and the squeegee will cause the stencil to be shifted in the direction of squeegee movement,with the danger that the printing position may be displaced. In high-speed printing, the printing pressure is particularly high, and problems of excessive pressure occur
readily.

(1) Squeegee angle(3) Paste viscosity (amount)

Figure 2

Factors giving rise to internal pressure in solder
paste.

Flift may be found by differentiating Equation (1) in the solder paste region as represented by Equation (2)Flift = ∫psqueegeedr = f(α)f(Q)ηV
where:Q is the amount of solder paste.

(2)

To achieve satisfactory printing it is necessary to wipe
the paste on the stencil cleanly away, and with thesqueegee currently in use it is necessary to apply printingpressure sufficient to overcome the internal pressure of
the solder paste. There is an appropriate value for thisprinting pressure, which is determined by squeegee angle,
printing speed, and the amount and viscosity of the solder
paste.
2.2 Problems with the Squeegee Currently in Use
With the squeegee currently in use, the printing pressure
is set to the optimum value by a test run prior to the actual
printing operation. This not only takes time, but since conditions such as the amount and viscosity of the paste canchange as the process goes on, destroying the balance
between the internal pressure of the paste and the printingpressure. If printing pressure is either too high or too low,stable printing becomes impossible.
Inadequate printing pressure results in improper wipingof the solder paste leading to improper transfer.
Conversely, when printing pressure is excessive, the solder paste will seep to the other surface of the circuitboard, causing smudging and bridges. And where stencil

76

2.3 Concept of the New-Type Squeegee Unit
Although printing pressure is not something that has adirect effect on printing performance, stable printing cannot be obtained with the squeegee currently being usedwithout extremely thorough monitoring. The authors therefore proposed to develop a new type of squeegee unit thatwould maintain printing performance equal to or better
than the current type, while eliminating the concept ofprinting pressure. In the new unit, rolling of the solder
paste is retained, while the inclined section that applies
the pressing force (solder paste internal pressure) within
one unit is separated from the section for the wiping of thesolder paste. Figure 3 shows a conceptual diagram.
The wiping section was designed so that it would be difficult for the counterforce from the solder paste to beexerted during printing. To achieve a structure in which
the internal pressure in the solder paste would produce
rolling and be no less than in the squeegee currently inuse, finite element analysis was carried out. Figure 4(a) is
a speed distribution diagram representing the results of ananalysis of solder paste behavior during printing with thecurrent squeegee; the angle formed by squeegee andstencil was 60°, printing speed was 10 mm/s, and solder
paste viscosity was 200 Pa•s. Figure 4(b) is an analogous
diagram showing speed distribution during printing with
the new squeegee; the values for inclined portion angle,
printing speed and solder paste viscosity are similar to
those for the current model. In both cases flowing of thesolder paste can be observed on the stencil andsqueegee surfaces, allowing the mechanism of rolling to
be known. Figure 4(c) and (d) show the internal pressure
distribution of the solder paste for the current and newtype squeegees respectively. It will be seen that in bothcases maximum pressure is exhibited at the point of intersection between squeegee and stencil, and that the pressure diminishes in accordance with the distance from thepoint of intersection.

Furukawa Review, No. 21

2002

(a)

(b)

Speed distribution

SqueegeePrinting direction
Solder paste

Squeegee
Solder paste

Stencil

Stencil(d)

Pressure distribution(c)Current squeegee

Figure 4

New-type squeegee

Simulated speed and pressure distributions in currently used and new-type squeegees.

0

Current squeegee

0.5
0.6

φ 0.3

0.7

New-type squeegee

0.8
0.9
1
0

50

100

Printing speed (mm/s)

Figure 5150

200Units: mm

Measurements of effect of depth of shift and internal
paste pressure.

Printing pressure x 10 5 (N/m)

0.3

1.0

Depth of shift (mm)

0.2

0.4

4500

Depth of shift

0.1

4000
3500

Current squeegee

3000
2500
2000
1500
1000

New-type squeegee

500
0
0

0.05

0.1

0.15

0.2

Printing speed (m/s)

Figure 6

To confirm the above results, printing was done at various printing speeds using metal circuit boards 1-mm thickin which through-holes 0.3 mm in diameter had been
formed, and the depth of solder paste transfer was investigated. A comparison was then made of the internal pressure of the solder paste using the current and new-typesqueegees. Figure 5 shows the results.
It will be seen that at all printing speeds the new-typesqueegee gave greater depth of transfer than the currently
used type, thereby confirming that the new squeegee
structure shown in Figure 3 provides ample solder pasteinternal pressure.
Since in the new-type squeegee the wiping section isperpendicular to the stencil surface, the force shown inEquation (2) is not exerted on the wiping section, so that
resetting of printing pressure at changing model is not
required, irrespective of printing speed or solder paste viscosity. That is to say printing can be carried out at low
printing pressure, independent of printing speed. Figure 6
shows the results of an investigation of the printing pressure required to remove solder paste cleanly from theRelationship between printing speed and printingpressure.

stencil at various printing speeds.
With the squeegee currently in use greater printingpressure is required as printing speed increases, whereas
it can be seen that with the new-type squeegee the solder
paste can be wiped from the stencil at a low level of printing pressure that is independent of printing speed.
Accordingly even if the internal pressure of the solder
paste is reduced due to changes in the properties oramount of the solder paste, the problems of excessive
printing pressure associated with the squeegee now in
use are eliminated, and stable printing is possible.
2.4 Print Characteristics of the New-Type Squeegee
To check the print characteristics of the new-typesqueegee both low- and high-speed tests were carried out
and comparisons made with the type currently in use. The
stencil design used consisted of two patterns: apertures
0.3 mm in diameter in addition to a QFP pattern with a77

Printing direction

0.3 mm diameter

0.3-mm pitch;
aperture width 0.15 mma) Printing speed: 50 mm/s

Photo 1Printing direction

0.3 mm diameterb) Printing speed: 200 mm/s

Results printed with currently used squeegee.

Printing direction

0.3 mm diameter

0.3-mm pitch;
aperture width 0.15 mma) Printing speed: 50 mm/s

Photo 2

FUNCTIONS AND FEATURES OF NEW
TYPE FPX-46A PRINTER

The newly developed printer, to be known as the FPX-46A
is shown in Photo 3. It has the following functions andcharacteristics.
3.1 Basic Specifications
Table 1 shows the basic specifications of the FPX-46A
solder paste printer. It is capable of handling large circuit

78

Printing direction

0.3 mm diameter

0.3-mm pitch;
aperture width 0.15 mmb) Printing speed: 200 mm/s

Results printed using new-type squeegee.

lead pitch of 0.3 mm and an aperture width of 0.15 mm.
Printing speeds were 50 and 200 mm/s. Printing pressure
was set prior to beginning the print run for the currently
used squeegee only. Results are shown in Photos 1 and
2.
With the currently used squeegee, printing failuresoccurred at the 200 mm/s speed with the pad perpendicular to the printing direction, whereas the new-typesqueegee printed at lower printing pressure, producing
satisfactory conditions irrespective of direction at both 50
and 200 mm/s.
Thus it was possible to realize a new-type squeegeethat provided print characteristics equal to or better thanthose of the currently used type, without the pre-setting ofprinting pressure that was heretofore needed to obtainstable printing.

3.

0.3-mm pitch;
aperture width 0.15 mm

Photo 3

FPX-46A solder paste printer.

boards measuring up to 510 mm in the length or transport
direction and 460 mm in the width or transverse direction.
Both the transport direction and transport reference canbe freely selected at manufacture. Printing stencils canmeasure up to 800 mm in the transport direction and 750mm transverse.
3.2 Reduced Cycle Times
To achieve more rapid cycling a change was made in themethod for aligning the positions of the print stencil andthe circuit board, from the currently used two-camera system consisting of a movable stencil camera unit mounted

Furukawa Review, No. 21

2002

Table 1

Basic specifications of the FPX-46A printer.

Depth

1600 mm

Height1625 mm

Weight (approx.)

1400 kg

Table 2Comparison of alignment axis specifications
between new and currently used designs.

Control axis

±5 mm

540 mmResolution

0.49 µm/pulse

0.98 µm/pulse

Max. speed

50 mm/s

533 mm/s

X StrokeLine accommodation

Freely accommodates to transport direction

Processing standPrinting stencil

Center-registered (max. 800(l) x 750 (w) mm)

Max. dimensions

510(L)x750(W) mm

Min. dimensions

50(L)x50(W) mm

Thickness

0.5-3.0 mm (below 1.0 mm by option)

Printing method

Contact printing with stencil detaching speed control

Printing functions

Current

New

Print table±5 mm

808 mm0.49 µm/pulse

0.98 µm/pulse

Y Stroke
Resolution

50 mm/s

533 mm/s

θ Stroke deg

Max. speed±1.0 deg(±5.41 mm)±1.5 deg(±8.1 mm)

Resolution

2.26x10-5 deg/pulse

4.52x10-5 deg/pulse

Max. speed

4.8 deg/s

9.6 deg/s

Z Stroke

155 mm46 mm

Perpendicular to direction of circuit board transport

Resolution

0.407 µm/pulse

0.098 µm/pulse

Squeegee control

Bi-directional printing with double squeegeeMax. speed

222 mm/s

40 mm/s

Squeegee speed

Variable from 5 to 200 mm/s

Print direction

Cycle time
Board support

Approx. 8 s (not including printing time)

T Stroke

36 mm±2 mm

34 mm±2 mm

Resolution

0.407 µm/pulse

0.12 µm/pulse

Max. speed

222 mm/s

50 mm/s

By magnetic pin and insertion pin method (standard)
Camera T Stroke

Power supply

200 V AC, 3-ph, 5 kVA

540 mmResolution

2.44 µm/pulse

Max. speed

1000 mm/s

Camera Y Stroke

850 mmResolution

2.44 µm/pulse

Max. speed

1000 mm/s

Cycle time not including

8

printing time (s)Operation

22

Time (s)
5

10

15

20

Transporting circuit board
Clamping circuit board
Recognition
Alignment

Photo 4Single camera unit.

Printing
Detaching stencil
Ejecting circuit board

on the print table and a fixed circuit board camera unit tosystem in which an XY camera shaft is mounted between
the stencil and the print table so that the stencil camera
and circuit board camera are accommodated in a single
camera unit (see Photo 4).
Table 2 shows a comparison of the alignment axis specifications of the currently used and new units. In the currently used type the printing table moves in the X and Y
directions relative to the circuit board camera, requiringthat the travel distance is large. In the new type, on theother hand the only travel is that required for fine adjustments. This eliminates movement of the printing table,
which has high inertia, and by moving the lightweight camera unit at high speed making it possible to significantly
shortening the cycle when not printing.
As a result it was possible to achieve a non-printing
cycle time of some 8 s for the new type, versus 22 s forthe type currently in use (see Figure 7).

Figure 7

Cycle time.

3.3 High-Accuracy Alignment
The use of a single camera unit makes possible the simultaneous detection of the alignment marks of both the printing stencil and the circuit board. Figure 8 shows themethod used.
The stencil camera and the circuit board camera are
independent, superimposed on a vertical shaft. Images
are transmitted through the respective lenses, and directed by the pentaprism to the CCD cameras. The verticaloffset between the two cameras is adjusted automaticallyduring printing by a calibration jig provided at the escapeposition of the camera unit. This design makes it possible
to recognize the stencil and circuit board automaticallyeach time, rendering it unnecessary for test runs to detectand compensate for offset errors.

79

StencilY-axis
CCD cameras
Z-axisPentaprism
Z-axisLens

Y-axisCamera axes

Circuit board
X-axisθ -axisPrinting table axes

Figure 8

4.

Alignment method using single camera unit.

SUMMARY

The new squeegee unit used in the printer developed inthis work achieves printing performance equal to or better
than that obtained from the currently used squeegee unit,
while eliminating the need to set printing pressure. And
the use of an alignment system that provides simultaneous detection of stencil and circuit board makes it possible
to achieve both increases alignment accuracy and higher
speed.
This has made it possible to market a printer offering
the high levels of performance envisaged in the original
development targets.

5.

CONCLUSION

Solder paste printers are of increasing importance in surface mounting technology. The new printer developed inthis work may be distinguished from competitors' productsin that it accepts LL circuit boards, while providing highspeed printing and, despite its compact dimensions,enables major options to be built-in. It is expected that byidentifying target users and conducting a systematic marketing campaign, it will be possible to equal the market
share of reflow furnace systems.

ACKNOWLEDGMENTS
The authors would like to express their thanks to Mr.
Hiroyuki Kobayashi of Furukawa Information Technologies
for assistance with finite element analysis.

REFERENCE
1) D. E. Riemer; "The theoretical fundamentals of the screen printing process", Hybrid Circuits, No. 18, pp. 8-15, 1989.

Manuscript received on June 21, 2001.

80

Furukawa Review, No. 21

2002