Reflow Oven for a Pb-Free Soldering Process
by Kosuke Nakao *, Atsushi Hiraizumi *2 and Etsuko Iwasaki *From the standpoint of environmental protection, we cannot move forward without adopting Pb-free processes for the soldering of electronic components.
Furukawa Electric, a leading manufacturer of reflow soldering systems, has now developed anoven designed for the Sn-Ag process, which at present seems to show the best chance of actual
implementation. Compared to the Sn-Pb eutectic crystal solders used conventionally, the liquid
phase temperature of Sn-Ag solder is about 30°C higher. The temperature for soldering must be
at least 230°C. If the joints of the components having the highest thermal capacity are raised tothis temperature, there is a major problem with the heat withstand properties of those with lower
thermal capacity. The issue as far as the reflow oven is concerned is how small a temperature
difference can be achieved between component joints. Other concerns addressed in the development work were bare-board temperature distribution, temperature profile reproducibility andobtaining wetting time, and satisfactory results were obtained. Results were especially good interms of joint-to-joint temperature difference, which was reduced to about one-third of the former
value.

ABSTRACT1.

INTRODUCTION

During the 20th century mankind has been polluting theenvironment at a fearsome rate. The pollution of rivers byindustrial and household effluents; the depletion of theozone layer by CFC gases that has received so muchattention in the 90s; acid rain caused by gaseous emissions from factories and automobiles; and global warming.
If immediate attention is not given to such environmental
problems we will not be able to bequeath to our children
and grandchildren a world where they can live in security.
Turning our attention to the field of electronics, we seethat product life cycles have been dramatically shortened.
Computers and cell phones are perfect examples of products that are soon replaced by models that have morefunctions at a lower price, and the ordinary consumer cannot wait to buy into the new convenience, carelessly discarding the outmoded item.
Sulfurous oxide gas and other components of factory
and automobile exhaust gas combine with atmosphericmoisture and fall to the ground as acid rain, causing trees
to wither where they stand. Acid rain also attacks metals.
The Pb used to solder electronic components is leachedout by acid rain, becoming ground water that, when used
for drinking purposes, builds up in the human body.
It was in the late 1990s that manufacturers began to feela responsibility to reduce the harmful effects of Pb by*

Engineering Sec., Industrial Equipment Dept., Electronics
Components Div.
*2 Process Development Dept., Production Technology
Development Center, Plants & Facilities Dept.

developing Pb-free soldering processes. The first candidates were tin-silver (Sn-Ag) and tin-zinc (Sn-Zn) solder,but Sn-Zn soldered joints have no track record when it
comes to reliability and zinc is a base metal exceedingly
subject to oxidation, with the result that only a few manufacturers have had it under study.
The Pb-free oven developed in this work is specificallydesigned to use Sn-Ag solder.

2.

DEVELOPMENTAL CONCEPTThe following are the main requirements for a reflow oven
designed for Pb-free soldering.
(1) Uniform temperature distribution on bare boards: In
measuring temperature profile, normal practice is tomeasure the temperatures of joints of components
having large thermal capacity and small thermal
capacity (ordinarily represented by substrate temperature), and estimates can then be made based on thisdata for components of intermediate size. If, however
the bare board temperature distribution is large, thetemperature may be too high or too low in unexpected
locations making the oven difficult to use.
(2) Low value of ∆T: In reflowing using Sn-Pb eutectic
crystal solder, soldering can be comfortably accomplished even when the upper temperature limit for thesmallest components was about 240°C and joint temperatures for the largest components was 210°C (∆T= 30°C). To implement Pb-free (Sn-Ag) soldering,
however, the maximum temperature of the smallest
components must be held to that same 240°C, while77the temperature of the components having the largest
thermal capacity must be raised to about 230°C, so
that ∆T is only 10°C. Thus the problem is not theheating capacity of the oven itself, but how far thejoint temperature of the largest components can beraised.
(3) Excellent reproducibility of temperature profile: Whilein Sn-Pb eutectic crystal soldering there was a marginof approximately 30°C with respect to the melting
point, Sn-Ag soldering allows for only 10-15°C. Whatthen becomes important is the reproducibility of thetemperature profile. PCBs are not manufactured
under individually set conditions. They are run in continuous batches, and with irregular insertion intervals.
Under such conditions of production, the oven cannot
be effectively used unless the temperature profile
obtained is identical with that obtained under condition setting.
(4) Assured wetting time: Heat transfer involves considering both the thermal capacity of the object being heated and its thermal resistance. In ball grid arrays(BGAs) and other packaged components, the package and PCB are heated first, and only then is thejoint heated by conduction, so that time is a crucial
parameter in the temperature rise characteristics. For
example, even if the package surface is exposed to a
powerful flow of gas heated to 230°C, the joint insidewill only get warm over a period of time. Thus it ismore effective if the reflow zone can be kept at peaktemperature while awaiting the rise in the temperatureof the joints inside.

DETERMINING THE HEATING METHOD

3.1 Heat Transfer Mechanism
Figure 1 shows heat transfer when soldering PCBs in a
reflow oven. Heat transfer takes place in three ways--radiation, convection and conduction. Of these, it is the rates
of radiation and convection that can be controlled at theoven. Figure 3 shows the heat transfer rate due to far
infrared heating, and, as can be seen, the infrared heat

Low thermal
capacity∆ T1

Temperature

3.

energy is significantly greater than that of the hot gas flow.
Salamander® ovens are designed for the reflow soldering of high-density PCBs of high thermal capacity, so that
infrared serves as their primary heat source and hot gas
as an auxiliary source. Nonetheless if only infrared is used
for heating, there will be too large a difference between
the temperature of components with low and high thermal
capacity (∆T1 in Figure 2). Accordingly gas at a temperature intermediate between the two is applied, acting tocool the hotter component (the one with low thermal
capacity) and further heating the component with highthermal capacity. This results in a reduction in the temperature difference to ∆T2. This is the foundation of the theoryof heating used in the Salamander series, and it will beobvious that to realize this effect the temperatures of thetwo heat sources must be independently controlled.
Furthermore, to enhance the temperature equalizing effectof the hot gas, heat transfer coefficient α in the equation
for convection heat transfer in Figure 1 should be on thehigh side. The value of α will change depending on theway in which the hot gas impinges on the object being
heated and on the rate of gas flow. It is generally considered that a higher gas flow rate is desirable, but an excessively powerful gas stream can disturb the position ofcomponents on the PCB. Gas flow rate should normallybe kept to no more than 4 or 5 m/s to avoid displacing surface-mounted components, and in the present work it was
decided to consider the direction in which the gas
impinged, without assuming any increase in gas flow rate.
Figure 3 shows the relationship between gas flow direc-

Infrared heating∆ T2
Gas temperature
High thermal
capacityTime

Figure 2

Temperature equalizing effect due to infrared and hot
gas heating

Nozzles

Boundary layer
Heat transfer by
convection

Diverging
point

Heat transfer by radiation
Laminarflow region

Turbulentflow region

Thermalconduction

Shape factor F12(Heater for substrate)
Radiation
Emissivity ε 1 . ε 2:QR = F12 . ε 1 . ε 2 . σ (T14-T24) Stefan-Boltzmann coefficientσ = 5.68x10-8(W/m2K4)
Convection heat transfer:QC = α (T∞-T2)
Heater temperature T1
Thermal conduction
Substrate temperature T2:Q = - λ . A . ∆ T/ ∆ X
Gas temperature
T∞

Figure 1

78

Heat transfer in a reflow oven

Heat transfer rate (W/m2)
T2=200°C, T∞=250°C

Figure 3

Parallel

PerpendicularNozzle flow

flow

flow

(diverging point)

1560

2175

6500

Direction of hot gas flow and heat transfer rate (forcircuit board)

Furukawa Review, No. 20

2001tion and the rate of heat transfer. In conventional ovens a
nozzle flow was applied at the center of the PCB with aparallel flow at the edges, but it can be seen from the figure that the use of a nozzle flow system, having a large
number of nozzles results in an increase in the heat transfer rate.

and the results are shown in Figure 5. For these experiments the heating conditions were set so that the joint
temperature was at approximately 100°C for a specified
period (70 s). Figure 5 shows the results for (left to right)
infrared (IR) heating, hot-gas heating and a combinationof the two, with data for a conventional heating system inthe upper row and data for the unit developed in this workin the lower row. It is obvious that the new system resultsin improved hot-gas heating performance, with a marked
reduction in the temperature difference at the joint
between a PCB (represented by the temperature of thecomponent having the smallest thermal capacity) and thecomponent. The temperature difference between the two
was further reduced when infrared heating was used in
combination.

3.2 Verifying Heating Structure
Photo 1 depicts the panel heater used in the present system showing the arrangement of nozzles. The heating
structure was based on the use of such units. Figure 4
shows the improvements achieved in terms of heat transfer rate α. In comparisons using the same gas flow rate,
the new system (with nozzles) showed an improvement ofapproximately 40%. This might at first sight be considereda disappointing increase, but in nozzle flow the heat transfer is extremely high at a point directly beneath the nozzles (the diverging point) and drops sharply toward the
periphery so that on average this figure may be considered satisfactory. An evaluation of the heating characteristics of a QFP 144P mounted on a PCB was carried out,

3.3 Effectiveness of Reflowing in Inert Atmosphere
Photo 2 shows the reflow condition of Sn-Pb eutectic crystal solder and Pb-free (Sn-Ag-Cu) solder, demonstratingthat the Pb-free solder is far inferior in wettability (i.e., it
has a higher surface tension when melted). It has beenshown that wettability was slightly improved by reflowing

Heat transfer coefficient (W/m2K)

60
55

Conventional system
Present system

50
45
40
35
30
25
20
15
10
0

0.5

1

1.5

2

2.5

3

3.5

4

Gas flow rate (m/s)

Figure 4a) Conventional heating system

Panel heater used in the present system showing
arrangement of nozzlesi) IR heatingii) Hot-gas heatingiii) Combined (IR + hot-gas)

Gas flow rate: 2 m/s, hot-gas temperature 170°C

Gas flow rate: 2 m/s, hot-gas temperature 95°C
IR heater 180°C

120

120

100

100

80

80

60

60

60

40

40

40

20

20

120
100

Circuit board

80

Joint

Body

20

0

0
0

10

20

30 40 50
∆ T = 16°C

6070b) New heating system

0
0

80

10

20

30

40 50
∆ T = 22°C

60

70

80

Gas flow rate: 2 m/s, hot-gas temperature 120°C

0

120

120

100

100

100

80

80

80

60

60

60

40

40

40

20

20

20

0

0
0

10

20

30 40 50
∆ T = 14°C

60

70

20

30

40 50
∆ T = 11°C

60

70

80

0
0

80

10

Gas flow rate: 2 m/s, hot-gas temperature 95°C
IR heater 180°C

120

Time (sec)Figure 5

Thermal coefficients of conventional and present
heating systems

IR heater: 200°C

IR heater: 200°C

Temperature (°C)

Photo 1

10

20

30

40 50
∆ T = 10°C

Body
QFP 140P

60

70

Joint

80

0

10

20

30

40 50
∆ T = 7°C

60

70

80

Board

Temperature difference (∆T) between joint and substrate for conventional and present heating systems

79

in a nitrogen atmosphere. Many manufacturers are introducing equipment to automate external appearance
inspections. Since this equipment inspects the shape offillets at the ends of the leads (joints) of the electronic
components, it is expected that if wettability is so poor thatthe fillets are malformed, more components will be rejected. This data suggests the possibility that by reflowing in anitrogen atmosphere, the frequency of wrong decisions by
the inspection equipment can be reduced. Also, when
reflowing the second surface of a two-sided reflowed
PCB, oxidation during the first pass can be inhibited,
improving soldering quality.

Photo 2

Photo 3

Wettability of Pb-free (Sn-Ag-Cu) solder

Appearance of the Salamander XN III-525 PZ systema) Salamander XN III-525
Fan

Motor

Panel heater

4.

THE SALAMANDER XN III SERIESSalamander XN III is a line of nitrogen gas reflow ovens
that have been commercially introduced, incorporating the
new heating system described above for Pb-free soldering. Photo 3 shows the appearance of the Salamander XN
III-525 PZ system, and Figure 6 shows a comparison of
the heating zone structures of the Salamander XN III-525
and XN III-725. Both infrared and hot-gas heating areused in the zone in which rapid heating of the PCB
occurs, with hot-gas heating used in the zone in which thetemperature is maintained constant. Of the heating zonesof the 525, zone 5 measures 450 mm in length and theother zones 600 mm, while in the 725, all zones are 450
mm. The 725 was developed to provide a more flexible
temperature profile, the larger number of zones making it
possible to achieve a greater variety of temperature profiles.
4.1 Features
The Salamander XN III series has the following features:(1) The nozzle-flow hot-gas heating system reduces temperature differences between components: Figure 7
shows the temperature difference between PCB andcomponent (a 45x45 mm BGA) for the XN III and theprevious model. It can be seen that the difference has
been reduced to one-third.
(2) Thermal energy is controlled independently for
infrared and hot gas: The same level of temperature
profile reproducibility was obtained for the
Salamander XN III as for the XN, probably due to theaccuracy with which the infrared and hot-gas heatsources are controlled. Figure 8 compares the change
in temperature when processing PCBs individuallyand continuously, and demonstrates the outstanding
stability of the oven when insertion conditions arechanged. And optimum conditions can be set according to the thermal characteristics of the PCB and themounted components simply by changing the distribution of heating energy.
(3) Small differences in temperature profile for the samesettings in any given oven type: This is a majoradvantage to users who operate volume-productionlines. Once conditions are determined for one oven,the others can be run under the same conditions,25

20

Gas flow rate

20

2.0

15
12
10

7

Trapezoid

1.0

Gas flow rate (m/s)b) Salamander XN III-725

Temperature difference ∆T (°C)

Nitrogen supply

50

XN 445 (40 Hz)

Figure 680Comparison of heating zone configurations of theSalamander XN III-525 and XN III-725

Figure 7

XN III-525 (30 Hz)

Temperature difference ∆T between center of 45x45mm BGA (at 235°C) and PCB

Furukawa Review, No. 20

2001above that is essential for Pb-free soldering.
(6) Flux-collecting system: Figure 11 shows the flux-collecting system and the energy-saving structure. Flux
from the fumes in the recirculated air in the oven isdeposited at points where the temperature is low(below the solidifying point). This can contaminate
equipment and cause malfunctions, requiring regularcleaning. In the present system fumes are first led toa bypass duct, the walls of which are chilled. Whenthe amount of solidified flux increases, it passes
through the walls and collects in a vessel. This system greatly reduces the frequency of flux cleaning.
(7) Energy-efficiency: This system has lower power consumption (Figure 11). As described above,
Salamander systems control infrared rays and hot gas
independently, so thermal interference cannot be
avoided. One disadvantage of conventional ovens, inwhich heat is actively radiated from the walls of theoven making it impossible to control gas temperature,
has been avoided. It has been found, however, thata) Triangular profile
250

Temperature (°C)

greatly reducing the time required for set-up. This is
the result of the accuracy of infrared and hot-gas control.
(4) Excellent bare-board temperature distribution: Figure
9 shows the temperature distribution on a bare PCB.
Because of the uniformity of gas flow from the nozzles, disparity of temperature stays within a range of
5°C.
(5) Trapezoidal temperature profiles can be achieved: In
packaged components such as BGAs, where thejoints are inside the package, heat energy cannot be
conveyed to the joint directly by radiation or convection. Thus in reflow soldering of such components the
package or substrate is first heated and heating proceeds by conduction. The package, however has its
own thermal capacity and heat resistance so that theheat applied is not transferred to the joints immediately but rises gradually over time. Figure 10 shows acomparison between the ordinary triangular (peaked)
temperature profile and the trapezoidal profile. It canbe seen that at the flat portion of the trapezoidal profile the temperature at the center of the BGA gradually
approaches that of the substrate, and it will be realized that the longer this flat portion is the less will be
the temperature difference between the two. Thus
with the trapezoidal temperature profile it is possible
to bring components having high thermal capacity totheir maximum temperature while limiting the temperature rise of components of low thermal capacity (i.e.,
reduce ∆T), and to obtain the hold time at 220°C or

200

235
Substrate temperature
Center of BGA

12°C

223

150
Time at 220°C or above:
Substrate: 28 sec
Center of BGA : 13 s

100
50
0
0

50

100

150

200

250

300

350

400

Time (sec)b) Trapezoidal profile
250

Conditions: 250 x 330 x 1.6 mm; interval between PCBs: 5 cm

Temperature (°C)

Peak temperature (°C)

240

1°C
235

Individual

Figure 8

Continuous processing
1st 2nd
4th

200

8th

Time at 220°C or above:
Substrate: 28 sCenter of BGA : 13 s

100
50

50

100

10th

Stability of temperature profile during continuous
processing of circuit boards

7°C
230

150

0
0
6th

237
Substrate temperature
Center of BGA

150

200

250

300

350

400

Time (sec)Figure 10 Comparison between triangular (peaked) and trapezoidal temperature profiles

Heat-insulating sheet

250 mm
10

10

330 mm

10
227

227

225

224

227

224

Flux-collection
vesselBlower
10 227

Figure 9227

225

Temperature distribution on bare PCB• Energy is saved by normally retaining heat in the oven by means of a heatinsulating plate.
• Heat is dissipated by the blower when it is difficult to control hot gas temperature.

Figure 11 Flux collecting system and energy-efficient structure81

Table 1

Specifications of typical systems
XN III-525PZ

Dimensions (LxHxH) (mm) 4750x1200x1500
Weight (kg)Permissible component
heightXN III-725PC

1400

5

7

PLC+touch panel

PLC+computer

50 patterns

Up to hard disk capacity

RS-232C

EthernetCommunication circuit
Nitrogen supply capacity& oxygen concentration
Power requirement

300 ppm @ 200 l/min
40 kVA

47 kVAdepending on the temperature settings, there is aregion in which interference presents no problem
even without radiation. In the present system, therefore, the outer walls of the oven are covered in heatinsulating sheets and radiation is suppressed in theregion where thermal interference is not a problem, sothat when control of gas temperature becomes impossible air is conveyed to the oven walls and insulating
sheets promoting heat dissipation and limiting energy
waste.
(8) Low nitrogen consumption: In Salamander systems
the oxygen concentration in the oven is controlled by
the nitrogen gas flow rate. Thus when the oven isused at a high concentration of oxygen the supply of
nitrogen can be curtailed, reducing running costs. If
the supply were curtailed too much, however, the static pressure in the oven would drop to the point where
oxygen concentration would be subject to the effects
of external disturbance. The new ovens are equippedwith a nitrogen flow rate optimizer that provides sufficient nitrogen to keep oxygen concentration at thelimit of stability, assuring optimum nitrogen gas flows
at all processing conditions.
4.2 Specifications
Table 1 shows the specifications of the Salamander XN
III-525PZ and XN III-725PC systems, which are typical of
the series. The 725PC has in its control system a personal
computer running Windows NT, for easy adaptation to a
network environment.
4.3 Model C Control System
Figure 12 is a schematic diagram of the Model C control
system, in which process condition management, data
entry, current status display, etc., are effected by personal
computer. The monitor is a TFT color liquid crystal panel(touch panel included) for visual display of system status.
Data for system operation is fed to a programmable logic
controller (PLC), which provides continuous control of thesystem. The computer used is normally an MSDOS

82

RS-232C

Keyboard
Top 13 mm/bottom 13 mm (specifiable)

Control method

production conditions• File management• Status display• Condition input• Production condition input

Personalcomputer

250x330 mm

No. of zones

Storage capacity for

TFT color
touch panel

5150x1200x1500

1200

PCB dimensions

Ethernet

Programmable
logic controller• Input/output control• Temperature control• Conveyer speed control• Error monitoring• Oxygen concentration control

Figure 12 Schematic of Model C reflow oven system

machine, backed up by a notebook computer in case of
malfunction. Network connection is easy so that software
upgrades can be downloaded. Work is currently going forward on providing an on-line diagnostic capability.

5.

CONCLUSION

It is expected that the use of Pb-free solder in the mounting of electronic components will expand greatly from thisyear on, but there are a number of problems that remain
unsolved in terms of solder cream and the heat withstandproperties of the components. For example a trapezoidal
temperature profile is extremely effective in limiting the
maximum temperature of components, but the time above
a given temperature is extended raising the danger ofchanges in the capacitance of electrolytic capacitors.
Process optimization cannot be achieved solely by theefforts of equipment manufacturers; electronic component
makers, materials manufacturers and assemblers mustalso be intimately involved. Furukawa Electric, as a leading equipment manufacturer, is a participant in the NEDO
Pb-free solder project, and is supplying and acquiring
information for practical implementation of Pb-free soldering. We intend that in future we will serve not only as an
equipment manufacturer but will also cooperate with users
with respect to the process as a whole. In closing wewould like to express our appreciation to the staff of the
Planning Dept., Hiratsuka Works for cooperating so effectively in this development work.

REFERENCES
1) Shozo Asano: Background and Present Status of Pb-free Solder,
Engineering Forum for COX Ultra-high Density Mounting '94,
Science Forum (in Japanese)
2) Data Sheets for Heat Transfer Engineering (in Japanese)

Manuscript received on June 20, 2000.

Furukawa Review, No. 20

2001