Development of New Model Reflow Ovenfor Lead-Free Solderingby Motohiro Yamane * , Nobuaki Orita * 2 , Koichi Miyazaki * 2 and Weiming Zhou * 2
The main issue with regard to improving the heating ability of reflow ovens is tominimize the temperature difference ∆ T among the various parts of the printed
circuit board (PCB) during the reflow soldering process. We need to control temperature within amuch narrower process margin when we use lead-free soldering. Accordingly, our development
target in this work is to achieve a significant increase in heat transfer coefficient α , fundamental
to the heating performance of a reflow oven. We have developed a new model of reflow ovenfor lead-free soldering with a much higher heating ability by redesigning the hot air blowing
unit, specifically by changing the design of the hot air panel. We were able to achieve this target
in only five months by using process diagnostic techniques such as direct measurement ofheat transfer coefficient and flow visualization, which we have developed and used for various
applications.

ABSTRACT1.

INTRODUCTION

As the process and equipment for lead-free soldering
have become more widely used, the characteristics
required of reflow ovens for lead-free soldering havebecome clearer and more advanced.
The status quo for lead-free surface mounting
technology is:a) An Sn/Ag-based composition with a high melting
point has become the de facto standard as thelead-free material. This is because problems arestill encountered with materials of Sn/Zn-based
composition, which have lower melting points.
b) It is difficult to improve the heat-resistance of any
parts in the short term. In practice, most parts
have to be mounted under conventional reflow
temperature conditions due to the parts that have
low heat-resistance.
c) The problems relating to the solder material andheat-resistance of the parts should therefore beovercome by improving the characteristics of the
reflow oven.
The main issue with regard to improving the heating
ability of reflow ovens is to minimize the temperature
difference ∆ T among the various parts of the printed
circuit board (PCB) during the reflow soldering process.
This is equivalent to improving the basic heating ability of
the hot air.

2.

REQUIRED PROPERTIES AND TARGETFOR REFLOW OVEN IMPROVEMENT

Heating ability, which is fundamental to the performanceof reflow ovens can be estimated by the accuracy withwhich it conforms to the required temperature profile.
Figure 1 shows a typical temperature profile for thelead-free soldering process. Assume that the upper
temperature limit, based on the heat-resistance of theparts, is 240°C, the process window for Sn/Ag/Cu-based
solders will be 20°C. Again, assuming that the lower
temperature limit, based on the melting point of thesolder is 230°C, the allowable ∆ T is less than 10°C, whichis narrower by 25 % than that of conventional Sn/Pbbased solder. The temperature control within this narrowprocess margin must be achieved.
The following is the main issues in improving reflow
oven properties:
1) improving ∆ T by improving heat transfer coefficient αof the hot air;2) improving cooling ability;
3) improving flux collection; and
4) reducing N2 consumption (for N2 atmosphere ovens)
The most fundamental of these issues is improving heattransfer coefficient α , but improving cooling capacity isalso important when we consider a lead-free process.
This is because we have to control the high temperature
exposure time according to the heat-resistance of theparts.

* Industrial Equipment Dept., Electronic Components Div.,Electronic Mounting and Components Co.

* 2 Production Engineering Development Center, Plant &Facilities Div.

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Development of New Model Reflow Oven for Lead-Free Soldering

Temperature

2 Upper temperature limit restricted by theheat-resistance of components (240˚C)
High-temperature∆Tcomponents
1 Melting temperature of
Low-temperaturesolder/melting time,components
e.g. 230˚C/10 sec

1 Preheating

3 Holding time restricted by

the heat-resistance of components

Time

Figure 1

3.

Typical temperature profile for lead-free soldering.

IMPROVING BASIC HEATING ABILITY

Basically there are three mechanisms of heat transfer:1) conduction, in which heat is transferred by contact
between a high-temperature body and a low-temperaturebody; 2) convection, in which heat is conveyed bymoving fluid; and 3) radiation, in which heat energy isconverted to electromagnetic waves and transferredwithout any transfer medium. Of these mechanisms thetwo that we can control in terms of the heating ability ofa reflow oven are convection and radiation. Conventional
SALAMANDER reflow ovens have achieved the desired
heating ability by using both forced convection heating
via impinging jets and radiation heating via a far infrared(IR) heater. In this development, however, heat radiation
has not been used as a measure of improving the heating
ability. We focused purely on redesigning the jets, inorder to achieve the improvement target. There are two
reasons for this: 1) it is not efficient to use an IR heater
when we want to heat the PCB uniformly because theIR heater tends to heat those parts with smaller heat
capacity more rapidly; and 2) it is not desirable to insert
a body with a higher temperature than the upper limitof the allowable temperature range of the parts. Under
forced air convection heating, the PCB will not be heatedbeyond the temperature of the hot air, which is “morefriendly” on the parts.
Hot air, temperature: Tgas

Thickness: s
Surface area: A
Substrate, temperature: Tboard

Figure 2

Heating model of PCB by convection.

Now consider the heating model shown in Figure
2, where a PCB with a surface area of A (m 2), and a
thickness of s (m) is heated by hot air at a constanttemperature of Tgas (°C). Assuming that the temperature of
the PCB is Tboard (°C), the amount of heat conveyed fromthe hot air to the PCB per unit time Q (W) will be
Q α A(Tgas Tboard) ρ AsCp

∂Tboard∂t

Cp: mean specific heat capacity(J/kg/K).

Heat transfer coefficient α is not a constant, but takes
a value determined by the kind of fluid and the statusof the flow. This α is the index of heating ability and isdetermined by designing the status of the flow impinging
on the PCB, and this can be designed, by modifying
the shape, size, diameter and the pitch of the blowing
nozzles. The greater the jet velocity, the higher α will be,
but jet velocity must be so limited that the parts will not be
dislodged during the reflow process.
Equation (1) above is a first-order differential equationand can be solved analytically by
TboardT0

Tgas 1 exp

α t
ρ sCp

(2)

where,
T0: temperature of the PCB (°C) at the time t=0.
The coefficient of t in the exponent (α /ρ sCp) represents
the response speed of PCB temperature. That is, thegreater α and the less the specific heat capacity C p,
the more quickly the PCB will be heated. By using thischaracteristic, namely, by passing a PCB the various
parts of which have different heat capacity and measuring
the difference ∆ T between the maximum and minimum
peak temperatures, we can evaluate the heating ability of
the reflow oven. We can emulate a surface-mounted PCB
by using a stainless steel substrate and partially changing
its thickness.
Figure 3 shows the simulated result of the temperatureof a PCB heated by two main heating zones for various
values of the heat transfer coefficient. The horizontal axis
represents the time (s) and the vertical axis represents
PCB temperature (°C). The blue line represents thetemperature of a PCB with a thickness of 1 mm, thered line represents the temperature of a PCB with athickness of 2 mm, and the gray line represents the gas
temperature. We have assumed that the board stays 25
seconds in one zone before moving to the next, and thatthe board is to be preheated to 190°C for the 1-mm partand 180°C for the 2-mm part.
The gas temperature in the first zone is to be set so thatthe temperature of the 1-mm part at the exit of the firstzone is 240°C, and the gas temperature in the second
zone is 240°C, which means that only the 2-mm part isheated in the second zone. The difference in temperature∆ T at the exit of the second zone is 11.0°C for a heattransfer coefficient α of 110 W/m2/K. ∆ T can be reduced
to 7.2°C for a heat transfer coefficient α of 150 W/m2/K.
Our target in this development was keep ∆ T below 10°C.

(1)where,
α : heat transfer coefficient (W/m2/K)
ρ : density of the PCB (kg/m3)

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Development of New Model Reflow Oven for Lead-Free Soldering

α =110 (W/ (m2•K))of flow per unit area of the PCB.

∆ T=11.0˚C

Substrate temperature (˚C)

260
N2
Hot air heater

220
Nozzle diameter

200

Tgas
s=1.0

180

160

Substrate

s=2.0

0

10

20

30

40

Figure 44∆ T=7.2˚CHeat transfer coefficient (arbitrary unit)

α=150 (W/ (m2•K))
Substrate temperature (˚C)

260

240

220

200

Tgas
s=1.0

180

160

s=2.0

0

10

20

30

40

Nozzle diameter: small
Nozzle diameter: medium

2

1

0

50

Nozzle diameter: large

30

Figure 5

Relationship between heat transfer coefficient α andtemperature difference ∆ T.

MEASURES FOR IMPROVING HEATTRANSFER COEFFICIENT α

As shown above, improving the heating ability isequivalent to improving the heat transfer coefficient, but
under the constraint of jet velocity it is not good to raisethe volume of the hot air flow excessively. We have totake into account how to design the flow from impinging
jets so that the heat is transferred effectively. The main
measure we took was to redesign the hot air blowing
unit, specifically by changing the design of the hot air
nozzle arrangement-- nozzle diameter and nozzle pitch-- in order to obtain more efficient heat transfer. The firststep was to examine the relationship between the hot air
nozzle arrangement and the heat transfer coefficient bymeans of model experiments (see Figure 4). Hot nitrogenis blown via four nozzles of various diameters and pitches
on to PCBs provided with thermocouples. The rate ofnitrogen flow is controlled by a mass flow controller sothat the jet velocity on the PCB stays constant for any
nozzle arrangement. The heat transfer coefficient can becalculated by the heat-up time of the board.
Figure 5 shows a summary of these experiments. We
found that the heat transfer coefficient was inversely
proportional to the ratio of nozzle pitch to nozzle
diameter, that is, the more the nozzle pitch is decreased
and the nozzle diameter is increased, the higher the heattransfer coefficient will be.
This is equivalent to increasing the volume hot air rate

2

4

6

8

(nozzle pitch)/(nozzle diameter) ratio

Time (s)Figure 3

Schematic representation of heating modelexperiment.

50

Time (s)4.

Nozzle pitch

240

Relationship between (nozzle pitch)/(nozzle
diameter) ratio and heat transfer coefficient.

Let us now consider the balance between blowing
and suction on the panel in an actual reflow oven. The
relationship derived above only takes into account theheat transfer coefficient of the blowing nozzles. In orderto get a uniform flow of hot gas on the PCB from a large
area of blowing nozzles, we need to have an equally large
area of suction apertures, but this results in the loss ofheating ability at the suction area. We therefore present a
new design of blowing panel, in which the suction means
has been changed from slits to pipes, arranged between
the blowing nozzles. This achieved a good blowing/
suction balance without losing heating ability.

5.

EVALUATION OF HOT AIR BLOWING
PANEL

To evaluate the heating ability and status of flow for the
newly designed blowing panel, we first carried out theheating model experiments described above, and thenscaled up to a one-zone experimental unit to analyze thefull-scale panel. More specifically, we applied process
diagnostic techniques, including 1) direct measurementof the heat transfer coefficient, 2) evaluation of uniform
heating ability by thermal imaging camera, and 3) flow
visualization using the smoke method, all of which havebeen used in various applications in our section. It isnoteworthy that we were able to minimize both the time
and the money required for development. Detail of thetechniques are as follows.

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Development of New Model Reflow Oven for Lead-Free Soldering

Seven heating zones

Two cooling zones

thermal images at 0, 10, 20, and 30 sec. Figure 7 showssome of the images taken in this way. It can be seen thatthe newly designed panel with matrix pipe suction has asuperior ability to heat the PCB uniformly that is superior
to that of the conventional design with slit suction.
Thermal imaging camera

Glass-epoxy substrate

Photo 2
Photo 1

5.1 Direct Measurement of Heat Transfer Coefficient
The heat transfer coefficient can be calculated frommeasured results for the temperature of the PCB and of
the atmosphere. We used a blank PCB made of stainlesssteel and provided with two thermocouples, which wasconveyed manually through the one-zone experimental
unit and heated at 200°C. We compared a number ofpanel designs by changing the panel installed in theunit. Figure 6 shows some of the measured results. We
adopted the pipe suction type panel having the highest
heating ability as the basic design for the production
model.
Slit suction
Temperature (˚C)

180
160
140
120
100
80
60

Gas temperature
Substrate temperature

40
20

40

60

80

100

120

140

Time (s)Matrix pipe suction

Temperature (˚C)

180
160
140
120
100
80
60

Gas temperature
Substrate temperature

40
20

40

60

80

100

120

140

Time (s)Figure 6

5.2

Thermal imaging by thermal imaging camera.

One-zone experimental machine.

Measured results for PCB temperature and gas
temperature.

Evaluation of Uniform Heating Ability by Thermal
Imaging Camera
The experimental setup of evaluating is shown in Photo
2. The central part of the lid of the one-zone experimental
unit was made of quartz glass. After heating the unit to a
specified temperature, we inserted blank PCBs, and took

Figure 7

Slit suction
Matrix pipe suction
Typical thermal image 30 sec after PCB insertion.

5.3 Flow Visualization Using Smoke Method
To visualize the jet flow using the smoke method, we
made the top and sidewall of the one-zone experimental
unit from acrylic material. We first introduced the smokefrom a blowing nozzle and illuminated by a sheet laser
beam created by a polygon mirror, and then took
images with a CCD camera. Photo 4 shows a typicalimage taken in this experiment. Based on the resultsof the visualization, together with in-depth investigationof the jet velocity measured by a wind velocity probe
and a 3-dimensional micro stage, we have concluded
that the jets from the blowing nozzles change direction
near the PCB pass line and are sucked into the suction
nozzles efficiently. This means that the hot air that hascooled inside the oven after heating the PCB is retrieved
immediately, and also that there is no interference
between the jets blown from the top panel and thosefrom the bottom panel in each zone. Consequently, we
can set the temperatures of the top and bottom panels
independently. This has the advantage of raising thetemperature of the soldering joint without heating the
body of the parts themselves. We can also achieve
lower consumption of the nitrogen used in controlling
oxygen concentration than in the conventional reflow
oven because the flow of the jets does not fluctuate even
if there are PCBs on the board pass line. We therefore
conclude that the structure that creates this phenomenon
is efficient on the whole.

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Development of New Model Reflow Oven for Lead-Free Soldering

Polygon mirror

Sealing region (blowing nozzles)

CameraPhoto 6
He-Ne laser

Photo 3

Sealing region (row of blowing nozzles) on the newly
designed panel.

Setup for flow visualization experiment.
With blowing
nozzles

Without blowing
nozzles

Substrate surface

Zone-to-zone interference is not seen

35 mm

Photo 7

Zone-to-zone interference is seen

Effects of the providing sealing region.

Blowing panel

Photo 4

6.

Flow visualization by smoke method.

7.

NEW DESIGN OF BLOWING PANEL

We have determined a new design for the hot air
blowing panel, as shown in Photo 5, based on the modelexperiments and evaluation procedures describedin Section 5. The main difference with respect to theconventional design is the modification of the suctionmeans: from slits to pipes arrayed between the blowing
nozzles. The overall heat transfer coefficient of the panel
was significantly increased by this modification comparedto the previous design which loses heating ability at thesuction slit.
There is an additional row of blowing nozzles at boththe entrance and the exit of the zone (see Photo 6). Theserows work as a sealing region which prevents zone-tozone interference. We have confirmed the efficiency ofthese rows by flow visualization (see Photo 7).

CONCLUSION

We were able to achieve the developmental target ofachieving a significantly increased heating efficiency
by adopting a newly designed hot air blowing panel
without changing the main features of Furukawa's leadfree reflow soldering system, i.e., forced heat convection
through impinging jets. Work for the future would include
1) continued improvement of basic heating ability, 2)
improvement of the heating ability between parts forboards with high-density mounting, and 3) stabilization of
atmospheric control inside the reflow oven (reduction of
nitrogen consumption). In order to continue presenting
the market with reflow ovens of ever higher performance,
we will not only apply the process diagnostic techniques
we have developed but also create more sophisticatedflow visualization (for high temperature, large area, etc.)
and computer simulation of flow analysis.
The new model reflow oven is now under a final
investigation in detail preparing for mass production at

Suction nozzles

Blowing nozzles

Conventional design (slit suction)
SUS sheet, ∆T=11˚C /Substrate, ∆ T=13.5˚C

Photo 5

New design (pipe suction)
SUS sheet, ∆ T=8.7˚C/Substrate, ∆T=9.6˚C

Newly designed panel.

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Development of New Model Reflow Oven for Lead-Free Soldering

Furukawa Electronics (Suzhou) Co., Ltd., China. The costadvantages of transferring the manufacturing activities toChina is significant.
We must anticipate the trend to lead-free soldering andcontinue to develop reflow ovens of higher-performanceand lower-cost. Finally we would like to express our deep
appreciation for the support of numerous people, bothwithin and outside Furukawa Electric.

REFERENCES
1) M. Shibamura: Development and evaluation of reflow oven for leadfree soldering process, Jisso Gijutsu Periodical, 219, 48 (2003). (inJapanese)
2) T. Mizushina, H. Ogino: Transport Phenomena, Sangyo Tosho
Publisher (in Japanese)
3) K. Nakao, A. Hiraizumi, E. Iwasaki: Reflow Oven for a Lead-Free
Soldering Process, Furukawa Review, 106, 25 (2000).

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