8

Solder Reflow Profile

The most commonly used solder

reflow method is accomplished in

a belt furnace using convection

heat transfer. The suggested

reflow profile for automated

reflow processes is shown in

Figure 25. This profile is designed

to ensure reliable finished joints.

However, the profile indicated in

Figure 25 will vary among differ-

ent solder pastes from different

manufacturers and is shown here

for reference only.

Figure 25. Suggested lead-free reflow profile for SnAgCu solder paste.

Figure 26. Stencil outline drawing (mm).

Stencil Design Guidelines

A properly designed solder screen

or stencil is required to ensure

optimum amount of solder paste

is deposited onto the PCB pads.

The recommended stencil layout

is shown in Figure 26. The stencil

has a solder paste deposition

opening approximately 70% to

90% of the PCB pad. Reducing

stencil opening can potentially

generate more voids underneath.

On the other hand, stencil open-

ings larger than 100% will lead to

excessive solder paste smear or

bridging across the I/O pads.

Considering the fact that solder

paste thickness will directly affect

the quality of the solder joint, a

good choice is to use a laser cut

stencil composed of 0.127 mm

(5 mils) thick stainless steel which

is capable of producing the re-

quired fine stencil outline. The

combined PCB and stencil layout

is shown in Figure 27.

Figure 27. Combined PCB and stencil layouts (mm).

TIME (SECONDS)

300

250

200

150

100

50

0

50

100

150

200

250

300

0

PEAK = 250

± 5°C

MELTING POINT = 218

°C

RAMP 1

RAMP 2

REFLOW

COOLING

PREHEAT

0.60

1.80

3.20

0.67

0.36

0.40

0.40

0.27

0.30

2.90

1.60

STENCIL

OPENING

0.36

0.40

0.46

0.60

0.95

0.40

0.36

0.36

4x – R0.14

1.60

0.70

0.9550

1.80

0.27

0.36