PCB solderability changes with time—what finish is best?
Title
Bob Willis
“When you evaluate any PCB surface finish, you must consider the reality of real-world produc-
tion processes. Although printed boards and components go into your process, ideally they come
out the other end of the production line; however there can be many stops along the way.”
PCB solderability changes
with time—what finish is
best?
During manufacture, circuit boards can the manufacture or assembly. To provide
be subjected to many of the steps outlined
Test conditions used on the
repeatable results, the wetting balance may
below, with each step impacting the solder-
wetting balance
be used to assess changes in solderability on
ability of the surface finish. If solder-levelled
• Solderability testing conducted
test pads, audit panels or even pre-produc-
boards are used, either tin/lead or lead-free,
with 200mg SAC pellet
tion boards.
and the thickness of the coating is main-
• One pellet per test site, pellet
Using these techniques, assembly
tained, there is limited impact. Tin, copper,
change during multiple testing
companies and printed board manufacturers
silver and gold will see a change in the
• Temperature set point 260˚C
can use the wetting balance to guarantee the
degree of wetting.
• No hold for pre-heat
solderability of boards and components in
• Baking boards
• Speed 0.1mm/second
their drive for zero defect manufacture. The
• PCB pre wash
• Flux ACTIEC5
test has often been used by the author for
material evaluations, the impact on assem-
• Multiple reflows
bly process stages, the change in flux activity
• In process cleaning or wash off
and supplier audits. It is also a useful tech-
• Adhesive curing
the same as the dip and inspect test used
nique as part of product development or to
• Reflow then wave or selective solder-
for surface mount components. It is very
provide customers with technical support.
ing
difficult to demonstrate the subtle changes
which take place on boards as they are sub-
Use of this test method eliminates the argu-
Sample boards previously subjected to one
jected to multiple heating operations during
ments which often arise when the soldering
or two reflow cycles were tested 10 days after quality does not provide the expected solder-
the first reflow using a MUST 2 wetting
balance. The following graphs show the
changes in solderability after one reflow
operation; the results are exaggerated due
to the hold time prior to testing. However it
is well known that some alternative surface
finishes do not hold up to one reflow cycle
and extended hold time before the second
soldering operation. This is a critical issue
when selecting surface finishes. Some
companies build their products or contract
out the assembly and only complete the
final assembly when orders are received.
There are many delays that can occur in the
manufacturing process—some are unavoid-
able, and some are actually planned but not
considered during a surface finishes evalua-
tion phase.
Rotary dip and solder float solderability
Figure 1. Close-up of gold test coupon used on the wetting balance, the pads shown have previously been tested. The
test methods are used by the printed board
parallel lines also seen on the sample are used for solder paste dot spread test for shop floor in production evalua-
industry but are very limited and basically
tions.
4 – Global SMT & Packaging – September 2008
www.globalsmt.net
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