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
be subjected to many of the steps outlined
test conditions used on the
below, with each step impacting the solder-
wetting balance
ability of the surface finish. If solder-levelled
• Solderability testing conducted
boards are used, either tin/lead or lead-free,
with 200mg SAC pellet
and the thickness of the coating is main-
• One pellet per test site, pellet
tained, there is limited impact. Tin, copper,
change during multiple testing
silver and gold will see a change in the
• Temperature set point 260˚C
degree of wetting.
• No hold for pre-heat
Figure 1. Close-up of gold test coupon used on the
• Baking boards
• Speed 0.1mm/second wetting balance, the pads shown have previously been
• PCB pre wash
• Flux ACTIEC5
tested. The parallel lines also seen on the sample are
used for solder paste dot spread test for shop floor in
• Multiple reflows
production evaluations.
• In process cleaning or wash off
be used to assess changes in solderability on
• Adhesive curing
test pads, audit panels or even pre-produc-
These complement his own practical hands-on
• Reflow then wave or selective solder-
tion boards.
workshops. For further information on how Bob
ing
Using these techniques, assembly
may be able to support your staff with on line
It is well known that some alternative sur-
companies and printed board manufacturers
consultancy, in house training go to
face finishes do not hold up to one reflow
can use the wetting balance to guarantee the
www.ASKbobwillis.com.
cycle and extended hold time before the
solderability of boards and components in
second soldering operation. This is a critical
their drive for zero defect manufacture. The
“Overlapped package structures—
issue when selecting surface finishes. Some
test has often been used by the author for
another option for space savings”
companies build their products or contract
material evaluations, the impact on assem-
continued from page 4
out the assembly and only complete the
bly process stages, the change in flux activity
the concerns over yield. In summary, new
final assembly when orders are received.
and supplier audits. It is also a useful tech-
solutions in the realm of three dimensional
There are many delays that can occur in the
nique as part of product development or to
interconnections continue to pour forth as
manufacturing process—some are unavoid-
provide customers with technical support.
the electronics industry pushes ahead in it
able, and some are actually planned but not
Use of this test method eliminates the argu-
continuing effort to meet the long stand-
considered during a surface finishes evalua-
ments which often arise when the soldering
ing expectations of the consumer market.
tion phase.
quality does not provide the expected solder-
The benefits that accompany this increas-
Rotary dip and solder float solderability
ing yields in production.
ingly more universal approach to design
test methods are used by the printed board
Further information on the solderability and fabrications are many and the payoff
industry but are very limited and basically
test equipment and testing services can be will be significant over the long term for all
the same as the dip and inspect test used
obtained at www.gen3systems.com/index.php. participants.
for surface mount components. It is very
difficult to demonstrate the subtle changes
Bob Willis is a process engineer providing
Verdant Electronics founder and president
which take place on boards as they are sub-
engineering support in conventional and surface
Joseph (Joe) Fjelstad has more than 35 years of
jected to multiple heating operations during
mount assembly processes. He runs special
international experience in electronic interconnec-
the manufacture or assembly. To provide
production features at exhibitions and also
tion and packaging technology and is also a well
repeatable results, the wetting balance may
offers his seminars and workshops worldwide.
known author writing on the subject of electronic
interconnection technologies.
6 – Global SMT & Packaging – September 2008 www.globalsmt.net
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