General guide to solder balls in wave soldering—now you see them, now you don’t
Figure 3. Solder ball on the edge of the mask and after removal to show the surface of the mask and the flux ring. Surface analysis of the mask surface after ball removal can
show surface damage and also tin oxide in the surface layer.
mance of the knife and blow solder out of
through holes. It is particularly difficult if
a wide range of plated through holes sizes,
non plated through holes or cut outs are
present on one board design.
consistent/repeating balls
Solder balls are often seen in a no-clean
process positioned between terminations
on SOIC, QFP and SOT terminations or
on through-hole components like integrat-
ed circuits, IDC post headers or sockets.
Figure 2. Random solder balls on the surface of two different solder mask coatings.
The ball is positioned between adjacent
pins as the solder separates. The solder web
samples in Figure 3. the balls from the base of the board
between two pins does not always separate
Solder ball on the edge of the mask directly after wave soldering. Some wave
in the middle, so the ball is not always in
and after removal to show the surface of solder suppliers still offer brush modules as
the centre of the pins. On wave soldering,
the mask and the flux ring. Surface analy- part of an outfeed conveyor system.
the solder ball tends to drain from back to
sis of the mask surface after ball removal
front; on multi-row connectors like IDC
can show surface damage and also tin
headers, the solder can drain to the side at
oxide in the surface layer Bob Willis is a process engineer working in
45˚, leaving the ball in a different location
A combination of flux and solder
the electronics industry, providing training,
on the mask. This is more noticeable on
mask are the most often used parameters
selective solder nozzles where the width of
consultancy and product failure analysis. Bob
to reduce and eliminate consistent/repeat-
the solder jet is often equal to the group
offers countless different workshops worldwide.
ing balls with the mask being the major
of pins.
He runs and optimises production lines for sup-
factor. Side by side trials have shown that
Normally close examination shows the
pliers at exhibitions and customer sites. Bob will
a rougher surface is better to reduce solder
solder balls bonded to the solder mask
be running workshops at APEX in Los Vegas,
ball adhesion; however there are many
with a thin flux layer around the base. This
SMT Nurnberg, Germany, SMTA Interna-
competing parameters.
is not normally seen in nitrogen or on
tional and at ITRI in England in the coming
SEM examples of solder balling show-
random balls. Careful mechanical removal
months.For further information on lead-free
ing the flux ring on the surface of the
of the ball often shows some surface bond training workshops, training materials
solder mask
was present, indicating a softening/attack
In the early days of no clean, one op-
and lead-free process support visit
of the mask surface, as illustrated by the
tion was to use a rotary brush to remove
www.ASKbobwillis.com
2009
Do you have a process or technology or a really strong case
call
study that you want to share with Global SMT & Packaging’s
readers? Global SMT & Packaging is currently seeking technical
articles and case studies for publication in its print magazines
for
and website. We are specifically looking for papers that offer a
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www.globalsmt.net Global SMT & Packaging – February 2009 – 5
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