This page
sponsored by
 |
Wave Soldering Defects - Solder Skips
Unsoldered surface mount joints are referred
to as solder skips where the termination does not have any solder.
It is caused by incorrect chip wave height or gassing of the flux
on the underside of the board.
As Figure 1
shows, the resist thickness can also cause the problem or make
it worse. Solder resist
or mask
should be level with the pad surface or below it for the ideal
assembly conditions. If the mask is thick it creates a cavity
around the pad where flux vapours are trapped forming a bubble.
The solder
cannot displace the vapour easily to form a joint.
 |
| Figure 1: Resist thickness contributed to this solder
skip. |
While any surface mount solder joint not wetted
with solder is generally referred to as a solder skip, some people
refer to these as insufficient solder joints, which is misleading.
During assessment of the
problem, it is important to check if a fresh coating of solder is
on the pad or on the component termination. Often, the presence or
lack of a new solder
coating on the leads can indicate the root cause of the problem.
The most common causes of skipping are incorrect chip wave
height, gassing of flux
under the board surface or excess resist thickness. Each of these
faults
can be easily fixed, although the resist may take a little longer
if you have a large stock of boards, but review your PCB specification.
In Figure 2, adhesive has contaminated the pad surface.
Although no adhesive deposit is visible, some adhesives allow an invisible
film
to bleed out from the dot
area during curing. Try evaluating your adhesive.
 |
| Figure 2: Adhesive contamination on the pad surface
caused this solder skip. |
The example in Figure 3 is clearly not
soldered, and the waves could not have been that badly adjusted in
a professional company, could they? It is more likely that the board was
not
correctly
positioned in the
finger conveyor or in the pallet. The pallets may be distorted or the
clips are not holding the board flat.
 |
| Figure 3: It is likely that the board was not correctly
positioned on the finger conveyor or in the pallet. |
Wave Soldering Defects:
| 
