May 7, 2018
Solder Balls During Selective Soldering
When selective soldering the secondary side, we are producing solder balls on the primary side. We are also reflowing surface mount components on the primary side. What can be done to prevent this from happening?
Solder balls on the top side during selective soldering could
indicate the preheating of the liquid flux is not sufficient.
If solvent remains after the preheating zone, it could lead to
explosive removal of the solvent. This outgassing would bring with it some solder to the top-side which will form solder
Removing solvents being IPA with the alcohol based fluxes or
water in VOC-free fluxes is critical to avoid solder balls but also other issues such as insufficient hole-fill or bridges.
The solder heat should be working the activators and not vaporizing flux carrier solvents. Alcohol based fluxes tend to
dry up more readily during the preheat time and are preferred.
Applying the least amount of flux to insure adequate wetting without
solvent entrapment is also beneficial.
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Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.
The main reason for these issues it looks to be inadequate heat
applied to the assembly (in the pre-heating process and/or soldering process) - basically the assembly gets too hot.
Items to consider: the geometry and the population of the board
and the parts that needs selective soldered. Depending on these things, you
will have to carefully choose your fluxing, preheat temperature, slope and
duration, nozzle size, dwell time, etc.
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Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
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Solder balls can be
created from several different factors during the process. Sometimes it's
just a matter of too much flux. But my opinion is when this occurs on the
top side while you are soldering in the bottom side, it's probably because of
too much moisture int he board.
As far as the reflowing,
this is typically because you have too much heat from the selective nozzle
while soldering and/or you have heated N2 set too high (if you are using this
feature). It's very important to use the right size nozzle for the job.
Many times people will have a nozzle that is too small. A smaller
nozzle has less energy and you need to dwell too long to get a good joint (By
the way, this can also make solder balls). When ever possible, use a bigger
size nozzle. Hopefully you have a machine with dual nozzles so you can
have a big nozzle and a small nozzle at the same time.
option if your volume allows this is to use selective stamp soldering.
This is a process which solders the entire PCB at once using dedicated
volumetrically controlled nozzles (with no pumps) which is very fast.
With this type of process there are virtually NO machine generated solder
balls, and you will NEVER have a problem with reflowing surface mount components.
The process is controlled such that you can expect very high FPY.
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Mr. O'Neil has been in the electronics manufacturing industry for over 20 years.