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September 27, 2016

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?

C.W.

Experts Comments

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 balls.  

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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Peter Biocca
Senior Market Development Engineer
Kester
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
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.
This is a 2 part answer
  1. Assuming that the balls were not present from soldering the first side the question is where is the solder that forms the balls coming from? If the via ID is oversized relative to the T/H lead then possibly both flux and solder are passing through. Flux can pool on the top side underneath component bodies. When the solder which is a pumped stream and pressurized passes through the via it will rapidly vaporize the flux causing a splatter of solder (solder balls).

    So, things to try... check for excess flux, lower the solder temp, slow the travel upwards of the solder nozzle as it contacts the via, lower the Z stroke of the nozzle to prevent excess solder pumping through the via, preheat top side of the board to dry of the flux. There are more possibilities but these ought to see positive results.

  2. This question really requires a better understanding of the particular board characteristics. However, reflowing of components is usually an issue with board design where components are simply too close to T/H visa. This criteria is known as "the keep away distance" and usually about 1-2 mm of unmetalized land surrounding the TH solder site for safe SS processing.

    There are always exceptions but If the "keep away" is violated then reflowing the adjacent components is likely. Using the correct nozzle size and shape and the correct use of heated nitrogen is the is key to close keep away soldering.
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Alan Cable
President
ACE Production Technologies
Alan Cable, Past President-Retired, ACE Production Technologies has over 40 years experience in the electronics manufacturing arena. Alan's expertise is high production manufacturing automation, equipment design and process engineering. For the past 25 years Alan has focused specifically on soldering issues relating to component solderability, lead tinning and selective soldering, owning several companies with this focus.

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.  

Another 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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Todd O'Neil
Business Development Manager
Scienscope
Mr. O'Neil has been in the electronics manufacturing industry for over 20 years.
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