Ask the Experts
February 3, 2020
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?
Expert Panel Responses
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 solderballs.
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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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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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 canhave 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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