Defect of the Month – Wave solder shorts
Hello, my name is Bob Willis and welcome to Defect of the Month with WNIE TV. Here is another video to watch and a short article with photographic examples to add to your collection of training material and help your team understand process and product failures. Plus how to correct them.
Images above show typical solder shorts on a SOIC, chip components and pins on through hole component terminations
Solder shorts on wave soldering and selective soldering do occur for a variety of reasons. They are seen on through hall components and form on surface mount parts on the bottom side of the board. Let’s assume that the design is correct, we’ve done everything to optimise good design for surface mount or through hole soldering. I have to say there are loads of tricks that are not in the textbooks to reduce wave solder shorts
The reason that we can see shorts is solder hasn’t separated from multiple pins on the board it has not separated from the board cleanly. Quality or contamination within the bulk solder alloy can affect drainage characteristics, even if the processed parameters haven’t changed. The solder itself has changed over time, so look for contaminants which might affect its characteristics. Changes in the solder alloy percentage will impact the solder drainage. Please remember that some parts of what you are soldering is progressively dissolving into the bath. That is why we take solder samples for analysis every month or three months depending on production volume
Three examples of solder shorts above the first on a large PGA socket and was caused by the incorrect preheat temperature required for large mass component sinking heat. The second flag short caused by the excessive component pin length passing through the wave likely to have resulted in limited flux during pin drainage. The third was also limited flux left during separation of the terminations from the wave.
We hopefully all monitor the amount of flux we apply to the printed circuit board and the effect of preheat on the board prior to going into the solder. Variability in the amount of flux which applied to the bottom side of the printed circuit board may affect drainage characteristics during the soldering operation and that then would lead to poor separation and solder shorts. So not enough solder draining off of the terminations to eliminate the excess and possibility of solder shorts.
If we have too much preheat, we can actually affect the flux’s characteristics. The flux can deteriorate, it may not allow the soldiers to drain as easily as it would have done if the process parameters were right to start off with. We can actually have too much flux, but that doesn’t necessarily relate to shorts, but too little flux or not enough activation of that flux or over activation will affect drainage. The simplest way to check flux application is to use a high temperature glass plate. In this way we can see coverage, the flux present after preheat and the amount of displacement over a single or double wave. Over the many years of running Wave Soldering Master Classes with Electrovert, then Speedline I was always surprised how many people had never seen glass plates being used. They are used and covered in my interactive wave soldering guide free to readers
FREE Interactive Wave Soldering & Defect
Board assemblies should always be held fairly rigidly. They should not be allowed to flex; sag and they should not be allowed to move as the board goes through. If the board moves in the pallet or fingers, it’s going to give an inconsistent process and potentially cause solder shorts because the solder is not draining evenly. There will be different lengths of contact with the wave and with it a different heat transfer and flux displacement
Now in terms of setup the wave should be stable, it should be moving at the same speed as the board as it exits the wave or slightly faster just to promote that energy which is pulling the solder away from all terminations. When you are analysing wave soldering faults, there is no magical way of doing it. It’s using your eyes, watching as much as you can, I have used video and high-speed video photography over the years to look at how solder drains away from termination. To understand how I can improve the process because it’s very difficult if you can’t actually see what’s happening in a process. So those are just some of the things to consider when you’re looking at solder shorts and it’s exactly the same with selective soldering. But again, we’ve got, fortunately far more parameters. More processed variables we can actually change.
We are quite fortunate that with selective soldering we don’t have the same limited parameters and processes that we have options to change on wave soldering. But the key thing is getting those design process parameters right to start off with. And when you are running your prototype builds, keep an accurate record of shorts opens and the process parameters that you’re using. Consider process changes as you scale up production to get the best possible yield.
We have many Defect of The Month videos and WNIE Online articles which we hope will help you solve your process and product failures. We have listed a small selection from over 100 plus videos below created over many years with NPL, IPC & WNIE
BGA popcorning
Dendrite formation
Open solder joints
Solder skips
Coating bubbles
Ultrasonic damage
Missing components
Incomplete past print
Component cracking
Tombstone chips
Solder balls
Solder bead formation
Solder shorts
Sulphur corrosion
Crimp connection failures
The list goes on and on, one month at a time.
FREE Wave Soldering Defect Poster Set
If you would like to download any of our Process Defect Guide booklets or my three soldering text books FREE just drop me an email bob@bobwillis.co.uk We also have and interactive guide to wave soldering and colour posters on wave soldering defects to use at your manufacturing site for training
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