Causes of Blowholes

Causes of Blowholes
Are old components such as resistors and circuit breakers more likely to cause blowholes during wave soldering compared to newer components?
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Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
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Phil Zarrow
Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.


And welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers. Pick and place. Today, Jim, we have a question from D.B. He is asking, are old components such as resistors and circuit breakers more likely to cause blowholes during wave soldering compared to newer components?

Well, blowholes to me are caused by a material vaporizing in the barrel of the hole in the wave. The barrel of the hole and the lead are heated up to soldering temperatures by immersion in the wave. Of course, the most common ones are caused by inadequate preheat, not evaporating all of the solvent in the flux and that solvent boils and creates blowholes when it hits the wave.

Here he is comparing new and old components. I am assuming that the preheat would be the same, so that should not be an issue. My feeling is that if they are observing blowholes with older components it is probably some contamination during the long storage process. Some contaminant material has gotten onto those leads that is not cleaned off by the flux and vaporizes at soldering temperatures during immersion in the wave.

Flux is not going to clean off your contamination though. Oxidation, particularly if you are using water soluble, that should really take it off, especially on those local contaminants. As Jim is saying, it is contamination. Yeah, that could be an issue.

If you store your components properly in bags and containers that contamination cannot get on the leads. The only issue with older components that have been stored a long time is additional oxide. If your flux is inadequate or you don't put enough on, you may not clean all of that off.

But the defects that you would see would be poor wetting and maybe inadequate hole fill. But not what I would call blowholes. The oxidation reduction does not particularly produce excessive gases. It is contamination of solvent materials.

Right, but we would also suggest that you take a hard look at your wave solder profile, particularly the preheat. Is your preheat in compliance with what the flux manufacturer's spec says with regard to rate of heating and also how you are heating it as well, whether it is halogen-free, water-based, alcohol-based.

Look at the data sheet for the flux. If you have any questions definitely call the tech support of the manufacturer of that flux. That is where you may be running into problems.

Preheat is definitely the most common cause of blowholes. I am assuming that they are comparing all of the new parts and that it is the same for both.

Well, we hope that we gave you some insight, D.B. Beyond that I would say regardless of how your wave soldering process is doing, whether you are getting blowholes and everything else, however you are doing it, please don't solder like my brother.

And please, don't solder like my brother.


The bottom line is that blow holes are caused by defects in the PWB, during storage or in the components / materials introduced into the holes. A fault tree or Ishawaka (fish bone) diagram will help identify the other potential contributors. If the PWB had representative sectioning performed, there would be an indication of the contribution of the PWB.
Don Vischulis, Woodward, Inc.
I'm not a believer in the main problem is voids in plating especially if boards are stored for long periods. I believe its down to HASL fluids left in the barrels that are extremely hygroscopic attracting moisture causing blow holes and shallow inverted joints.
Gregory York, BLT Circuit Services Ltd
Barrel plating is the most likely source of wave solder blow holes. Tracking that down to get a response from your supplier can be difficult. The fab house is going to refer to the data they collected that says everything is ok. They have the same problem you will in that finding a barrel defect with cross sectioning can be difficult because you need grind in the right spot and angle. You have a little more information because the defect shows you where to look. We have had success recently using CT X-Ray to examine a defective holes on a assembled PCB and used that information to inform our cross-section location. If you can run fabs from another lot or date code with success it will help you narrow down and indict the date code of PCBs giving you trouble.
Don Adams, Bose
If all else fails may I suggest you check the PCB barrel plating i.e. no holes or cracks in the side wall, no glass fiber penetrating the barrel and no voids behind the barrel. See ANSI/IPC-A-600F for examples - it appears to be free on line.
Robert Hills, Tait International
Nice photo used in the article 99% of the time outgassing in vias and through plated holes is related to the thickness of the plating and the amount of volatile material, moisture in the board. If you wanted to test with leads in place this can also be done but better to test these separately as yes plating particularly tin can outgas for a few seconds. To test boards on site or compare different suppliers sue my simple test method shown online"
Bob Willis, EPS

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