Ask the Experts
November 14, 2017 - Updated
September 21, 2009 - Originally Posted

Problems with blow holes

We are having problems with what appear to be blow holes on one particular part of an assembly we run frequently. We have experimented with different profiles and different no-clean fluxes with no results. We also baked the boards. Could the cause be due to an improper ratio of the lead to the hole? Are there specifications for lead to hole ratios for wave soldering?

R. G.

Expert Panel Responses

Lead To hole Ratios are fairly important but not normally a cause for Blow Holes. If you are using Single Sided PCBs then many things are blamed on lead to hole ratios, when in fact the problem is incorrect drilling leaving a laminate burr inside the hole wall which then naturally de wets solder. If the problem is with PTH boards then this can be caused by HASL fluids attracting moisture into the holes, sometimes baking works, but sometimes there is too much high boiling solvents left to bake out as they are there to withstands high temperature so stands to reason they cannot be baked out. This can be tested for as if you leave an Iron tip for 4-5 seconds on the blown joint you will see an acrid smoke come off with the solder fizzing and popping and a black spec appear on the molten solder normally. If it is only in a certain area of the PCB then it could more likely be thin plating so you get hole wall break out, get that area micro sectioned and wall plating including Copper thickness tested. If the PCB is Silver Finish then this too can produce blow holes and the only thing that really helps is to flow the PCB twice, we believe there is resist sometimes improperly removed or leached/bled over the holes and by soldering it twice this actually removes the residue and gives an improved solder joint. Would be helpful to know what solder finish PCB is and what type i.e. single or double sided. Hope it helps

Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

Blow holes have many causes. One possible cause is air escaping from between the layer of the PCB through pin holes in the barrel of the plated through hole. This will happen no matter how much you pre-bake the board because it's air and not moisture being forced out as the board heats. The only solution is to look to your board vendor to make sure the through-hole plating is 100% with no pin holes. The IPC publishes manyquality and inspection documents for the electronics manufacturing industry. I think theIPC-2221A "Standard for Printed Board Design" will have the hole to lead ratio recommendation.

Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 39 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes and well as moisture sensitive component storage solutions.

Many times this can be caused by humidity or any water on the PCB.

Todd O'Neil
National Sales and Marketing Manager, North America
Business Development Manager, DAGE | X-Ray component counting
Mr. O'Neil has been in the electronics manufacturing industry for over 20 years.

Reader Comment
Number one cause is the PCB and the platingquality or thickness. it will happen on wave, selective and intrusive reflow asshown in my IPC online video see
Bob Willis,

Reader Comment
The issue you describe is a blow hole but notone as a result of a PCB fab issue. What is happening in your case is the volatilesin the flux normally escape through the through hole but the clinch of thecomponent lead actually traps some of the gases causing them to release in thesolder wave and explode.
Ray Chartrand

In order to express my opinion, more information isneeded:
  • Did youhave problems from the very beginning with this assembly or they just showed upnow?
  • If theyjust started - what did it change from the last time when you've ran goodboards
  • If youhave the issue from the very beginning, a complete root cause analysis is needin order to find out what the issue is.
From your comments it looks like you might have done somechange overload trying different thingswithout a specific and detailed,structured plan.

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
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