|Ask the Experts|
November 14, 2017
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?
|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
Technical Sales Manager
BLT Circuit Services Ltd
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.
Senior Project Engineer
Electronic Controls Design Inc
Many times this can be caused by humidity or any water on the PCB.
National Sales and Marketing Manager, North America
Business Development Manager, DAGE | X-Ray component counting
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 http://www.youtube.com/watch?v=xaauETZkBPsBob Willis, bobwillisonline.com
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:
Engineering and Operations Management
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