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The Knowledge and Know-how Connection
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May 21, 2012
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Solder balls are popping up from via holes causing bridging. What might be causing this and how can we prevent it?
We are facing issues with a double sided board assembly, after bottom side reflow.
In some of the fine pitch component locations, solder balls are popping up from via holes causing bridging.
What might be causing the solder ball problem and how can we prevent it?
N. C.
Expert's Panel Responses
This could be attributed to couple things:
Is the solder finish HASL? As solder finish may not be applied to clean copper and the HASL thickness could be plugging the via holes.
Other options which could affect solder via hole intrusion.
Solder aperatures in the stencil are allowing solder to enter the hole.
Solder is wicking from pad of logic device, into via hole. You might allow your assembler or PCB fabricator to solder mask the holes. Tenting the vias on the topside will prevent solder paste intrusion in to the pads.
Rodney Miller
Capital Equipment Operations Manager, Specialty Coating Systems
Rodney is currently Operations manager at SCS coatings, Global Leader in Parylene and Liquid Coating equipment. Rodney applies his 20+ years of diverse manufacturing to the Equipment Business at SCS Coatings. We provide unique value added coating equipment solutions for our customers. Including conformal, spin and Parylene coating expertise.
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It sounds like you are overheating the previously reflowed side of the board, during the second reflow operation. This is causing the solder to flow out of the vias onto the PCB.
Try turning the temperature down on the subzone of the reflow oven.
Edward Zamborsky
Regional Sales Manager, OK International Inc.
Mr. Zamborsky serves as one of OK's technology advisers to the Product Development group. Ed has authored articles and papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead Free Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.
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It sounds like you have molten solder in a blind hole, and air pressure is pushing the solder out of the hole in the shape of a ball. Are these blind vias or is there a solder covering both sides of the hole?
You may also want to try baking the boards prior to this soldering operation. If you have moisture in the board it may be looking for a way out, in this case you would have steam in the hole expanding and pushing out solder from the hole.
A simple bake over 100 deg for an hour prior to soldering would tell you if its a moisture problem or not.
Steven Adamson
Market Specialist, Nordson ASYMTEK
Market Specialist for Nordson ASYMTEK. Mr. Adamson worked for Kodak, Motorola and Plessey, ICL in the UK with 5 US and 2 UK patents. He was awarded a HNC in electrical engineering and was 2008 President of IMAPS. Mr. Adamson was a respected mentor in the electronics industry. He passed away October, 2011. Learn about the Steve Adamson Memorial Annual Scholarship Fund.
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