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September 2, 2009

Mysterious Solder Balls

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.

Experts Comments

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.

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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 BS in Computer Integrated Manufacturing from Purdue University, along with 20+ years of Electronic manufacturing and Equipment Assembly, to direct the Equipment business at SCS Coatings. "We provide unique, value added coating equipment solutions for our customers". Including conformal, spin and Parylene coating expertise.

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.

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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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