Ask the Experts
August 4, 2008 - Updated
August 4, 2008 - Originally Posted

Reflow PCB with BGA's and CSP's

We are trying to reflow a PCB that contains both BGA and CSP's. During reflow we are seeing solder being sucked into vias, rather than being consumed by the BGA balls. Is there a way to keep this from occurring?

J. P.

Expert Panel Responses

Unused vias should be protected from pulling the solder off BGA pads and any other pads for that matter and this is usually accomplished by using solder mask or solder resist to tent the unused vias. Check IPC-A-600 REV G, page 40, 41, and 42 for information on how this should be requested from your board suppliers.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

Couple of ways to approach this problem.
  1. Design the boards w/o via in BGA pads
  2. Overprint the BGA pad with oversized aperture or thicker (step stencil)
  3. Print on vias only, reflow, and then process board through printing / reflow process.

Bill Coleman
Vice President Technology
Photo Stencil
For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.

There are two common solutions; one would be to apply solder mask over the open vias, and the other is to redesign the board with dog bone shapes so that there is a small trace between the pad and the via. This will prevent the solder from migrating across the pad and down the via. Can you turn down the temperature of the subzone of your oven? This might be the easiest solution, as solder flows to areas of highest temperature. This might just work.

Edward Zamborsky
Regional Sales Manager
OK International Inc.
Ed Zamborsky is a Regional Sales & Technical Support Manager for Thermaltronics, located in New York. His position requires frequent customer visits throughout North America and the Caribbean and his position encompasses not only sales but the role of trainer and master applications engineer for all of Thermaltronics products. His expertise includes such specialties as hand soldering, convection and conduction reflow techniques, array rework, fluid dispensing equipment, and fume extraction. Ed has authored many articles and has presented many papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead-Free Hand Soldering, High Thermal Demand Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.
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