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December 20, 2010

Square vs. Round BGA Apertures

What is your opinion of using round vs. square apertures for BGA patterns in our solder paste stencils?



S. L.

Experts Comments

AIM has conducted extensive printing tests on BGA's using a Kho Young volume measurement instrument. Square apertures repeatedly outperform round apertures in regards to repeatability of pause-to-print volume and high-speed volume transfer.

AIM used four different types of stencils during testing and found that in all but one case there was a remarkable improvement using square apertures. The one stencil that did not demonstrate much variation outperformed all the other stencils on its own.

It is believed that the round apertures do not perform well due to the uniform tension on the paste deposit (i.e. trampoline affect). The square aperture has non-uniform tensions and peals away from the stencil aperture requiring less adhesion force to the circuit board pad.

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Karl Seelig
Vice President of Technology
AIM
In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.

Click here to see the results of a test comparing paste release from round and square apertures. E-FAB (Electroformed); L-A (laser-cut w/o electropolish); L-EP (Laser-cut with electropolish); L-EP/NP (Laser-cut with electropolish and Nickel plate)

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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.

Round apertures are easier to etch and etch more accurately. In addition, you get better paste release from round apertures.

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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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