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