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Microvia Failures: The Pathologist's Viewpoint
June 2, 2009 | Pete Starkey, I-Connect007Estimated reading time: 1 minute
Paul Reid, IST Program Coordinator with PWB Interconnect Solutions, Inc., shared his experiences of microvia reliability testing and failure analysis with PCB007 Editor Ray Rasmussen when they met at APEX in Las Vegas, Nevada. PWB Interconnect Solutions is the Canadian company that developed the Interconnection Stress Test (IST) technique for quantitative measurement of interconnect quality, now incorporated into IPC TM-650, and Reid saw his role as that of a pathologist--a specialist in examining failure modes and mechanisms, a forensic scientist conducting autopsies on defective structures to determine causes of death.
The incidence of microvia failure was on the increase. Was that due solely to the introduction of lead-free soldering, Rasmussen asked? Reid believed there was a combination of factors and, although lead-free was significant, a real consideration was that microvia interconnect was no longer reserved for high-end product and had grown substantially in more commercial high-volume applications. An area of concern, however, was that thermal cycle testing to the established 150° upper temperature could allow potential defectives to escape detection. He recommended increasing the temperature to 190°, and quoted examples of apparently robust product previously passing 800 cycles failing after only 10 cycles under the more aggressive conditions.
Whereas the OEM had originally been the driver for compliance testing by IST, the technique was now recognised by fabricators, chemical suppliers and material suppliers as a quantitative comparison method to support design and process engineering development.
Reflecting upon the anatomy, geometry and morphology of the perfect microvia, Reid considered that, all other factors being equal, a laser ablated hole, with an aspect ratio less than 1.2:1, was more robust than the mechanically drilled equivalent.