Journal of SMT Article

Laminate Resistance to Pad Crater Defects: Comparative Spherical Bend Testing

Authors: John McMahon, P.Eng., and Brian Gray, P.Eng.
Company: Celestica Inc.
Date Published: 7/30/2011   Volume: 24-3

Abstract: The increased temperatures associated with lead free processes have produced significant challenges for PWB laminates. Newly developed laminates have different curing processes, are commonly filled with ceramic particles or micro-clays and can have higher Tg values. These changes which are aimed at improving the materials resistance to thermal excursions and maintaining electrical integrity through primary attach and rework operations have also had the effect of producing harder resin systems with lower fracture toughness.

Celestica has been conducting an ongoing testing program to assess lead free (Pb-free) compatible materials in combination with area array packages against the currently accepted levels of process strain which were established when the dominant and limiting failure mode was interfacial fracture (IFF) in complex intermetallic compound (IMC) layers at the solder / package interface. This program has identified "Pad Crater / Pad Lift" as the dominant failure mode in Pb-free materials. Subsequent testing has enabled comparisons between OSP and Pb-free HASL surface finish on a single laminate. Comparisons have also shown the effect of laminate stack and component construction on the safe working strain levels. Finally, testing of various Pb-free compatible laminates currently in production to assess their resistance / susceptibility to the Pad Crater phenomenon shows that there are real differences between resins systems and suggests that mechanical testing should be included in new Pb-free laminate qualifications. Test methods, test results, and failure analysis are discussed.

Keywords: 

Pad Crater, Spherical bend test, Mechanical Failure Mode, Laminate Testing, Process Strain.



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