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22 The PCB Magazine • March 2014 F e a t u r e by John McMahon, P.Eng and Brian Gray, P.Eng celestica (canada) Abstract The increased temperatures associated with lead- free processes have produced significant challenges for PWB laminates. Newly devel- oped laminates have different curing processes, are commonly filled with ceramic particles or micro-clays, and can have higher T g values. These changes designed to reduce Z-axis ex- pansion and improve the materials resistance to thermal excursions through primary attach and rework operations have also produced harder resin systems with reduced fracture toughness. Celestica has undertaken an extensive "Spherical Bend Test" program to assess lead (Pb) free compatible materials and area array pack- ages. This work has confirmed "pad crater/ pad lift" as the dominant failure mode in Pb- free materials in agreement with observations from multiple streams of field returned prod- uct. This work discusses the multiple phases of testing and the implications for mechanical reli- ability of Pb-free product. The initial phase was designed to confirm or refute the established re- lationship between strain rate and safe working strain in Pb-free materials. The second phase studied the effect of extended thermal excur- sions for an extensively used standard loss lami- nate material. The third phase was designed to directly compare standard loss laminate materi- als and has confirmed the impact of filled resin systems identified by other investigators. This new work seems to confirm the relation- ship between board thickness and safe working strain established by in IPC/JEDEC-9704: "Print- ed Wiring Board Strain Gage Test Publication." Data is only available for a limited number of package designs but these selected packages are believed to generate conservative strain limits for manufacturing process guidelines. The de- sign of the most recent test plan was intended The Application of Spherical Bend Testing to Predict Safe Working Manufacturing Process Strains

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