Mechanical Failures in Pb-Free Processing
This paper evaluates the effect of pad crater defects on process strain limits for BGA devices. Methods, results, analysis and techniques are discussed.
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Authored By:
John McMahon
Brian Gray
Celestica Inc.
Toronto, ON, Canada
Summary
The increased temperatures associated with Pb-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.
Industry guidelines for mechanical stress limits were developed for materials processed using eutectic SnPb solders. A series of design and material implementations have gained wide acceptance by the industry to address mechanical failures at the corners of area array packages. Current accepted levels of process strain were established when the dominant and limiting failure mode was interfacial fracture (IFF) in complex intermetallic compound (IMC) layers at the solder / package interface. Changes in packaging processes, conversion to Lead (Pb) free solders and the subsequent compensation by laminate suppliers have produced significant shifts in failure mode occurrence and the "Pad-crater" failure mode has become far more common than IFF.
By conducting a testing program that focuses on materials and geometries consistent with high complexity "Enterprise Computing and Telecomm" electronic assemblies, and evaluating the results against the current industry guidelines it is possible to determine whether the dominance of the "Pad-crater" defect mode will require revision of process strain guidelines. Test methods, test results, failure analysis and likely mitigation techniques are discussed.
Conclusions
1. Mode 10 - "Pad Crater" is the dominant fracture mode for solder ball array devices when assemblies are fabricated using common Lead free process capable materials.
2. The Pad crater failure mechanism follows the strain rate dependent nature well established for brittle materials.
3. Safe working limits for monotonic failure in solder ball array devices can be established by board thickness for these materials.
4. Secondary fracture modes can be introduced by extended thermal exposures typical for hot gas rework of BGA devices.
Initially Published in the SMTA Proceedings
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