Mechanical Failures In Pb-Free Processing: Evaluating The Effect Of Pad Crater Defects On Process Strain Limits For BGA DevicesAuthors: John McMahon and Brian Gray
Company: Celestica Inc.
Date Published: 5/17/2010 Conference: ICSR (Soldering and Reliability)
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.
Key words: Pad crater, spherical bend test, mechanical failure mode, process strain.
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