Material Testing And Mitigation Techniques For Pad-Crater Defects
Authors: Brian Gray, P. Eng., and John McMahon, P. Eng. Company: Celestica Inc. Date Published: 10/16/2011
Abstract: The increased temperatures associated with lead-free processes have produced significant challenges for printed wiring board (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 material’s 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. Currently 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. Celestica has been conducting an ongoing testing program to assess these lead-free compatible materials and area array packages against these standards. This program has identified “Pad crater / Pad Lift” as the dominant failure mode in lead-free materials. Testing has also confirmed the more severe impact of filled resin systems identified by other investigators. As a result Celestica has modified internal guidelines for process strain limits, reflecting an increased sensitivity to strain rate. Testing of more lead-free compliant product continues while initiating a parallel testing program to evaluate mitigation techniques which increase strain margins and prevent the defect. Specifically we evaluate two approaches: Pad design changes aimed at dispersing the stress concentration at the solder ball; and the integration of a high modulus polyimide film into the board stack up to prevent crack initiation at the board surface. These evaluations are performed on structures that are consistent with high complexity “Enterprise Computing and Telecom” electronic assemblies. The polyimide materials are of interest because they have suitable electrical and thermal characteristics for electronics. If successful, they offer a thin film solution for new designs as well as drop-in improvements in designs which exhibit a propensity for this problem. Test methods, test results, and failure analyses for both primary attach and hot gas (HG) rework conditions are discussed.
Pad Crater, Spherical bend test, mechanical failure mode, polyimide film, process strain.