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Study of the Thermomechanical Inelastic Energy Response of Backward Compatible Solder Joints Made With Sn-3.8Ag-0.7Cu versus Reballed Sn37.0Pb Components

[+] Author Affiliations
Lorraine M. Renta

University of Wisconsin-Madison, Madison, WI

Ricky Valentin, Pedro Quintero

University of Puerto Rico-Mayagüez, Mayagüez, Puerto Rico

David Ma

Lockheed Martin Co., Sunnyvale, CA

Alan Hovland

Lockheed Martin Co., Denver, CO

Paper No. IPACK2011-52036, pp. 123-129; 7 pages
doi:10.1115/IPACK2011-52036
From:
  • ASME 2011 Pacific Rim Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Systems
  • ASME 2011 Pacific Rim Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Systems, MEMS and NEMS: Volume 1
  • Portland, Oregon, USA, July 6–8, 2011
  • ISBN: 978-0-7918-4461-8
  • Copyright © 2011 by ASME

abstract

Conflicting results in reliability tests for backward compatible and Pb-free soldered assemblies has motivated RoHS-exempted industries to practice reballing. Reballing is the name given to the process of removing Pb-free solder balls from the copper (Cu) pads of the Ball Grid Array (BGA) components received through the supply chain and replacing them with SnPb solder balls. Recent studies on the subject of reballing have shown the possibility that the removed Pb-free solder ball leaves behind some intermetallic remnants of the Pb-free solder alloy and the Cu from the pads. A modeling approach based on physics of failure (PoF) is presented that quantifies the interactions between different thermal cycles applied to reballed Ball Grid Arrays (BGA) with remnants of the Pb-free solder alloy on the Cu pads. These resulting interactions are compared to backward compatible Sn-3.8 Ag-0.7Cu (SAC) balls soldered with eutectic SnPb paste for the same thermal cycles. For the latter, the risk of having improper mixing during the assembly process is also studied. The approach is formulated at the microscale, incorporating physical mechanisms of the intermetallics created with Cu, and at the macroscale, capturing the creep phenomenon of the bulk solder as dominant failure driver. Simulation results show that the reballed cases have higher inelastic energy density per cycle averaged over damage volume near the copper pads and that the inelastic energy density is higher across the bulk of the improperly mixed backward compatible solder balls when compared to properly mixed backward compatible solder balls. The results of this study permit extrapolation of laboratory results to field life predictions and to explore the design of accelerated re-balled or backward compatible BGA tests that relate better to application-specific usage environments.

Copyright © 2011 by ASME

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