Journal of SMT Article

Copper Pad Dissolution and Microstructure Analysis of Reworked Plastic Grid Array Packages in Lead-free and Mixed Assemblies

Authors: Lei Nie, Michael Osterman, and Michael Pecht
Company: Center for Advanced Life Cycle Engineering (CALCE)
Date Published: 4/30/2009   Volume: 22-2

Abstract: An experimental study was conducted to examine the impact of rework processes on the quality and reliability of plastic ball grid array packages in lead-free and mixed assemblies. In this study, 676 IO plastic ball grid array packages with Sn3.0Ag0.5Cu solder balls were assembled with Sn3.0Ag0.5Cu solder paste and eutectic SnPb solder paste. Selected parts on circuit boards were subjected to rework processes one time, three times, and five times. The ball grid array voids, the microstructure and composition of the intermetallic compounds, and the copper pads were investigated to determine the impact of the rework processes.

The rework process includes multiple liquid solder state periods, such as component removal, site redressing and component reflow. Thus, more copper is consumed in the rework process, which makes the intermetallic compound growth trend an interesting topic. Copper pad dissolution was found in the samples after multiple rework processes. Lead-free assemblies consumed more copper than mixed assemblies because of the higher concentration of Sn in lead-free solder. After five times rework processes, the copper pad thickness of lead-free assemblies was only about half of that of mixed assemblies. Copper pad over-consumption was found at the connection area between the copper pad and the copper trace after the rework processes were applied three times and five times, and thick intermetallic compounds formed at the same locations, which may lead to reliability concerns. The thickness of the intermetallic layer increased as the number of rework processes increased. The IMC growth in reworked assemblies was very fast due to multiple liquidus solder period in the rework process, including component removal, site redressing, and component placement. The IMC morphology in the reworked BGAs was fragmentary, which was different from that in as-assembled BGAs.

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