Effects of multiple BGA rework on strength of solder joints

摘要

Purpose - The purpose of this paper is to investigate effects of the multiple rework of ball grid array (BGA) components on mechanical strength of BGA balls, as well as any possible intermetallic (IMC) embrittlement, and obtain data correlated with possible estimation on the maximum permitted limits of BGA rework. Design/methodology/approach - In this paper, mechanical strength of BGA components assemblies with multiple numbers of rework operations was evaluated. Mechanical evaluation was conducted using BGA ball shear tests and four-point bending tests of BGA assemblies. Test samples were prepared under the following conditions: virgin, one, two, three and five BGA reworks. Failure mechanism was evaluated using cross-section and SEM analysis. Findings - The results show that both ball shearing tests and four-point bending tests indicates that strength of BGA solder ball itself was not reduced significantly after repair/rework operation from one to five cycles. The IMC structure layer after rework is a mixture of IMC, Sn-rich and Pb-rich phases. This mixture layers with thickness even more than 10μm in thickness does not show reduction of strength of BGA solder balls and do not cause premature embrittlement. However, the bonding strength of the copper pads to the laminates is reduced with rework/repair operation, with the great reduction coming from the first and second rework operation. Practical implications - In general, the industry recommends two rework cycles for BGA components on the same spot. This study indicates that further rework (up to five) causes little degradation, therefore there is room to increase the total rework cycle limit beyond recommended two for plastic BGA components. Originality/value - Presented test results shows that in most cases industry overestimates risks associated with increased embritlement of the BGA solder joints due to the intermetallics growth after multiple BGA rework operations. Strength reduction of BGA assemblies is mostly associated with reduction of bonding strength of the copper pads to the laminates is reduced with rework/repair operation and number of reworks could be increased in most cases.