BGA Rework Challenges Of High Thermal Mass, Large PCBA’s

ABSTRACT Rework of area array devices on large, heavy and non-uniform thermal mass PCBAs presents a significant challenge due to the potential of damage to the PCB. PCB bow and twist experienced during rework process development posed a significant risk to quality and cost given the high price of the PCBA. A systematic root cause analysis through process of elimination was performed with the objective of identifying the cause of the problem and eliminating bow and twist at rework. This paper will focus on the discussion of the key factors that were analyzed during root cause analysis, including: temperature differential (ΔT) across the board during the rework process, effect of baking prior to rework to remove moisture, board support during rework and its effect on thermal expansion, thermal imbalance analysis from a PCB design point of view, bottom side pre-heat coverage and design considerations for the high thermal mass assembly. The results of the analysis allowed Plexus to narrow down the problem and identify a solution that successfully eliminated board bow and twist during rework.