Accelerated Life Testing of Surface Mount Connections on an Inter-Board Bond-Strap and Design Implications for Pallet-Mounted Circuit Boards

摘要

The Low Noise Amplifier (LNA) is a critical out-sourced electronic sub-assembly used in wireless basestation systems. The initial mechanical design of a particular commercial LNA contained a pair of separated epoxy-glass FR-4 printed circuit boards (PCBs) bolted to a common aluminum heat-sink pallet. The earliest deployed LNAs exhibited a relatively high failure-rate, due to fractured surface mount (SM) solder connections on the two strain-reliefs, or bond-straps, that interconnect the pallet-mounted boards. A mechanical redesign program for the bond-straps resulted in a new configuration with significantly improved compliance. A critical part of the redesign program was the comparative robustness evaluation of SM connections on the initial and improved bond-straps. Accelerated thermal cycling of production LNAs induced about 85% cumulative failure of solder joints on the original bond-straps. By comparison, none of the connections on the redesigned bond-straps failed. There were no failures of deployed LNAs with the redesigned bond-straps due to degraded SM connections. The life-testing failure statistics showed a Weibull slope close to unity, indicating a near-uniform attachment failure-rate, compared to the typical increasing failure-rate associated with the fatigue wear-out of solder connections on traditional SM leaded packages. The reduced Weibull failure-rate suggests that the cyclic thermal load on the SM connections is mitigated by the attachment of the PCB to the pallet relatively close to the two bond-straps. Finite element structural modeling of the pallet-PCB assembly over temperature indicated that the local board displacement near the bond-straps is reduced to nominally 40% of the corresponding free thermal expansion of the unconstrained PCB. Application of the Engelmaier predictive model for SM attachment reliability showed that such a reduced thermal expansion can increase the median life of solder connections by about 50 times, compared to similar assemblies with unconstrained PCBs deployed in typical thermal environments for outside-plant wireless basestations. This study provides insight into the attachment failure statistics for SM strain-reliefs on mechanically constrained PCBs. The life-testing results add to the industry understanding of SM interconnection reliability, currently based on traditional SM perimeter packages on substrates with free thermal expansion. Pallet-mounted circuit boards with a systematically designed array of bolt-downs and relative positioning of "neutral centers" can mitigate the attachment hazard for known at-risk SM packages during aggressive operational thermal cycling.