Copper Trace Failures in Ball Grid Array (BGA) Packages under Sequential Harmonic Vibration and Temperature Cycling

Abstract

Solder interconnects in ball grid array (BGA) packages can present substantial fatigue degradation risk under combinations of mechanical (vibration) and thermo-mechanical cyclic loading. Durability tests to investigate such risks typically use either sequential or simultaneous combinations of vibration loading and temperature cycling. The interconnect system consists of not just the solder joint itself but also the associated features, such as the interfacial IMC layer, the copper pads on the substrates and the copper traces and vias connected to the copper pads. The copper traces are known to be particularly vulnerable to mechanical flexural loads, such as those experienced due to vibration loading. Recent studies have shown a combined failure mode consisting of cracks in both the solder and copper pad. However, there is limited information in the literature on the vulnerability of copper traces under sequential vibration and temperature cycling loads. Therefore, this work examines the influence of loading sequence (harmonic vibration followed by temperature cycling and vice versa) on copper trace cracks in a Printed Wiring Assemble (PWA) that consists of a centrally located BGA component on a multilayered organic printed wiring board (PWB). In addition, the width of Copper traces connected to the copper pad was varied (45µm, 100µm and 200µm) to determine how the fatigue degradation risks scale with trace width.In order to understand the loading conditions that lead to copper trace cracks, the test matrix consists of (i) room-temperature harmonic vibration tests close to the fundamental mode of the PWA at 150g’s and 175g’s; (ii) temperature cycling (TC) tests (-40°C to 125°C) and (iii) sequential application of items (i) and (ii). Failure analysis conducted on the tested specimens revealed that copper trace cracks were highly likely to occur during the harmonic vibration segment of individual and sequential tests. In contrast, temperature cycling loads did not lead to copper trace failures, although it did show evidence of accelerating the propagation and growth of cracks that had already initiated during any prior vibration exposure.