Safe working process strain limits for large area array packages: Observations from spherical bend testing

The increased temperatures associated with lead free processes have produced significant challenges for PWB laminates. Newly developed laminates have different curing processes, are commonly filled with ceramic particles or micro-clays and can have higher Tg values. These changes, designed to reduce Z-axis expansion and improve the materials resistance to thermal excursions through primary attach and rework operations have also produced harder resin systems with reduced fracture toughness. This reduction in toughness has increased the incidence of the “Pad crater/Pad Lift” defects which are now reported as the dominant failure mode for Pb-free compliant materials. Observations from multiple streams of field returned product are now available to confirm this change in failure mode. This work discusses multiple phases of bend to failure testing and the implications for mechanical reliability of Pb-free product. The various phases were intended to investigate relationships commonly accepted for SnPb processes when they are applied to Pb-free compatible materials. The relationships between safe working strain, strain rate and board thickness are examined in the first phase. The second phase directly compares commonly used standard loss laminate materials and the impact of micro clay fillers in these resin systems which has been identified as a significant factor by other investigators. The effect of extended thermal excursions for an extensively used standard loss laminate material is also investigated. Assembly process and test methods are discussed in addition to selected results, failure analysis and distribution parameters. The effectiveness of a simplified predictive model that includes board and package parameters will also be discussed.