During the past two years there has been a steady migration toward lead-free (RoHS) compliant printed wiring boards (PWB). The affect of RoHS compliance has been felt primarily in the areas of PWB design, fabrication and assembly. An increasing number of high density interconnect products, fabricated with most of the commonly available materials have been subjected to reliability evaluations by thermal cycle testing, following exposure to multiple lead free assembly and rework cycles. Failure analysis on test vehicles preconditioned to simulate lead free assembly and rework has demonstrated a shift in traditional failures from central zone barrel cracks to knee cracks. There is also a marked shift in the hierarchical influences effecting interconnect reliability. The shift transitions from the focus on copper quality, to a balanced influence between copper quality and material robustness. Certain base materials appear unable to withstand more than 3 or 4 exposures to the elevated (245°C to 260°C) temperature extremes required by lead-free assembly and rework. Lead-free processing has significantly increased the incidences of material delamination (both adhesive and cohesive), decomposition, degradation and "pad cratering" are becoming more common. Large chip arrays, especially those on smaller grids (0.8 mm), are more prone to material related issues. Lead-free assembly and rework has narrowed the process window, and PCBs that would have been marginally acceptable is now failing. Lead free thermal excursions are proving to be much less forgiving of process variation. Statistical analysis has demonstrated that lead free assembly and rework is typically reducing the cycles to failure of well-made PWBs by as much as 65%, while coupons with marginal quality can fail quickly. This article embraces an overview of effects of lead free assembly and rework on PWB reliability and identify some of the reliability trends.
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