The trend towards tighter pitch, smaller features, and shrinking pad sizes have put a strain on the standard photolithographic processes for soldermask patterning on substrate packaging and rigid Printed Circuit Boards (PCB). These constraints have limited the ability of the package/board designer to shrink features and have resulted in yield losses by the fabricators. To date, the answer to this situation has been to re-capitalize with expensive Laser Direct Imaging (LDI) systems or modified semiconductor step and repeat equipment. This paper discusses direct laser ablation of soldermask as an alternative technology to enable fabricators to achieve very tight soldermask feature control. Presented in this paper is the process and performance evaluation of the direct laser ablation process on soldermask for very fine pitch package placement on printed circuit boards. Results of solder adhesion, pre-surface finish pad morphology, and solder joint reliability to shock, bend, and thermal cycling are reviewed across design and process parameters. The evaluation looked at fabrication process variations across different soldermask types and soldermask thicknesses. Design implications between soldermask defined pads, metal defined pads and via in pad when used with soldermask direct laser ablation were also investigated.
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