Reaching detente in the design and material selection for Hi Rel WLCSP's

As WLCSP packaging technology travels the road to ubiquity a transition is unfolding. WLCSP designs and material selection paradigms are moving from a process integration focus to one of product integration. The transition is driven by the application of WLCSP technology to larger die and enabled by the improvement in materials properties and process requirements. Both the driver and the enabler are consequences of the relentless pressure on cost that is the hallmark of the recent industry cycle. Achieving the goal of a low cost, high reliability, product-driven package solution required the balancing a variety of needs and characteristics. Often these are in conflict and detente is needed to ease the tension. The design goals of a Hi Rel WLCSP must provide superior protection, mechanical strength and electrical contact. A successful solution necessitated the need to avoid special tools or processes. The solution also needed to exceed the reliability requirements of the end customer. A key component of the WLCSP structure is the dielectric which is interposed between the solder bumps and the die. It must provide most of the protection functions while supporting the bumps and buffering the bump strains. A negative tone, photosensitive polyimide was ultimately selected. This polymer was designed with the goal of achieving thermal and mechanical cured film properties fully compatible with the stresses and thermal excursions associated with WLP thin-film processing and bumping. Integration of the new polymer require careful characterization and process optimization to assure critical material qualities such adhesion were achieved. Similar optimizations were run for assembly operations. Based on test results described herein, a double-layer polyimide, copper redistributed, eutectic SnPb or Pb-free bumped WLCSP was developed that exceeded all the design goals. Salient differentiators of the WLCSP solution are identified. Improved reliability was achieved, and new design standards were established relative to RF capability and power density.