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This talk will compare the electromigration performance of wafer level packages having standard SnAgCu solder with those having solder balls with polymer cores. Information on package construction, process, and board level reliability of packages having balls with polymer cores was presented earlier (2009 International Wafer Level Packaging Conference, San Jose, CA). In the new ball variation, the structure is made of a large solid polymer core, which is plated with a thin layer of copper and covered with a layer of SnAg. The polymer core within the solder ball ensures that the standoff height remains constant during board assembly, and acts as a stress absorption layer between the Si and the PCB during any thermal excursion and drop testing. Such characteristics allow extension of the wafer level package from small pin count (~30 I/O) to higher pin count (100+ I/O) without the need for redistribution layers. For electromigration, bump resistances were monitored continuously at different current densities and temperatures for the two package types. Equivalent performance was obtained. Similar failure modes were observed in both cases, with pancake-like void formation growing outward from the pad corners due to current crowding. Equivalent performance was obtained, with both packages exhibiting MTTF of 1200 hrs at 150C, and 550 hrs at 165C. Application to a modified Black's equation to predict MTTF and account for current crowding, heating, and stress will also be discussed.

Electromigration Performance of WLPs with Standard and Polymer Core Balls