DEVELOPMENT AND SCREENING OF POLYMER COLLAR WLP1 CANDIDATES FOR LEAD-FREE SOLDER SPHERE TECHNOLOGY TO ENHANCED RELIABILITYAuthor: David Luttrull et al.
Company: FlipChip International, LLC - A RoseStreet Labs Co
Date Published: 11/3/2005 Conference: IWLPC (Wafer-Level Packaging)
In general, good polymer collar material performance is defined where first; the solder (sphere) effectively wets the copper pad. Secondly, the polymer forms a fillet or collar at the solder ball/die interface, without encapsulating the solder ball. Based on the viscoelastic data on several polymer candidates (from two different epoxy/flux families) as compared to die yield percentage, it is clear that minor changes in the ratios of components in the polymer candidates we tested have a huge effect.
We have also concluded that a very low complex modulus [G* = ((G’)2 + (G”)2)1/2 ] slope change over time (?G*/?t) and temperature, produced by the thermoset reaction through the solder-melt temperature range, is required for good performance and thus, good die yield. Finally, this paper briefly highlights three different ways to inspect the solder spheres for proper wetting to the underlying pad and to inspect for polymer residue on the top of the solder spheres. The inspection techniques discussed herein are (1) optical (AOI), (2) SEM, and (3) laser/fluorescence scanning and imaging. This paper briefly discusses the advantages and drawbacks with each inspection method.
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