Abstract
A potential laser etching method has been investigated and implemented in decapsulating wafer level chip size package (WLCSP). A chemically and physically clean surface could be obtained and lead to successful solder reflowing. Firstly, fast and cost-saving transfer molding was suggested to replace underfill dispensing in encapsulation. An industrial-grade KrF excimer laser was utilized to remove the compressed molding compound (epoxy resin and silica fillers) on Au/Cu or Pb–Sn/Cu bumps. An oblique angle of incidence was found effective in achieving a filler-free and chemically clean surface. Explosion forces from epoxy underneath the fillers, together with the transient thermal expansion of whole substrate, were attributed to the higher filler removal efficiency than at normal incidence. In addition, etching rates at angular irradiation were found more balanced between the surrounding areas and the bump. Helium, in comparison with oxygen, helped to reduce polymer molecules left over the bump top. In higher laser fluence or oxygen environment, carbonyl groups with stronger bonds to metal surface (bump) were found on bump top so that success rate of solder reflowing was greatly lowered. A success rate higher than 80% was achieved at an optimal laser fluence and scanning strategy, with inert gas assistance.
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