Several wafer level chip scale package (WLCSP) technologies have been developed which generate fully packaged and tested chips on the wafer prior to dicing. Many of these technologies are based on simple peripheral pad redistribution technology followed by attachment of 0.3-0.5 mm solder balls. The larger standoff generated by these solder balls result in better reliability for the WLCSP's when underfill is not used than for equivalent flip chip parts. Rambus™ RDRAM and integrated passives are two applications that should see wide acceptance of WLCSP packages.
The adhesion of several commercial underfills (Dexter Hysol FP4511, FP4527, and CNB775-34) to BCB (CY-CLOTENE) 4022, 4024) has been studied through die shear testing and subsequent failure analysis. Die shear values range between 69-90 MPa. Failure analysis by optical microscopy, profilometry and XPS spectroscopy indicates mixed mode failure at various interfaces. From the die shear data collected before and after 24 h water boil for Cyclotene 4022 and 4024 (with AP3000 adhesion promoter) and underfillers FP 4511 and 4527 we find the die shear strength decreases an average of 11% for the four comparisons. Adhesion promoters based on vinyltriacetoxysilane or 3-aminopropyltriethoxysilane show equivalent die shear performance. Substrate surface cleaning based on UV ozone treatment reveals oxidation of the BCB surface which by SIMS analysis remains 0 1 m deep after 10 min of treatment.
As IC scaling continues to shrink transistors, the increased number of circuits per chip requires more I/O per unit area (Rent's rule). High I/O count, the need for smaller form factors and the need for better electrical performance drove the technological change towards die being interconnected (assembled) by area array techniques. This review will examine this evolution from die wire bonded on lead frames to flip chip die in wafer level or area array packages and discuss emerging technologies such as copper pillar bumps, fan out packaging, integrated passives, and 3D integration..
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