Toshiya Satoh

Development of low-cost and highly reliable wafer process package

A wafer level chip-scale-package (WLCSP) is expected to reduce the manufacturing cost of CSPs, but reliability of a solder joint for a large chip size of about 100 mm/sup 2/ without underfill assembly is still in question. To meet this needs, we have developed a highly reliable and low-cost WLCSP named wafer process package phase 2 (WPP-2). The package includes a built-in stress-relaxation layer for reducing the strain of the solder bumps. To lower the manufacturing cost of the package, the stress-relaxation layer is formed by printing. The Youngs modulus and the thickness of the stress relaxation layer were optimized by finite element analysis. The package was assumed to have 10/spl times/10 mm chip and 54 Sn-Ag-Cu solder balls of 400-/spl mu/m diameter placed as a grid array with the minimum pitch of 0.8 mm, and be mounted on a FR-4 motherboard. It was found that a thickness of 75-/spl mu/m and a Youngs modulus of 1000 MPa are necessary for assuring no failure up to 1000 cycles under temperature cycling between -55 and 125/spl deg/C. Accordingly, a resin with a Youngs modulus of about 1200 MPa at -55/spl deg/C was developed for the stress relaxation layer. High reliability of the simulated WPP-2 structure was confirmed by simplified test samples made of the developed resin. Fully processed WPP-2 samples were fabricated on an 8-inch wafer. The lifetime of the solder joints mounted on the FR-4 motherboard was evaluated by the temperature cycling test. The contact resistance of none of 50 samples increased by more than 20% even after 1400 cycles, and their lifetime to 50% failure was more than 3000 cycles.

Improvement of adhesion between Si3N4 thin films and polycarbonate substrates by preparation of an interpenetrating layer using microwave plasma enhanced chemical vapor deposition

The adhesion between a silicon nitride layer and polycarbonate substrate was improved by employing a silicon interfacial layer which was deposited by a microwave plasma enhanced chemical vapor deposition technique. The silicon interfacial layer had an interpenetrating layer composed of the two materials, which provided an anchoring effect. In the interpenetrating layer, it was assumed that chemical bonding between carbon of the polycarbonate substrate and the silicon interfacial layer occurred through the oxygen when the radio frequency power was used in the fabrication process. The oxygen originated in the polycarbonate substrate. During the silicon interfacial layer deposition, carbon was sputtered and oxygen reacted with silicon.