Reliability of flip chip packages with high thermal conductivity heat spreader attach

Abstract

Copper heat spreaders are often used in flip chip in package construction. While providing high thermal conductivity, Cu has a significantly higher coefficient of thermal expansion than Si. In this work, two heat spreader attachment materials, indium for high power and polymeric adhesive for medium power applications, have been investigated. For In solder based attach, the Cu heat spreader was metallized with Ni/Au. Two thin film metallizations, Ti/Ni/Au and Ti/Au, have been studied for the Si backside. A nearly void free heat spreader attach has been achieved with vacuum soldering. For Ti/Ni/Au backside metallized Si die, there was no significant shear strength change after 1000 hours aging at 120degC and there was no significant shear or pull strength variation after five lead free re flow cycles. The shear and pull failure mode was within the indium layer. For Ti/Au die backside metallization, the initial die pull strength and failure mode were a function of Au thickness. With 3000 A of Au, there is no significant variation for shear and pull strength after 600 hours aging at 120degC or after five lead free solder reflow cycles. Failure was in the indium layer. For both types of die metallization, 24 mm times 24 mm Cu heat spreaders assembled on 22 mm times 22 mm Si die, exhibited no delamination after two lead free solder reflow cycles followed by 500 air to air thermal shock cycles (-40degC to 85degC). At 1000 cycles, slight delamination was found at the edges of the assembly for both die metallurgies. For adhesive based flat heat spreader attachment, a thermally conductive adhesive was used as the thermal interface and a non-thermally conductive adhesive was applied at the substrate corners to provide mechanical reinforcement of the heat spreader. After pre-conditioning then aging at 100degC for 500 hours followed by 500 air-to-air thermal shock cycles (0degC to 100degC), no delamination was observed and there was no significant degradation in pull strength.