Palladium coated copper wire wedge integrity to withstand extended high temperature storage stress test

Abstract

Copper (Cu) wire bonding technology had been widely accepted as a interconnect material in semiconductor packaging. The main advantage of Cu as a interconnect material is cost, thermal and electrical performance as compare to gold. However effect of corrosive elements on bare Cu wire remain a big challenge for the reliability of Cu wire packages. Market trend is pushing for Palladium coated copper (PCC) wire to be the alternative for bare Cu wire, due to the better bondability process and resistance to corrosive elements. Nevertheless there are many researches suggest PCC wire is not suitable for High Temperature Storage (HTS) reliability stress test more than 150oC due to the intrinsic degradation of PCC wire. From literature review the main contribution to the degration of PCC wire was due to the crack or void on the Palladium (Pd) coating. Cu underneath the Pd coating will diffuse through the crack to the surface and weaken the integrity of the wire. Through sparking parameter optimization, optimal Pd coverage on the FAB can be achieved hence first bond integrity can be secured. On the other hand, 100% Pd coverage on the second bond wedge is impossible to obtain due to the extensive mechanical contact of capillary on the wedge during formation. In this paper, the study will focus on the wedge PCC wire on roughen leadframe with NiPdAuAg surface finishing. The study will cover difference 3 main factors influencing the integrity of wedge, i) wedge formation, ii) leadframe AuAg plating thickness iii) ionic elements from mold compound. Sample with various wedge formation on different AuAg plating thickness with and without mold compound will be subjected to HTS 200oC up to 900 hours and comparison of the degration rate for different samples can be identify. The aim of the study is to identify the dominant factor of the PCC wedge degration on HTS stress so that improvement for package beyond AEC Q100 Grade 0 can be achieved.