COMPARISON OF RELIABILITY OF COPPER , GOLD , SILVER , AND PCC WIREBONDS UNDER SUSTAINED OPERATION AT 200 C

Semiconductor packaging industry is transitioning to use of alternate lower cost wirebond materials to replace gold (Au) wire which is often used in high-reliability applications. Typical wire diameters vary between 0.8mil to 2mil. Recent increases in the gold-price have motivated the industry to search for alternate materials candidates for use in wirebonding. Three of the leading wirebonding candidates are Silver (Ag), Copper (Cu), and Palladium Coated Copper (PCC). The new material candidates are inexpensive in comparison with gold and may have better electrical, and thermal properties, which is advantageous for fine pitchhigh density electronics. The transition, however, comes along with few trade-offs such as narrow process window, higher wire-hardness, increased propensity for chipcratering, lack of reliability knowledge base of when deployed in harsh environment applications. Relationship between mechanical degradation of the wirebond and the change in electric response needs to be established for better understanding of the failure modes and their respective mechanisms. Understanding the physics of damage progression may provide insights into the process parameters for manufacture of more robust interconnects. In this paper, a detailed study of the electrical and mechanical degradation of wirebonds under high temperature exposure is presented. Four wirebond candidates (Au, Ag, Cu and PCC) bonded onto Aluminum (Al) pad were subjected to high temperature storage life until failure to study the degradation of the bond-wire interface. Same package architecture and electronic molding compound (EMC) were used for all four candidates. Detailed analysis of intermetallic (IMC) phase evolution is presented along with quantification of the phases and their evolution over time. Ball shear strength was measured after decapsulation. Measurements of shear strength, shear failure modes, and IMC composition have been correlated with the change in the electrical response. Change in shear strength and different shear failure modes for different wirebond systems are discussed in the paper. INTRODUCTION Wirebonds are widely used first-level interconnects between the semiconductor-chip with the substrate of the package. Adaptability of the wirebonding morphology to a number of package types in addition to the trend towards low-profile formed wires has resulted in their continued use in newer chip-scale form-factor and stacked chip packages. Typical bond wires range in 0.8-2 mil in diameters. Copper wires may be thermosonic or ultrasonic bonded on aluminum pads either on-chip or on-substrate [1]. The intense interest in the viability and manufacturing process development of copper wires has been motivated by search for cost-effective alternatives and the increase in gold prices. Copper, silver and palladium-coated copper are amongst the top-choices for alternative materials. Electrical, thermal and mechanical properties of alternative wirebond candidate materials are shown in Table 1. Table 1. Material Properties of Cu, Ag, Au [2] Property Unit Cu Au Ag