Comparison of Gold and Copper Wire Bonding on Aluminum and Nickel-Palladium-Gold Bond Pads for Automotive Application

Abstract

New automotive requirements expect plastic packages to survive higher operating temperatures with extended thermal duration. Mission profiles for under-the-hood and transmission application historically specified minimal duration at maximum junction temperature, such as 50 total hours at 150C, while keeping most of the total operating duration at lower temperatures. Further module integration and more stringent environmental requirements push modules and thus plastic packages closer to the heat source. As such, new mission profiles include more than 3500 total hours at 150°C. To satisfy new automotive requirements, plastic packages must meet AEC Grade 0 or higher. One key limitation of the conventional plastic package is the use of gold bond wire on aluminum bond pad. Au-Al intermetallic degradation due to intermetallic transformation in high temperature storage condition remains the main reliability concern. More reliable intermetallic systems have been proposed that change the wire material and/or the bond pad metallization. An alternative wire material to gold, copper, has many benefits including low cost, high electrical and thermal conductivities and excellent reliability with aluminum pad metallization. Pad re-metallization using nickel/palladium, nickel/gold or nickel/palladium/gold over aluminum bond pad or copper bond pad offers a noble and reliable metal interconnect. This study focused on evaluating Au and Cu wire bonding on low-K-copper wafers having two types of bonding surfaces, the conventional aluminum pad and aluminum pad re-metallized with electroless nickel / electroless palladium / immersion gold. Ni thickness ranging from 1μm to 3μm was evaluated. Defects on as-plated Ni/Pd/Au bond pads such as color difference and surface roughness were determined to be due to nodule growth and plating non-uniformity. Wire bonded strip-level thermal aging was conducted to compare the high-temperature performance of the four interconnect types. Packages underwent extensive reliability stress conditions. Cross-sectioning through the ball bonds was also conducted to examine the welding region between the ball bond and bond pad. Defects in plating and wire bonding processes causing package reliability failures were identified. Recommendations for plating and wire bonding processes were derived to ensure high quality and reliable interconnect exceeding AEC grade 0 requirements.