Micross

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Aluminum bonding pad corrosion of wirebond packages
Keywords: Corrosion, Wirebond Package, Reliability
Corrosion of aluminum bonding pads in wirebonded packages often lead to device reliability issues especially in extreme and harsh conditions such as in automotive applications. Corroded Al (aluminum) surface is found to have a mud-crack type appearance with presence of relatively very thick aluminum oxide/hydroxide (> 0.1µm) compared to thin protective native aluminum oxide (< 7nm). Various factors contributing to the formation of mud-crack type corrosion are identified. Invariably in all cases of mud-crack corrosion observed for packaged devices, Cl (chlorine) was detected on the corroded Al pad. Corrosive impact of high levels of Cl on the Al pad leading thick aluminum oxide growth will be discussed. It is shown that concentration levels of Cl to form mud-crack corrosion is far beyond the level of ionizable Cl typically present in packaging materials such as mold compound and die attach material and hence the source of high levels of Cl inferred to be a foreign contamination. The effect of Cl on mud-crack corrosion is found to depend on other factors such as pH and applied bias voltage. Corrosion susceptibility is found to be significantly reduced even in the presence of high levels of Cl when the pH > 6 and close to neutral value. It is also found that Cl is not essential to form mud-crack corrosion. Certain conditions leading to the corrosion in the absence of Cl will be presented. As F (fluorine) is a known contaminant in semiconductor processing its influence on the Al pad corrosion is examined. The differences on the impact of F vs. Cl on mud-crack corrosion will be discussed. Corrosion was observed irrespective of type of wire, gold (Au) or copper (Cu), bonded to Al pad. However, type of bi-metallic contact influences the extent of corrosion. The impact of bi-metallic contact on the mud-crack corrosion will be presented.
Varughese Mathew,
NXP Semiconductors
Austin, TX
USA


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