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Explore High Bonding Reliability of Cu Wire Bonded Devices under Extreme Halide Contaminated Environments
Keywords: Bonding Reliability , Al bond pad corrosion , Corrosion Inhibition Treatment
Copper has rapidly replaced gold as the preferred wire bonding material in microelectronic packaging due to its higher electrical conductivity, lower cost, and better mechanical strength which leads to reduced pad size and pad pitch. However, halides induced corrosion-related failures, although at the low ppm level, need to be carefully controlled to ensure maximum bonding reliability. Specifically, Al bond pad are particularly susceptible to heavy corrosion with a characteristic “mud crack” appearance. Such corrosion can be a critical failure mode for Cu wire bonded assemblies, especially under harsh conditions such as in automotive environments. While this type of corrosion is often associated with the presence of contaminants such as halide (Cl-, Br-, F-) and or application of positive bias, the exact corrosion mechanism has not been established [1, 2]. In addition, the recent transition to non-lead SAC (Sn-Ag-Cu) solder necessitates the use of stronger flux (2-3%) that could leave flux residues and ion contamination. Furthermore, higher reflow temperatures (~ 260 °C) required by non-lead solder can cause stress delamination allowing corrosion agents to penetrate and initiate corrosion. The emerging trend of wearable electronics also imposes new, more stringent packaging requirements to ensure complete corrosion protection from sweat/mud/rain in all-terrain non-stop usage conditions. Literature reported corrosion studies of Cu wire bonding to Al pads mostly focus on intermetallic compounds (IMCs) like Cu9Al4, CuAl and CuAl2 especially in acidic chloride under moisture condition. However, these studies fail to correlate corrosion activation mechanism(s) with underlying operating parameters (such as chemical environments, bimetallic contacts, etc.) precluding their usefulness in developing effective corrosion prevention strategies. Utilizing a novel real time corrosion screening approach, we established that the bimetallic contact between Cu balls and Al bond pad, induced by chloride ion penetration, is the initial driver for the observed heavy Al pad corrosion. Most importantly, we identified, for the first time, H2 evolution was the coupling cathodic half reaction that fuels these vicious Al corrosion cycles eventually leading to mud-cracking on Al bond pads. With this improved mechanistic insights, we developed a corrosion prevention treatment specifically targeted on Cu/Al bimetallic contacts to prevent the acute Al bond pad corrosion [3]. In this presentation, we will report an effective corrosion inhibition strategy that utilize chemical vapor deposition (CVD) to selectively bond corrosion inhibitors to these critical Cu ball/Al pad interfaces. As revealed by the External Reflectance Infrared Spectroscopy (ER-IRS), the CVD process formed a strong chemisorbed corrosion inhibitor layer on the Cu/Al interface to raise the activation barrier of the H+/H2 and effectively stop the Al bond pad corrosion. ER-IRS spectra of the inhibitor coating remained unchanged after annealing in 260 oC for 25 minutes suggesting excellent thermal stability at the required solder re-flow condition. In addition, both ER-IRS and SEM-EDX characterization showed lead frame received no detectable trace of inhibitor compound after the CVD treatment, suggesting our corrosion prevention treatment will not lead to any delamination issues. Device failure due to Al corrosion generally arises when the package undergoes humidity testing such as PCT, HAST and THB during qualification procedures. To better evaluate the effectiveness of protection treatment, we developed a corrosion screening platform that is compatible with current packaging and molding operation. Utilizing a microliter syringe, tiny NaCl solution droplets were delivered to the die surface of Cu wire bonded devices. After drying, the NaCl salt crystals now act as the massive internal Cl- ion contamination source. After molding, the Cl- loaded test devices were subjected to selected thermal stress tests, like Autoclave/unbiased HAST and Temperature Cycle. The subsequent failure analyses, showed CVD inhibitor coating is highly effective of preventing Al bond pad corrosion even under the attack of a massive internal Cl- ion contamination. In this presentation, we will present electrical failure analyses to correlate with the extent of delamination provided by Scanning Acoustic Microscopy (SAM) imaging. Testing outcomes will be evaluated to further optimize the effectiveness of this unique corrosion prevention treatment especially in high temperature, to achieve highest bonding reliability goal under extreme condition.
Oliver Chyan, Professor
University of North Texas
Denton, TX

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