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Reliability of heavy gage aluminum wirebonds under high temperature aging
Keywords: aluminum wirebonding, high temperature aging, reliability
Aluminum (Al) wires are widely used in semiconductor industry to electrically connect the integrated circuits to the microelectronic package. Heavy gage Al wires (10 mils diameter or above) finds most use in analog power packages TO, SO, IGBT etc. due to its ease of use, low cost and excellent thermal and electrical performance. Ultrasonic wire bonding process is typically used to form thick Al wedge bonds in the power package. It is an advantageous method of interconnect formation as it does not require external heat application and can be performed at room temperature. Al wedge bonding on bare copper (Cu) substrate is a popular combination in the power package industry, however, little is known about the reliability of this system under high temperature aging conditions. The wirebonds are a critical component of the microelectronic package and therefore, it is important to understand the reliability risks associated with them under the conditions at which the package which be exposed to during reliability stress testing and also its lifetime. The reliability of Al wedge bonds/bare Cu leadframe substrate was evaluated in a TO220 type power package. Wirebonding parameters were optimized to ensure strong bonds at time zero (T0) conditions which was verified by high wedge shear force (> 800 g) and high wire pull force (> 200 g). All failure modes were acceptable, no bond lifts were observed. Molded TO220 package samples were subjected to isothermal aging conditions of 1000 hrs@ 150 C, 2000 hrs @ 150 C, 1500 hrs @ 175 C and wedge shear testing was performed to check for bond integrity. These thermal aging conditions were chosen to align with the automotive grade high temperature storage life reliability requirements. Results showed a significant drop in wedge shear force, as high as 80% for all testing conditions. The failure mode observed was bond lift. Failure analysis performed showed no Al remnant on leadframe, while Cu remnant was detected on backside of the lifted Al wire indicating failure at the Al/Cu intermetallic (IMC). Cross-section analysis using ion-mill/scanning electronic microscopy (SEM) was also performed to check for bond integrity. Results revealed gross cracking at the Al/Cu joint, which translated into lower shear force values. Non-uniform IMC growth was observed at the interface and IMC thickness as high as 2 m was measured in certain areas along the bond length. Brittleness of the Al/Cu IMC as such high thickness was identified as the root cause of bond integrity loss under high temperature aging conditions. The Cu/Al IMC has high brittleness and high electrical resistance which can cause significant increase in overall package resistance when it reaches critical levels like 2 m, putting the package reliability at risk. The results presented in this study can be used as guidelines to design a more robust microelectronic package capable of passing automotive grade reliability requirements.
Nishant Lakhera, Packaging and Materials Engineer
NXP Semiconductors
Austin, TX

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