Fine Pitch Wire Bonding for Automotive Requirements

摘要

All market segments including the automotive industry continue to put cost pressure on semiconductor packaging suppliers in order to stay competitive. Wire bond technology for IC devices has been the backbone of the semiconductor industry to serve automotive customers for many years. The drive for fine pitch wire bonding is to reduce silicon area, increase potential die per wafer and lower die cost. Fine pitch devices must pass the same package reliability requirements as devices designed at coarser pitches. In addition to being submitted to package-level stress testing such as Temperature Cycling, High Temperature Bake and Autoclave, packages are de-processed and the ball bonds are pulled and sheared. The acceptance criteria for wire pull and ball shear testing are to exceed the minimum specification limit and to have good failure signature (i.e. no ball lift). Silicon devices offered to automotive customers used to be designed with bond pad openings to receive ball bond diameters larger than 70um; silicon today is designed for ball diameters smaller than 50um. Although the ball bond diameter has reduced significantly, gold wire size (diameter) has not changed proportionally due to other manufacturability constraints (wire bonder equipment and tooling limitations and wire sweep leading to short-circuiting at mold encapsulation). Fine pitch geometry has created unique challenges in proving wire bonding reliability when using the ball shear and wire pull testing. High quality ball bonds had resulted in ball lift, or metal lift while recording high value of wire pull force. This paper will discuss challenges experienced during the qualification of an automotive device wire bonded with 48um ball diameter. Challenges range from wire pull test method and procedure, interpretation of the wire pull result and failure mode, and the sensitivity of package de-processing method to fine pitch balls. Alternative methods including mechanical simulations and extended package reliability testing were used to demonstrate the robustness of small ball bonds in an automotive application. A set of recommendations will be derived to ensure that quality fine pitch bonds can be implemented in applications demanding high level of reliability.