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Aluminum as an Alternative to Copper Wirebonding
Keywords: Fine wire aluminum bonding, Process improvement, Reliability with green compounds
This paper will present a paradigm shift in wirebonding for molded packages, presenting fine wire aluminum wedge bonding as an alternative to copper wirebonding. Copper wirebonding has taken the semiconductor industry by storm as the price of gold increased to stratospheric levels in the past few years. A number of large IDMs have converted to copper wirebonding for their molded packages, with subcontractors aggressively adding more capacity. Although process knowledge gaps are narrowing since 2006 when interest in copper wirebonding was rekindled, a number of issues still exist such as high risk of bond pad cracking or peeling since copper is 1.7x harder than gold, high temperature bonding requiring gas shielding of the bonding environment, low looping challenges, second bond repeatability, and long-term reliability when interacting with residual ionics in green mold compounds. The latter is a newly reported reliability challenge with copper wirebonding where copper reacts with residual ionics (such as Chlorine) in halogen-free compounds, leading to early bond failures in HAST testing. By considering aluminum wirebonding on the other hand, a number of those challenges are resolved. Indeed, with aluminum being 2.2x softer than copper, less stresses are imparted on bond pad structures, eliminating the risk of dielectric cracking or silicon cratering. Room temperature bonding removes the need for any gas shielding and the associated conversion kits. Furthermore, bonding aluminum to aluminum offers a mono-metallic system ideal for high temperature applications such as automotive under-the-hood uses, high brightness LED arrays, or downhole drilling. The need for over protective coatings such as ENIG, ENEPIG, or copper top on the bond pads is cost effectively addressed. One of the main drawbacks of wedge bonding in the past has been its throughput compared to the typical thermosonic ball bonding process. With the proper hardware and software modifications of a wedge bonding platform, UPH can be raised to levels comparable with current copper wirebonding machines. And, last but certainly not least, the price of copper is expected to increase with higher electronics demand. With the price of aluminum staying at least 3x lower than that of copper, significant cost savings can be obtained by switching to aluminum bonding. The paper will discuss the development process covering the advantages mentioned previously, including comprehensive reliability data on packages with green mold compounds.
Luu Nguyen, Senior Engineering Manager
National Semiconductor Corporation
Santa Clara, CA

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