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Fine Copper Wire Bonding For IC & Advanced Packaging Applications: Challenges & Solutions
Keywords: Cu wire bonding, Applications, IC
Wire bonding continues to be the predominant interconnect for Integrated Circuit Packaging. Cost motivation is driving Cu wire bonding gradually to penetrate into the assembly packaging production floors replacing gold wire. Though copper was one of the earliest elements known to man, when in it comes to Cu wire bonding the knowledge base is still in infancy stage. Recently Cu wire bonding is evaluated for various applications such as high I/O fine pitch (<60µm BPP), SiP /multi dice, stacked dice, Cu BSOB, Ultra low loop, over hang dice, low-k device, new first bond metallization (ex Ni-Pd-Au) and new treated second bond surface. Some of these applications are show in figures below. Fundamental physical and chemical characteristics of Cu wire are different from Au wire. These factors makes Cu wire bonding more challenging compared to Au wire bonding. Developments in wire bonders, wire, capillary and understanding Cu interactions with packaging materials are factors that could accelerate the Cu wire bonding implementation to various product and applications. Wire bonder equipment capability is one of primary factors in fine Cu wire bonding (1.0 mil and below). Cu FAB oxidation control is the key priority. Also, the Cu nozzle design and gas delivery systems are vital. Other key process criteria for Cu wire bonding are: minimizing the Al squeeze out, maintaining sufficient Al remnant below the bond pad, minimizing the bonding stress to under pad structures, and achieving good stitch pull values. On top of this, each application may require a special process control feature: Cu BSOB, where the bond pad undergoes multiple impacts by harder Cu wire requires low impact process with bump ball oxidation control. Over hang dice exhibit twisting and bending during bonding requires capillary positional feed back during the bonding motion. Low-k & ultra low-k chips could not with stand conventional Cu wire bonding stress requires a additional features to minimize the impact to the under pad. Work hardening nature of Cu makes bonding more challenging, and may require using multiple bonding segments which may affect the productivity. Getting a good bonding and time zero ball shear and wire pull values are only a preliminary step. Cu wire interaction with packaging materials impose a major reliability concern, especially with moisture induced reliability tests such as HAST and PCT. Recently Cu wire coated with Pd wire has been reported to have superior HAST and PCT performance. New capillaries are developed for Cu wire bonding, which is designed to enhances the ultrasonic coupling and minimize the bonding stress to the bond pad. In this paper we shall discuss the recent developments in Cu wire bonding, process requirements, key challenges and solutions for various applications.
Mohandass Siva Kumar, Technical Marketing Manage
ASM Technology Singapore PTE LTD.
Singapore 768924,

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