A brief review of selected aspects of the materials science of ball bonding

Overview of wire bonding using copper wire or insulated wire

There is growing interest in Cu wire bonding for LSI interconnection due to cost savings and better electrical and mechanical properties. Cu bonding wires, in general, are severely limited in their use compared to Au wires; such as wire oxidation, lower bondability, forming gas of N 2 +5%H 2 , and lower reliability. It is difficult for conventional bare Cu wires to achieve the target of LSI application. A surface-enhanced Cu wire (EX1) has been developed. It is a Pd-coated Cu wire and has many advantages compared to bare Cu wires. Stitch strength was much better under fresh conditions and maintained without any deterioration after being stored in air for a prolonged period of time. EX1 had a lifetime of over 90 days in air, although it was 7days for the bare Cu wire. Spherical balls were formed with pure N 2 (hydrogen-free), whereas the bare Cu produced off-center balls. Cost-effective and secure gas, pure N 2 was only available for EX1. The reliability for Cu wire bonding under conditions of high humidity was investigated in pressure cooker test (PCT). The lifetime for EX1 and the bare Cu was over 800h and 250h, respectively. Humidity reliability was significantly greater for EX1. Continuous cracking was formed at the bond interface for the bare Cu wire, although there was no cracking for EX1. Corrosion-induced deterioration would be the root cause of failure for bare Cu wires in PCT. EX1 improves the bond reliability by controlling diffusion at the bond interface. The excellent performance of Pd-coated Cu wire, EX1 is comparable with Au wires and suitable for LSI packaging.

Surface-enhanced copper bonding wire for LSI

Copper wire bonding of microelectronic parts has developed as a means to cut the costs of using the more mature technology of gold wire bonding. However, with this new technology, changes in the bonding processes as well as bonding metallurgy can affect product reliability. This paper discusses the challenges associated with copper wire bonding and the solutions that the industry has been implementing. The paper also provides information to enable customers to conduct qualification and reliability tests on microelectronic packages to facilitate adoption in their target applications.

https://www.tpbin.com/Uploads/Subjects/9e60b56e-e1c1-41bb-abcc-d2e01e0c4809.pdf

Copper Wire Bonding Concerns and Best Practices

Thermosonic ball bonding is a major interconnect process in microelectronics packaging and is positioned to remain one of the key process technologies available to package designers in the near future. However, the main wire material used in fine pitch (FP) and ultra-fine pitch (UFP) ball bonding is gold and with significant increases in gold price, gold ball bonding has become a more costly process that has a considerable economic effect on the assembly of packages used in consumer electronics. An alternative wire material to gold is copper, which is much cheaper and has several technical benefits including better electrical conductivity and has been widely used in discrete and power devices with wire diameters typically larger than 30μm in diameter for many years. However, copper wire behaves quite differently than gold due to its different physical properties, some of which are beneficial and others detrimental to bonding performance. In this article, we briefly review some of the advantages and difficulties with using copper wire advanced packaging and explain why copper cannot replace gold in many applications and why gold offers significant benefits.

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What is the future of bonding wire? Will copper entirely replace gold?

Although Cu bonding wire excels over Au bonding wire in some respects such as production costs, it has not been widely used because of its poor bondability at second bonds due to surface oxidation. We conceived an idea of electroplating oxidation-resistant metal on the Cu bonding wire to prevent the surface oxidation. The electroplating of Au, Ag, Pd, and Ni over Cu bonding wire all increased bond strengths as expected, but it caused problematic ball shapes except Pd-plated Cu bonding wire. The wire could produce the same ball shape as that of Au bonding wire. It was also proved to have excellent bondability sufficient to replace Au bonding wire. That is, it excelled in bond strengths, defective bonding ratio, and wideness of "Parameter Windows". It also showed the same stability as Au bonding wire in reliability tests, while bonds of Cu bonding wire were deteriorated in a few of the tests. In short, the Pd-plated Cu bonding wire can realize excellent bonding similar to Au bonding wire, while having much lower production costs.

A. Mizoguchi

Papers

The development of Cu bonding wire with oxidation-resistant metal coating