Improving the durability of wire bonds in EV batteries
HOW2POWER TODAY, January 2024
Dodgie Calpito TANAKA Kikinzoku International (America), Inc.
Shuichi Mitoma, Shizu Matsunaga, Kosuke Ono, Tsukasa Ichikawa TANAKA ELECTRONICS CO., LTD. (Japan, Tokyo)
Improving the durability of wire bonds in EV batteries
Wire bonding has been developed and has been used for a long time in semiconductor packaging, due to the inherent flexibility and programmability of wire bonding. These characteristics are the greatest strengths of wire bonding compared to other interconnection methods. When using wire bonding, wires made of gold, aluminum, copper, silver, and their alloys, or copper composite materials with palladium coating can be used to form electrical interconnections and wire bonds between silicon dies and their substrates.
Wire bonds are delicate and have limited flexibility, and are usually sealed in semiconductor devices with cushioning materials such as resin or molding compounds. These cushioning materials provide a certain level of durability and strength to withstand damage from vibrations. However, in most EV battery packs, wire bonds are used without any materials to protect them from vibrations, which leads to a loss of this certain level of durability and makes wire bonds more susceptible to damage.
This article examines ultrasonic wire bonding used for interconnecting cylindrical lithium-ion (Li-ion) batteries in EV battery packs. In particular, it describes a series of vibration tests conducted to measure the extent to which various aspects of wire bond design affect susceptibility to damage.
This paper describes the wire bond structure in EV battery packs, identifies its mechanical weaknesses, and introduces the test equipment developed in this study to assess the vulnerability of wire bonds to vibration. It also discusses five comparative studies conducted to examine the effects of vibration direction, materials, wire shapes, loop heights, and single versus multiple wires on this vulnerability.
The aim of this research is to analyze wire disconnection issues in EV battery packs and provide several mitigation measures to reduce or eliminate such failures. After presenting the test results, we will summarize key findings that will help pack designers achieve wire bonds with high durability in EV battery packs.
Vulnerability of Wire Bonds in EV Battery Packs
Currently, ultrasonic wire bonding is used for interconnecting cylindrical Li-ion batteries in EV battery packs. The negative terminal wire is bonded to the edge of the battery, while the positive terminal wire is bonded to the cathode at the center of the battery, making it prone to vibration. The battery can is made of nickel-plated steel, and the edge has a corrugated shape and a curved cross-section.
Wire bonding is designed for flat surfaces, making bonding to batteries with curved edges difficult (although recent designs have been flattened more). The surface roughness of the edges of batteries is also inconsistent due to crimping and is susceptible to corrosion and electrolyte contamination.
Wires may experience fatigue and eventually break due to vibrations caused by rough road conditions, rough handling of EVs, or sudden acceleration/deceleration. As a result, the heel area between the bond and the wire bond area may be damaged. This is a common cause of energy capacity degradation in EV battery packs. Figure 1 shows a typical wire bond used for interconnecting cylindrical battery cells.
Related Information
How was this article?
If you found this helpful, please share it.
