Facing rigorous pricing constraints on IC packaging materials (especially gold), semiconductor assembly and test service (SATS) providers worldwide are looking at lower-cost alternatives to packaging to improve their narrowing profit margins. According to Jean Ramos, chief technology officer at Unisem, her company’s main challenge last year was dealing with the customers’ price reduction requirements in face with increasing costs of gold and mould compound. One solution to this, she says, is a low-cost packaging technology that uses copper wire bonding instead of the traditional gold wires.
COPPER VERSUS GOLD
In power devices, heavy copper wires, measuring around 2mils in diameter, are usually used because of its high current carrying capacity and thermal stability at high temperatures. However, for most devices, gold is a preferred, albeit expensive, choice in most IC packages because of its excellent conductivity and ductility. The metal is used as wires for the connection of the IC to the leads of the package. Also, gold’s ductility makes the wires resistant to breaking during the encapsulation process for plastic-encapsulated devices.
Considering the economy, on the other hand, there has recently been a strong push to replace gold wires with copper in IC packaging. Compared to gold, copper has 25 percent better electrical and thermal conductivity than gold. This means a gold wire diameter can be replaced by a smaller copper wire diameter without affecting electrical performance, thereby making it possible to reduce bond pad size.
Copper is also 25 percent more thermally conductive than gold, meaning it can dissipate heat faster. Ramos says that as more companies stack up the chips to lower down cost, the increase of activity in the device because of integration leads to greater heat generation. Without proper thermal dissipation, this shortens battery life. According to Ramos, one thing that is driving the industry today is coming out with a better BoM for devices that are more thermally dissipative.
CHALLENGES
Copper wire bonding for IC packages, however, is still considered to be in its early stage. The key challenges encountered by Unisem in the process include the free air ball oxidation, because copper oxidizes easily and quickly; bonding method, because copper wire is harder than gold; and failure analysis, because it is difficult to get good contrast to distinguish copper lead frames from copper wires.
Ramos says there was no robust process to put that into production as of last year. This year, however, Unisem was able to come up with an optimized process. Working with suppliers and customers, the company expects copper bonding to finally kick off.
ELP
Unisem recently developed what it claims to be the next generation in QFN—the alternative multi-row leadless packaging called etched leadless package (ELP), which offers not only dual-row but also lower-cost packaging with the advantage of better wire bond stability while patronizing standard manufacturing processes.
Unisem owns five patents for the technology—three in the United States, one in Korea, and one in China. The ELP solution for low-cost multi-row packages enables different form factors and configurations, and is also made of copper, so it addresses heat dissipation issues as well.
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