SMTA International Conference Proceedings


COPPER AS A VIABLE SOLUTION FOR IC PACKAGING

Authors: Sheila Rima C. Magno, Jean Ramos, Eduardo Pecolera, and Chris Stai
Company: Unisem
Date Published: 10/11/2007   Conference: SMTA International


Abstract: The most commonly used conductor metals in the IC industry today are gold (Au), aluminum (Al), silver alloy and copper (Cu). Gold is the most widely used metal for IC wire bonding because of its resistance to surface corrosion and high productivity through the gold ball bonding process. However, the price of gold has risen significantly since June 2005, from US$420 per troy ounce to US$665 at present, more than a 50 percent increase in 18 months. The upward trend in gold base price has spurred customer interest in gold wire replacement, which helps reduce cost without diminishing conductivity, chip functionality and reliability.

Aluminum is presently used on discrete/power devices because of its current carrying capacity. However, there are two major tradeoffs: first is lower productivity of wedge bonding compared to gold ball bonding, and second is its lack of flexibility to cope with complex wire layouts such as multi-tier and long wire lengths.

Silver alloy is more conductive than gold and employs the same ball bonding process, but it has inconsistent pressure cooker test performance.

Copper is also more conductive than gold and considerably cheaper. It uses the same ball bonding process with the addition of a forming gas to provide an inert environment during free air ball (FAB) formation. Copper/aluminum (Cu/Al) intermetallics (IMC) have considerably slower inter-diffusion than gold/aluminum (Cu/Al) IMC, which prevents Kirkendall voiding, thereby ensuring better performance during high-temperature storage tests. Heavy copper wire, > 2.0 mils in diameter, is already widely used in the industry today for power applications. With these, copper is selected as the most suitable replacement for gold wire.

We will discuss some of the challenges and recommendations in implementing copper wirebond as a replacement for gold in high-volume manufacturing of QFN packages. The focus will mainly be on 1.0 mil wire replacement, which is the common wire size in IC devices.

Among the challenges is defining the optimum inert environment during the FAB formation. Copper bond quality begins during FAB formation, where a high-voltage spark is used to melt the copper wire to form a ball that requires a non-reactive environment to prevent copper oxidation. To create this inert region, the forming gas is supplied to the EFO tip area.

Determining the appropriate forming gas mixture is essential in achieving a round and blemish-free ball. The forming gas is a mixture of nitrogen (N2) and hydrogen (H2). The most common concentrations available are: 95 percent N2 with 5 percent H2 and 90 percent N2 with 10 percent H2. Another important parameter is the gas flow rate. This must be defined so that it provides an adequate and consistent inert surrounding with a reasonable gas consumption rate. Installing a closed-loop gas flow rate detection is a must for high-volume manufacturing (HVM) to prevent possible reliability issues (i.e., electrically open failures) arising from poor copper-to-aluminum adhesion due to copper oxidation.

Copper wire is harder than gold wire, so thermocompression bonding or force-dominant bonding is applied rather than thermo-sonic bonding or power-dominant bonding to prevent aluminum splashing, pad metal lift or silicon damage/cratering. The effect of aluminum pad thickness on pad metal lift will also be discussed.

Special failure analysis methodology to prevent copper from being etched during decapsulation was used during this evaluation.

Lastly, an assessment on the cost reduction impact was performed, taking into consideration the controls and the logistics necessary for its implementation.

Key words: copper wire, gold wire replacement, copper bonding challenges, copper wire in production, copper wire cost benefits.



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