Bondability and Reliability of Pd-Coated Cu Wire
Semiconductor Digest, April/May 2024
DODGIE CALPITO TANAKA Kikinzoku International (America), Inc.
TAKESHI KUWAHARA, YUKI ANTOKU TANAKA ELECTRONICS CO., LTD. (Japan)
High-reliability CHR PCC wire contains unique impurities that leave the original Pd coating on the ball surface.
Pd on the ball bond surface on Al pads prevents corrosion and bond defects by forming Cu9Al4 and ternary intermetallic compounds.
Copper (Cu) is the most common metal after silver.
Copper is known as a metal with high conductivity and is ideal as a bonding wire for semiconductor interconnections. However, copper is very prone to oxidation and can easily corrode from halogens with high reactivity, such as chlorine. Chlorine (Cl) can be found in uncontrolled environments and in electronic molding compounds used for sealing semiconductors. For this reason, palladium-coated copper (PCC) bonding wires have been developed, and this has become the most widely used bonding wire in place of expensive gold for fine wires with a diameter of less than 50 micrometers.
Ultrasonic thermocompression wire bonding is the most commonly used form of interconnection for semiconductor devices. This requires the formation of a molten free-air ball (FAB) at the wire's end, which is then thermocompressed with ultrasound onto the semiconductor bonding pad to form the first bond.
However, when using PCC wires, forming gas is often required during this process to prevent the oxidation of Cu, which often becomes exposed. The exposure of Cu occurs because the difference in surface tension between the intermetallics causes the Pd coating to flow inside the FAB (for more on this phenomenon, see the article "TANAKA – Wine Tears and Palladium-Coated Copper Wire – Marangoni Effect" – see pp. 33-34 of the March issue of Semiconductor Digest).
Over the past year, TANAKA has started manufacturing a new series of highly reliable PCC wires that prevent Pd flow and maintain Cu plating even during the formation of Fabs.
This article compares conventional wire and high-reliability PCC wire from the perspective of bondability and reliability.
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