Manufacturers traditionally apply a thick layer of soft gold on PCBs for gold ball wire bonding. This process involves plating the copper traces on the PCB with a layer of nickel, followed by an electroplated layer of soft gold, typically 30-50 µin in thickness. The properly plated gold layer provides an excellent surface for wire bonding. Although this method is ideal for fine-pitch gold ball bonding, it can be costly.
However, common tin-based solder alloys dissolve large amounts of gold during reflow, which can damage the gold conduction patterns and make the solder joint brittle or prone to fatigue cracking. Indium-based solders, on the other hand, perform better on gold films, as gold is largely insoluble in indium.
Traditionally, manufacturers use wire bonding to connect chips with chip carriers. A gold wire is bonded to an aluminum or copper pad, but the intermetallicity between gold and aluminum/copper can reduce the reliability of the bond. Additionally, bonding dissimilar metals weakens the overall process. Typically, an oxide layer covers the aluminum or copper surface, which must be removed by scrubbing or plowing to ensure proper contact between the gold wire and the pad, potentially causing mechanical damage to the pad.
With electroplated nickel and gold on the copper pad, bonding between the gold wire and the gold on the pad eliminates intermetallic formation, improving yield. Since the gold surface remains oxide-free, no scrubbing is needed to make a proper contact, enhancing the bonding quality. This method also allows for smaller pad sizes, leading to higher I/O counts for chip designs and enabling smaller chip sizes.
The electronic packaging and surface finishing industries are familiar with traditional nickel and gold electroplating processes. Nickel acts as the barrier layer, while gold forms the bonding surface. Manufacturers typically plate the nickel layer through photoresist onto the copper pads. After stripping the resist and etching the copper, the fabricator applies a polyimide passivation layer, which prevents corrosion of the copper sides beneath the nickel. Finally, the immersion gold is plated onto the nickel to create the top wettable surface.
Once the gold/nickel plating is complete, the stream PCB requires baking at 360°C for one hour to allow the passivation layer to cover the copper sidewalls. However, this process causes the diffusion of nickel into the gold, hardening the gold and making it more difficult to bond wires. As a result, the conventional plating approach is not ideal for creating durable gold wire bonding pads.
To address this, manufacturers use a selective plating process that employs titanium as a conductive layer. Through selective plating, the electroplated nickel and gold encapsulate the copper layer underneath to prevent corrosion.
The fabricator first deposits a TaTaN liner onto the copper layer using a sputtering process, followed by a photolithography process to apply photoresist on the layer. Unwanted copper between pads is removed through electroetching. After stripping the photoresist, the fabricator electroplates nickel and gold onto the copper pads through the TaTaN liner.
The final nickel thickness is approximately 1 µm, while the gold thickness is about 0.5 µm, with the copper layer fully encapsulated beneath the nickel and gold layers.
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