Estimated reading time: 2 minutes

The Plating Forum: EPIG—A Nickel-free Surface Finish for Next-generation Products

In recent years, electronic devices, such as smartphones and tablet PCs, have been miniaturized. Chip-size package (CSP) used inside the electronic devices have been miniaturized as well, and the spacing between the lines continues to diminish every year. Some of the latest packages have spacing as little as 15 µm or less. If electroless nickel electroless palladium immersion gold (ENEPIG) is used with an EN thickness of 5–6 µm, only 5 µm of spacing would be left, increasing the risk of shorts between the traces.

Electroless palladium immersion gold (EPIG), an alternate surface finish, eliminates the use of EN and allows for greater spacing between the traces. EPIG is both solderable and wire-bondable. EPIG finished parts do not exhibit the skin effect of EN, making it ideally suited for high-frequency RF applications.

In addition, eliminating EN dramatically reduces processing time at the manufacturing site. The plating process for EPIG is similar to ENEPIG except for the EN step (the longest step in the process), which is eliminated.

For EPIG, electroless phosphorous-palladium (P-Pd) is the better choice for plating on copper. The use of reduction-assisted immersion (RAI) gold as the top layer gives a better rate of deposition on EP, as well as allowing for a thicker gold layer to be deposited. At present, no IPC specifications for EPIG exist. For EP and IG thickness, refer to supplier recommendations for soldering and wire bonding applications.

Table 1 shows the plating process for EPIG on copper. Other variations of EPIG may be used for specific applications. These include EPIG on copper with a different activator; here, an immersion gold activator is used instead of the Pd activator. Another variation is to use a thin (0.15 µm) electroless nickel layer before applying EPIG. The latter variation will give a Ni/Sn intermetallic compound (IMC) while maintaining the advantages of EPIG like no signal loss due to excessive Ni thickness and does not interfere with the spacing for fine-line applications.

Soldering to EPIG

When soldering to EPIG (EP: 0.1–0.15 µm; IG: 0.1–0.2 µm), a Cu/Sn IMC is formed as contrasted with ENEPIG where a Ni/Sn IMC is formed. The Pd and the gold are dissipated into the molten solder, and the solder joint is formed on the copper surface. From the results of EDS, when using EPIG surface finish, Cu₆Sn₅was formed as layer 1, and Cu₃Sn was formed near Cu layer as layer 2. This solder joint is similar to immersion silver, immersion tin, and OSP solder joints, and is expected to demonstrate similar solder joint reliability.

To read this entire column, which appeared in the January 2020 issue of PCB007 Magazine, click here.