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PCB007-Jan2020

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84 PCB007 MAGAZINE I JANUARY 2020 In recent years, electronic devices, such as smartphones and tablet PCs, have been min- iaturized. Chip-size package (CSP) used inside the electronic devices have been miniaturized as well, and the spacing between the lines con- tinues to diminish every year. Some of the lat- est packages have spacing as little as 15 µm or less. If electroless nickel electroless palla- dium immersion gold (ENEPIG) is used with an EN thickness of 5–6 µm, only 5 µm of spac- ing 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 re- duces 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-palladi- um (P-Pd) is the bet- ter choice for plating on copper. The use of reduction-assisted im- mersion (RAI) gold as the top layer gives a better rate of deposi- tion on EP, as well as allowing for a thicker gold layer to be de- posited. 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 dissipat- ed into the molten solder, and the solder joint is formed on the copper surface. From the re- EPIG: A Nickel-free Surface Finish for Next-generation Products The Plating Forum by George Milad, UYEMURA Table 1: EPIG plating process (target thickness, EP: 0.05, 0.1, 0.2 µm; IG: 0.05, 0.1, 0.2, 0.3 µm).

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