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86 PCB007 MAGAZINE I AUGUST 2021 products that require a single thermal excur- sion at assembly. ENIG was the choice for sur- face mount and BGA (ball grid array) used for surface mount, where co-planarity was par- amount. Electrolytic nickel gold was used to plate hard gold over nickel for insertion fin- gers and with a thicker, so gold layer it was the choice for gold wire bonding. With the introduction of lead-free (LF) sol- der—an example is SAC 305—the reflow tem- perature was increased from 230 o C to 260 o C. Standard OSPs could not withstand the re- flow temperature of LF solder and a new gen- eration of high temperature (HT) OSPs came to the market. is class of OSPs could with- stand the increased reflow temp of LF solder and was capable of withstanding multiple ther- mal excursions. In the same time frame, the industry saw the introduction of immersion silver and immer- sion tin. Immersion silver (a metallic surface finish) is a solderable, coplanar contacting surface. Although silver had some limitations like tarnishing and creep corrosion, it has its place with certain process modifications to contain tarnishing and creep corrosion. Im- mersion tin became a viable finish, particular- ly for press fit type connectors where its lu- bricity came in play. Of course, it was solder- able for LF solder. For immersion tin, the IPC specified a thick layer (40 µin or one micron) to counter the natural tendency to form an IMC (copper-tin intermetallic) on storage or thermal excursion. If the IMC works its way to the surface it would render the surface non- solderable. With continued miniaturization, electrolyt- ic nickel gold for gold wire bonding became a problem as it required electrical continuity through bussing. ere was a need for a gold wire-bondable surface that was non-electro- lytic; this resulted in the development of the ENEPIG (electroless nickel, electroless palla- dium, immersion gold) surface finish. e pal- ladium layer between the nickel and the sur- face gold is a diffusion barrier to the migration of nickel to the surface, which can create bond failures. ENEPIG offered all the advantages of ENIG plus gold wire bonding. As the use of wireless networks and hand- held devices continued to grow, massive data is transmitted through printed circuit boards in the form of high frequency RF signals. It be- came clear that the electroless nickel layer in ENIG and ENEPIG will create signal loss. is brought about the development of nickel-free surface finishes. At present there is commer- cially available EPIG and EPAG, both nickel- free. Another finish that fits the bill is thicker DIG. I wrote about DIG in a previous column. A new addition to this group is ISIG, which is presently under development. On the manufacturing end, board shops choose the surface finishes they offer to their customer base. e newer finishes that are in limited demand are relegated to contract plat- ing shops. ese shops offer a wider variety of surface finishes to the board shop, which will, in turn, subcontract their services. As the de- mand increases, the board shop will eventually set up the capability in house. Designers have a lot of choices where they balance availability, cost, manufacturability, and performance. In some instances, specif- ic finishes are the only option (for example, nickel-free finishes for RF signal propagation). Designers also must specify the thickness that would meet their design criteria. e IPC Plating Committee has put out spec- ifications for ENIG, I-Ag, I-Sn, and ENEPIG. In the specifications, a thickness range for the ENIG had the advantage over OSP in that it withstands multiple thermal excursions required at assembly.

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