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One of the main challenges in PCB manufacturing is to create a stable, long-term coating of the copper surface to perform critical functions throughout the expected lifetime of the part. The surface coating is there to do two things: prevent the copper from oxidizing by coming in contact with the air and form a reliable contact for a soldered joint or wire-bonded connector. Following IPC specifications IPC-4552A, IPC-4553A, IPC-4554, and IPC-4556 will improve reliability and longevity. X-ray fluorescence (XRF) is a proven method—and, for this reason, has been written into these four specifications—to control processes for plating thickness of substrates to address oxidation and solderability.
IPC and ENIG Specification: IPC-4552A
The electroless nickel/immersion gold (ENIG) deposit is one of the most widespread surface finishes used in printed board manufacturing today. IPC released its first specification for ENIG in 2002, followed by revision A in August 2017. ENIG is an excellent surface finish for reliable solder joints and aluminum wire bonds and has a relatively long shelf-life; however, its high performance depends on the quality of nickel and gold layers. The thin outer layer of immersion gold is very stable and prevents oxidation of the underlying nickel for the life of a component.
The 2017 revision helps manufacturers to create a more reproducible and reliable ENIG surface finish and outlines printed board performance requirements, including the J-STD-003 solderability specification. The revision focuses on the thickness of the gold layer; the minimum allowable thickness has been reduced, and a new parameter for the maximum gold thickness was introduced. If the gold thickness is too low, the deposit may not remain intact once in use. This would result in corrosion and cause weak solder joints and board failure.
To read this entire article, which appeared in the February 2020 issue of PCB007 Magazine, click here.
