Surface Finish Layer Thickness Limits for High Density Circuit Boards
High density circuit board surface finishes require tight thickness boundaries to prevent nickel hyper-corrosion, black pad, and solder joint embrittlement.
Hard Gold
Finish Coating
Electrolytic finish options utilize a gold deposit alloyed with a secondary metal like cobalt or nickel to increase surface hardness. Printed circuit board manufacturers apply hard gold to edge connectors and slide-on contacts that undergo repeated insertion cycles. The alloyed structure offers superior wear resistance compared to pure gold finishes, which are too soft for sliding contact applications.
Plating Process
Deposition occurs via electroplating where the circuit board is submerged in an aqueous plating bath and connected to an electrical circuit. Controlling the cobalt or nickel co-deposition percentage within the hard gold matrix remains essential to prevent brittle contacts. The bath chemistry must be carefully monitored because excess base metal increases internal stress, leading to plating adhesion failures.
Thickness measurements using X-ray fluorescence verify that the deposited layer meets the specified thickness, typically between thirty and fifty microinches.
Mechanical Endurance
Insertion wear represents the primary degradation mechanism for gold-plated contact pads. Contact pads must endure hundreds of mating cycles with spring-loaded connector pins without exposing the underlying nickel barrier layer. If the nickel is exposed, it can oxidize quickly, leading to high electrical contact resistance and intermittent signal loss.
Engineers specify this finish for critical test points, board-to-board mating interfaces, and keypad contacts. This selection guarantees that the interface remains conductive and corrosion-free over years of operation. The durable alloy prevents micro-fretting corrosion that degrades signal integrity in high-frequency applications.