Gold Finger Plating For Edge Connectors
Edge connectors are the part of a board that is inserted into a socket or a card cage, and they take more mechanical abuse than any other feature. The gold finger plating on those contacts has to survive repeated insertion, abrasion against the socket contacts and the wiping action that happens every time the card slides home.
A gold finger plating stack is not simply gold on copper. It is a nickel barrier with a hard gold layer on top, and each layer has a specific job. Getting the thicknesses and the surface finish right decides whether the connector works for a hundred cycles or for ten thousand.
What Edge Connectors Need
A card edge contact has to do three things at once: conduct current with a low and stable resistance, resist wear as it slides against the socket, and resist corrosion over the life of the product. These requirements pull in different directions, which is why the standard solution is a layered finish rather than a single metal.
The contact also has to survive the assembly process. Gold fingers are usually masked off during soldering so that solder does not wet them, since a solder coated contact changes the geometry and the friction of the mating surface. Masking tape or a peelable resist is used, and the tape must be removed without leaving adhesive behind.
Hard Gold Versus Soft Gold
Pure gold is soft, and a pure gold contact wears rapidly under repeated insertion. Hard gold is a gold alloy containing a small amount of cobalt or nickel, which raises its hardness considerably and makes it suitable for a contact that will be mated many times.
Soft gold still has its place on pads that are only soldered or wire bonded, where ductility matters more than wear resistance. The two finishes are not interchangeable, and the specification should state which one applies to the edge contacts and which applies to the component pads. The way pad finishes are specified is described under PCB pad design standards.
The Nickel Barrier Layer
The nickel barrier sits between the copper of the board and the gold. It stops copper diffusing into the gold, gives the contact a hard mechanical base and prevents the gold from being absorbed into the substrate during plating. Typical nickel thickness for a connector is 2.5 to 5 microns.
Nickel quality matters as much as thickness. A porous or poorly bonded nickel layer lets corrosion reach the copper underneath and produces a contact that fails at a low insertion count. Where ductility is important, a sulfamate nickel bath is often preferred over a bright nickel bath.
Plating Thickness And Insertion Cycles
Insertion cycles are the figure that drives the gold thickness, and the relationship is close to linear for a well made contact. A typical guide is 0.4 to 0.8 micron of hard gold for a few hundred cycles, rising to 1.0 micron or more for thousands of cycles in a hostile environment.
Gold thickness is verified by X-ray fluorescence on the contact area itself rather than on a coupon, because the edge plating geometry can differ from the panel surface. Measurements should be taken at the top, middle and bottom of the finger to confirm uniformity along the whole contact.
Bevel, Chamfer And Masking
The board edge at a connector is bevelled so that the card enters the socket without catching. A typical bevel is 20 to 45 degrees, cut after plating or before it depending on the process, and it must not expose copper or lift the plating at the edge.
Masking protects the rest of the board during edge plating and protects the fingers during subsequent processing. The mask must be placed so that the plated area matches the drawing, because an under-plated contact is a reliability risk and an over-plated one can short to a neighbouring finger. How these features are dimensioned is described under board outline and mounting design.
Inspection And Common Defects
Inspection covers gold thickness, nickel thickness, plating adhesion, bevel geometry and the presence of pits or nodules on the contact surface. A burnish test or a tape test confirms adhesion, and a visual check under magnification finds nodules that would damage the socket.
The common defects are thin gold at the bottom of the finger, plating that has peeled at the bevel, solder wicking onto the contact, and nickel exposed by wear during a test insertion. Each of these is a functional failure rather than a cosmetic one. The way these requirements are written into the fabrication package is described under PCB design and fabrication.
Process Control and Verification
On a design of this kind, bevel is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.
Process Control and Verification
On a design of this kind, bevel is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
Process Control and Verification
On a design of this kind, bevel is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
FAQ
How thick should hard gold be on an edge connector? It depends on the number of insertion cycles. A few hundred cycles are served by 0.4 to 0.8 micron, while thousands of cycles in a corrosive environment call for 1.0 micron or more over a sound nickel barrier.
Why is a nickel layer needed under the gold? Nickel blocks copper diffusion and provides a hard base that stops the gold being displaced under load. Without it the contact resistance rises quickly and the coating wears through.
Can gold fingers be soldered? They can, but solder changes the contact surface and is usually unwanted. Fingers are normally masked during assembly so that only the connector interface keeps its gold finish.
