Connector Plating Selection For PCB Headers

The plating on a board mounted connector is a separable contact surface rather than a soldered joint. It has to conduct a signal or a current through a mechanical interface that may be mated once or thousands of times, and it has to keep doing so in whatever atmosphere the product will see.

This article covers what the plating has to do, how gold thickness is related to mating cycles, why tin behaves differently, and how the choice is made.

What The Plating Has To Do

A contact surface has three jobs. It has to be conductive, which rules out anything that forms a stable insulating oxide at the interface. It has to resist wear, because every mating cycle removes a little material. And it has to resist corrosion, because a corroded contact develops a film whose resistance rises with time and with the number of cycles.

No single metal does all three well, which is why contacts are built as a stack. The outermost layer is chosen for its low contact resistance and for its inertness; the layer beneath it provides hardness and acts as a diffusion barrier; and the base is the copper of the board or the brass of the pin. The physical mechanism of the deposit, and the defects that can appear in it, are described under copper plating defects prevention.

Gold Thickness And Mating Cycles

Gold resists oxidation almost completely, which is why a gold contact keeps a low contact resistance over time even at low signal levels. It is also soft, so it wears. The rate of wear depends on the thickness of the gold, the hardness of the deposit and the force at the contact, and the number of mating cycles the surface survives before the underlying layer is exposed is roughly proportional to the gold thickness.

Hard gold, deposited from a bath containing a small amount of cobalt or nickel, is significantly more wear resistant than pure soft gold, and it is used wherever repeated mating is expected. The thickness is chosen from the cycle count: a connection that is mated once at the factory needs a thin flash, while a card that is inserted daily needs a layer several times thicker. Below a certain thickness the gold is porous, and porosity allows the base metal to corrode through the pores, which defeats the purpose of using gold at all. The comparison between finishes that applies to soldered surfaces is treated under lead free versus leaded solder.

Tin, Whiskers And Fretting

Tin is cheap, solderable and widely used, and it has two failure modes that matter for a separable contact. The first is tin whisker growth: a tin finish can grow thin crystalline filaments over time, and a whisker that bridges two adjacent contacts creates a short. The growth is driven by stress in the deposit and is worse for pure tin and for tin deposited over a copper alloy, which is the combination used on many pins.

The second is fretting corrosion. Tin forms a hard oxide, and at a contact that experiences small relative movement, such as vibration, the oxide is broken and re-formed repeatedly until a resistive debris builds up in the contact area. The resistance rises and eventually the connection becomes intermittent. This is why a tin on tin interface is avoided in applications that see vibration, and why a lubricant or a gold interface is used instead. The same class of behaviour decides the reliability of a soldered joint as well, as described under the quality characteristics of a board design.

The Nickel Underlayer

Nickel is present in almost every contact stack for three reasons. It is hard, so it supports the gold layer and reduces the way the gold deforms under load. It is a diffusion barrier, so it prevents the copper beneath from migrating into the gold and raising its resistance. And it is corrosion resistant in its own right, so if the gold wears through, the exposed surface is a metal that still conducts rather than a metal that oxidises.

The nickel thickness is chosen to be thick enough to act as a barrier and thin enough that it does not introduce its own stress or embrittle the contact. Its quality matters more than its thickness: a porous nickel layer allows copper to reach the gold, and a stressed layer cracks at the contact area under load. Both defects are plating process issues, and both are controlled by the bath and by the current density distribution, which is the same problem as uniform plating in a small hole.

Choosing By Application

For a board to board connector that is mated once during assembly and never again, a tin finish on both sides is adequate and inexpensive. For a connector that is mated a few dozen times during the life of the product, a thin hard gold over nickel is the usual choice, and the thickness follows the expected cycle count. For an application with vibration, a gold interface is preferred regardless of the cycle count, because it removes the fretting mechanism entirely.

For a high current connection, the choice is driven by resistance rather than by wear, and the contact area and plating thickness are set to keep the voltage drop and the temperature rise within limits. For a very low level signal, the choice is driven by film resistance, and gold is again preferred because its surface does not develop a film. The specification should state the application class rather than only the finish, so that a supplier can choose the thickness that meets it. The general approach to setting such requirements is part of design guidelines for manufacturability.

Specifying And Inspecting

The specification for a connector plating should state the base metal, the underlayer and its thickness, the outer layer and its thickness, the hardness or the bath type for a gold, and any porosity requirement or test. Where whisker growth is a concern, the specification should call for a matte tin with a defined grain structure, or for a nickel barrier under the tin, which reduces the stress that drives whisker growth.

Inspection of a plated contact is by thickness measurement on a sample, by a porosity test such as a nitric acid vapour exposure followed by a check for corrosion sites, and by contact resistance measurement after the specified number of mating cycles. The last of these is the test that matters most and the one that is most often omitted, because it requires a mating cycle fixture. Where the connector is critical, the fixture is worth building: it converts an argument about finishes into a number of cycles at which the resistance crosses a limit.

FAQ

Is gold always better than tin? For repeated mating, low level signals and vibrating environments it is. For a connection that is mated once and soldered on the other side, tin is adequate and costs less.

Why does a gold contact sometimes fail early? Usually because the gold is thin and porous, or because the nickel beneath it is defective or too thin to act as a barrier. In both cases the base metal reaches the surface and corrodes.

Can a whisker problem be avoided with tin? It can be reduced by using a matte tin with a controlled grain structure, by adding a nickel barrier beneath the tin, and by annealing. Avoiding tin altogether on critical fine pitch contacts is the more certain answer where the risk is unacceptable.