Gold Finger: Key Checks Before Release

A gold finger is a row of plated contacts along the edge of a board, and the board plugs into a socket the way a card plugs into a computer. The fingers are the whole electrical interface of the product, so their plating, geometry and edge treatment decide how many times the card can be inserted before the contact resistance rises. They are also the part of the board that is touched, wiped and worn by the user.

What a Gold Finger Is

A gold finger is a flat contact on the edge of a printed circuit board, plated with gold and shaped so that it slides into a socket. The contact area is on the surface of the board, so the finger is a pad rather than a pin.

The interface is called a card edge connector, and the board acts as the plug. The socket contains a spring contact that presses on the finger, and the contact force and the geometry of the socket set the reliability of the connection.

The fingers are usually arranged on both sides of the board and along one edge, with a keying notch or a shorter finger to prevent the card from being inserted the wrong way round.

The plating has to survive abrasion rather than a soldering process, which is why a gold finger uses a different finish from the rest of the board.

Hard Gold and Soft Gold

Hard gold is an alloy of gold with a small amount of cobalt or nickel, deposited to give a hard, wear resistant surface. It is the standard for a card edge, where the finger will be inserted many times.

Soft gold is pure gold, which is used for a wire bond or for a surface that is soldered. It is softer and wears quickly under sliding contact, so it is the wrong choice for a finger that is repeatedly inserted.

The gold is deposited over a nickel barrier, which stops the copper from diffusing into the gold and stops the gold from migrating into the copper. Without the nickel the contact resistance rises as the layers interdiffuse.

The thickness of the gold is usually specified between 0.5 and 1.5 microns for a card edge, and the plating thickness should be given as a minimum rather than a nominal, because the wear life depends on the thinnest point.

Finger Geometry and Pitch

The finger width and the gap between fingers are set by the socket, and the values come from the connector datasheet rather than from a general rule. A finger that is narrower than the socket contact reduces the contact area; one that is wider can bridge to its neighbour.

The usual centre to centre spacing is 1.27 mm or 2.54 mm, and the finer pitch is used where the card has many signals. The gap between the fingers has to allow the socket contacts to be moulded, so the finger cannot be wider than about half the pitch.

The finger length is the insertion depth, and it has to be long enough that the contact still lands on the gold after the card has been inserted as far as the mechanical stop allows. A finger that ends under the socket housing is a finger that cannot be probed.

The pad is normally defined by the mask, so the gold is exposed only over the contact area. A finger that is exposed beyond the contact area wastes gold and is prone to damage from handling.

Bevelling and Insertion

The leading edge of a card is bevelled so that the board slides into the socket without catching. The bevel is cut at an angle, usually about thirty degrees, and it removes the sharp corner that would otherwise dig into the socket contact.

The bevel has to be cut after the plating, which means the gold is cut as well and the cut edge is bare copper or bare laminate. That is acceptable because the cut edge never contacts the socket, but the bevel must not intrude into the contact area.

The keying notch is cut in the same operation, and its position defines which socket the card will fit. A notch that is out of position either will not fit or will fit the wrong slot.

A card that is inserted without a bevel, or with a bevel that is too small, damages the socket contacts rather than itself. The pad standards and the mechanical drawing both have to describe the bevel.

Wear and Contamination

Contact wear is the result of the sliding motion during insertion and of the debris that collects on the finger. The gold wears through the thin plating at the edge of the contact, and once the nickel is exposed the contact resistance rises sharply.

Contamination comes from the environment and from the user. Fingerprints leave salts and oils that corrode the gold, and the corrosion product is an insulator that increases the resistance. A card edge should be handled by its edges rather than by the fingers.

The fingers are also sensitive to the surface finish of the socket contact. A tin plated socket contact mated with a gold finger forms a galvanic couple, and the tin corrodes in a humid environment. Gold against gold is the combination that lasts.

A lubricant is sometimes applied to the fingers to reduce the wear rate, and it also protects the surface from the atmosphere. The lubricant has to be compatible with the socket and it must not collect dust, which would increase the wear rather than reduce it.

The number of insertion cycles that a card can take is usually between a few hundred and a few thousand, and it is a specification that should be quoted. A card that is intended to be inserted daily needs a thicker gold.

Design Review

The review starts with the socket: its pitch, its contact geometry, its insertion depth and its rated cycles. The fingers are drawn from the socket datasheet, not from a template.

The second item is the plating, given as a hard gold thickness over a nickel barrier, with the thickness stated as a minimum. The rest of the board can use a different finish, so the drawing has to separate the two areas.

The third item is the mechanical preparation: the bevel angle, the keying notch and the handling instructions. These are as important as the artwork, and they are usually the part that is left off the drawing.

The last item is the inspection, where the gold thickness and the finger geometry are checked on a sample. The quality check on a card edge includes the bevel and the plating, and both should be measured rather than assessed by eye.

FAQ

Why is hard gold used on a card edge? Because the contact slides during insertion and pure gold wears through quickly. The alloy is harder and gives a much longer life.

Does the bevel need to be plated? The cut face is bare, which is acceptable because it does not contact the socket. What matters is that the bevel does not cut into the contact area.

How many insertions can a gold finger take? It depends on the gold thickness and the socket, and a range from a few hundred to a few thousand cycles is typical. The figure should be quoted in the specification.