Gold Finger Chamfer And Bevel Depth Control

A gold finger edge connector works by sliding. The board is pushed into a socket, and the leading edge of the board has to guide itself between the contacts without catching on them, without bending a contact spring and without removing metal from the plating. The feature that makes this possible is a chamfer cut along the insertion edge, and its angle and depth are specified for a reason rather than by habit.

This article explains what the chamfer does, how the angle and depth are chosen, how it relates to the bevel and to the insertion force, and how the result is inspected and controlled.

What The Chamfer Does

The chamfer is a cut that removes the sharp corner from the leading edge of the board, turning the first contact into a ramp. As the board enters the socket, the ramp meets the contact spring and pushes it aside gradually instead of striking it with a square edge. The result is a lower insertion force, less wear on the plating and a much smaller chance of a contact being permanently deformed by a careless insertion.

The chamfer also protects the plating at the leading edge. Hard gold is applied to the fingers to resist wear, but the laminate beneath is softer than the plated layer, and a sharp corner will chip and lift the plating over repeated insertions. Removing the corner removes the feature that starts the damage.

Angle And Depth

The typical chamfer is cut at 20 to 30 degrees from the board surface, to a depth that clears the plated fingers, commonly about 1 to 1.5 millimetres measured along the edge. The angle is a compromise: a shallow angle guides the board more gently but consumes more of the edge length, while a steep angle takes less length and pushes the contact aside more abruptly.

The depth has to be enough to remove all of the plating from the ramp. If the chamfer is cut through the board but leaves a sliver of gold at the top of the ramp, that sliver becomes the first part of the board to meet the contact, and the purpose of the chamfer is defeated. The depth is therefore checked against the finger layout, and the fingers are kept back from the chamfered area by a defined distance.

Bevel, Radius And Insertion Force

The chamfer may be a straight cut, a bevelled edge produced by a rotating cutter, or a radius turned on a router with a form tool. A radius gives the smoothest entry and the most uniform force, at the cost of a special tool, while a straight chamfer is easier to produce and easier to measure. For a connector that is inserted by hand the radius is worth the trouble; for a board that is inserted once by a machine the chamfer is usually sufficient.

Insertion force is the sum of the forces at every contact, and it rises with the number of fingers, with the stiffness of the contact springs and with the friction between gold and the contact material. The chamfer affects only the initial force, but the initial force is what damages contacts, so a board that is difficult to start into its socket is a board whose chamfer needs review even if the total force is acceptable.

Plating, Underplating And Wear

The fingers themselves are plated with a hard gold over a nickel barrier, and the gold thickness is chosen for the number of insertion cycles the product will see rather than for corrosion resistance. A nickel layer of 2.5 to 5 microns sits between the copper and the gold and stops the two from diffusing into each other, and the gold itself is typically 0.5 to 1.3 microns thick on a connector that will be mated many times.

The tail of the finger, away from the chamfer, is where the contact settles. That region should be flat, free of mask and free of any plating defects, because a contact that rests on a void or on an inclusion will have a higher resistance than its neighbours. The pad dimensions that support this are described under PCB pad design standards.

Fabrication Route And Tolerances

The usual route is to plate the fingers before the outline is cut, protect them with tape during the rest of the processing, and cut the chamfer as part of the routing operation with a form tool or a second pass at an angle. Cutting after plating risks lifting the gold at the ramp, so the sequence matters as much as the tool. The position of the chamfer relative to the fingers belongs on the drawing with a tolerance, and so does the depth.

The outline in general, and the way the connector edge interacts with the rest of the mechanical design, is described under board outline and mounting design. Where the board is part of a card cage with rails, the thickness tolerance of the board and the chamfer have to match the socket that the card will be inserted into, and that dimension is normally taken from the connector datasheet rather than assumed.

Inspection And Control

Inspection is visual with a low power microscope, looking for a chamfer that is too shallow, plating left on the ramp, nicks along the fingers and residue from the tape. The angle is checked with a toolmaker microscope or with a profile projector against the drawing, and the depth is measured from the board edge to the point where the chamfer meets the surface.

A functional check is more informative than a dimensional one. Inserting the board into a mating socket and measuring the insertion and withdrawal force confirms that the chamfer, the thickness and the finger height all work together, and it is the test that the customer will perform. Doing it at first article prevents a lot of finished boards from being rejected for a reason that is visible only when the connector is on the other side. The wider assembly context is described under PCBA development process.

Process Control and Verification

On a design of this kind, bevel is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

FAQ

Can the chamfer be cut after the fingers are plated? It can, and it often is, but the cut has to be made with a sharp tool and at a low feed rate, because the cutter lifts the gold at the edge of the ramp when it is blunt.

How deep should the chamfer be? Deep enough to clear the plating and to expose the laminate along the whole ramp, typically about 1 to 1.5 millimetres along the board surface. The depth is stated on the drawing.

Does the chamfer affect the impedance of the fingers? The fingers are not usually controlled impedance structures, and the chamfer is outside the mating area, so the effect is negligible. The mating region is where the contact settles and where the geometry must stay uniform.