Card Edge Connector Design Guide

A card edge connector removes the plug from the design by using the board itself as the contact. The socket springs press directly on pads along the card edge, and the board thickness, the pad position and the plating all become part of the connector specification. The arrangement is compact, cheap and reliable, and it fails in ways that a conventional connector cannot.

What a Card Edge Connector Is

A card edge connector is a socket with two rows of spring contacts that grip the edge of a board. The board carries the mating contacts as plated pads, so no separate plug is needed.

The socket provides the contact force and the mechanical retention, while the board provides the contact surface and the alignment. The two halves have to be designed together, because the tolerance stack runs from the socket housing through the board thickness to the pad position.

The arrangement is used in computers, in industrial racks and in instruments, and the standard pitches come from that history. The 2.54 mm and 1.27 mm pitches are the most common, with finer pitches available for a dense card.

The contact geometry is the same idea as the pad standards used elsewhere, but the load is applied by a spring rather than by solder, so the surface condition matters more than the joint.

Contact Force and Reliability

The contact force is what makes the connection work. It has to be high enough to break through the surface film and to keep the contact area stable under vibration, and low enough that the insertion force is manageable.

The force is provided by the socket spring, and it depends on the board thickness. A board that is thinner than nominal gives a lower force, and one that is thicker stresses the spring and can cause a permanent set.

A contact that operates with too little force is a contact that becomes intermittent, and the fault appears only when the product is warm or when it is tapped. That behaviour is the signature of a low force contact rather than a broken one.

The retention force is a different figure from the contact force, and it comes from the friction of the whole row of contacts plus any latch in the socket. A card in a vibrating rack needs either a high retention force or a mechanical latch, and the latch is the safer of the two.

The rated insertion force is a system figure and it should be checked for the whole card, since a socket with two hundred contacts can need a considerable force to seat. The extraction force is lower and is also specified.

Board Thickness and Tolerance

The board thickness is the most important mechanical dimension, because the socket is designed for a nominal value with a tolerance. A card that is outside that tolerance will either be loose or will not enter.

The thickness tolerance of a plated board is wider than most designers expect, and it depends on the laminate and on the press cycle. The outline and thickness rules should be checked against the socket specification before the stackup is fixed.

A thick board also changes the pad geometry, because the pad on the edge is on the surface of a thicker section. The finger position has to be dimensioned from the same datum as the thickness.

The pads on the edge are also thinner than the rest of the board, because the plating and the mask add a small amount to the profile. A card that measures at the top of the thickness range on the laminate can measure over the limit once the plating is included.

The socket may accept a range of thicknesses with a different spring, so the mechanical drawing should quote the target thickness and the tolerance rather than a nominal figure alone.

Keying and Polarisation

Keying prevents a card from being inserted into the wrong slot and prevents it from being inserted the wrong way round. It is usually a notch cut in the card edge that engages a moulded rib in the socket.

The notch position is part of the connector specification, and it has to be cut in a position that does not interfere with a finger. A notch that lands on a pad removes the contact rather than keying the card.

The keying also fixes the orientation of the card, so a card that is inserted upside down will not enter at all. That is a robust design, because the mistake is impossible rather than merely unlikely.

Where several cards of the same type are used in one system, a family of key positions gives each card a unique slot. The position of the notch must be documented, because it is easy to reproduce a board with the wrong key.

Plating and Environmental Effects

The plating on a card edge has to resist wear rather than oxidation, which is why a hard gold over nickel is used. The thickness is specified as a minimum and the value comes from the number of insertion cycles.

In a humid or a polluted environment the contact is also attacked by the atmosphere, and the corrosion product is an insulator. A gold surface is the most resistant, and a lubricated surface reduces the attack further.

The plating on the socket contact matters as well, because a galvanic couple between the two surfaces drives corrosion of the less noble metal. Gold against gold is the stable combination, while gold against tin corrodes in a humid room.

The contact resistance should be measured after the specified number of cycles and after the environmental test, since a new contact that measures well can measure poorly after a week in a humid chamber.

Design Review

The review starts with the socket datasheet: the pitch, the thickness range, the contact force and the rated cycles. The card is then drawn to those numbers rather than to a template.

The second item is the board thickness and its tolerance, checked against the socket range. The third is the gold specification, given as a minimum thickness over a nickel barrier.

The fourth is the mechanical preparation: the bevel, the keying notch and the finger length. These features are usually on the profile drawing and are forgotten in the artwork review.

The last item is the trial insertion, where the card is fitted into the real socket and the force is measured. The quality check on a card edge includes the plating and the bevel, and the plating thickness should be taken from a coupon rather than from a note.

FAQ

What board thickness does a card edge connector need? The socket specifies a nominal thickness with a tolerance, and the board has to be inside it. A board that is outside the range will be loose or will not enter.

Why is the card edge plated with hard gold? Because the contact slides during insertion. A soft gold wears through quickly, and the nickel barrier underneath then oxidises.

Can a card edge connector be used for a high current? It can, with a wider finger and a socket rated for the current. The contact force and the finger area both set the rating.