Strain Relief And Lead Forming For Through Hole Parts

A through hole component is held by its leads, and those leads have to do two jobs that pull in opposite directions. They have to hold the part in position during handling and soldering, and they have to absorb the movement that happens afterwards, when the board expands and contracts and when the assembly is vibrated or bent. Lead forming, and the strain relief that goes with it, is how the two jobs are reconciled.

This article explains why leads are formed, how the bend radius affects the risk of a crack, how the forming is done, and how the result is inspected.

Why Leads Are Formed

A lead that goes straight from the body to the hole and is soldered flush transfers every movement of the board directly into the seal or the body of the component. A ceramic disc capacitor, a glass diode or a metal can transistor will crack at that point, and the failure appears after the product has been in service rather than at assembly.

Forming the lead introduces a length of wire between the body and the joint that can flex. The shape, the length of that flexing section and the way the lead enters the hole together determine how much of the board movement reaches the body. On a product that will be exposed to vibration, a deliberate loop or a standoff is used to increase the compliance, and on a product that is only handled the forming is mainly for placement.

Bend Radius And Where Cracks Start

The lead is a solid metal wire with a plating on it, and bending it too sharply concentrates the strain on the outside of the bend. The plating cracks first, and the crack in the plating becomes a stress raiser in the wire. A minimum bend radius of about one to one and a half times the diameter of the lead is used for a tin plated wire, and the figure is larger for a component that will be bent repeatedly.

The bend should also be at a distance from the body. Bending immediately at the seal of a glass or ceramic component introduces stress into the seal and is the usual cause of a leak or a crack at the body. A standoff of a millimetre or two between the body and the start of the bend is enough, and the component datasheet normally states the minimum distance that the manufacturer has qualified.

Forming Tools And Fixtures

Hand forming with a pair of pliers produces leads that are different on every part, and the variation matters because the lead length and the angle set how the part sits in the hole. A forming die or a simple fixture holds the lead while it is bent and produces a consistent shape with a defined radius. Where the volume justifies it, the forming is done by a machine in the same step that cuts the leads to length.

The order of operations matters. The leads are formed before the part is inserted, and any cutting of the length is done before the forming rather than after, because cutting after bending leaves a burr that can damage the hole. Where the part is inserted by a machine, the forming is done by the same machine, and the shape is part of the program rather than of the operator’s judgement.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/8.jpeg" alt="Stress relief loop formed in a component lead” />

Stress Relief Loops And Mounting

A stress relief loop is an extra bend in the lead that increases the length of the flexing section and reduces the force that reaches the body. It is used on components that are mounted away from the board, on parts that are expected to see vibration, and on assemblies where the body is fixed to a chassis and the board moves relative to it. The loop is a defined shape rather than an improvisation, and its dimensions belong on the assembly drawing.

Where the component is large, the body is often supported separately. A bracket, a clip or a bead of adhesive holds the body to the board so that the leads carry electricity rather than load, and the lead forming then serves to accommodate the tolerance between the two fixings. The materials used for that purpose are described under potting, dispensing and adhesives.

Soldering And Its Effect On The Lead

The soldering operation changes the properties of the lead. A tin lead alloy soldered at 183 degrees does little damage, while a lead free alloy at 250 degrees anneals the copper of the lead and removes some of its work hardening. A lead that was formed and then soldered at a high temperature is more ductile and, in some cases, weaker than it was. That is not usually a problem, but it becomes one if the lead is also carrying a load, which is a reason to make sure the body is supported rather than relying on the lead.

The joint itself is affected by the lead shape. A lead that enters the hole at an angle produces a joint with an uneven fillet and can leave a gap on one side. A lead that is formed to a length that leaves too much of it protruding below the board creates a long tail that can bridge to a neighbouring joint, and one that is too short cannot be clinched. The alloy differences that affect the joint are described under lead free versus leaded solder.

Documentation And Inspection

The forming requirements belong on the assembly drawing together with the standoff height, the bend radius and the length of the lead below the board. A note that says the leads are to be formed to a standard does not define anything, and two shops will produce two different results from it. Where the shape matters, the drawing shows the shape.

Inspection is visual and dimensional. The lead is checked for cracks, especially at the point where it leaves the body, for the standoff and for the presence of the intended loop. A magnifier is used on a glass or ceramic component, because the crack that matters is small. The wider assembly sequence that the forming belongs to is described under PCBA development process.

Process Control and Verification

On a design of this kind, bend radius 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

How tight can a lead be bent? A radius of one to one and a half times the lead diameter is the practical minimum for a plated wire, and a larger radius should be used where the part will be bent more than once or exposed to vibration.

Do leads have to be clinched? They are clinched where the part has to stay in place before soldering and where the joint benefits from the mechanical retention. A clinched lead should not be bent so tightly that the bend falls under the board.

Can a formed lead be straightened and used again? It should not be. A lead that has been bent and straightened is work hardened at the bend and the plating is disturbed, and the component is usually replaced rather than reused.