IPC-A-610 allows a nick in a component lead of up to 10 percent of its diameter. On a 0.6 mm resistor lead that is 0.06 mm, which is thinner than a human hair and far shallower than the mark a serrated jaw leaves.
That single figure explains a set of faults most people blame on their soldering: a joint that reads open after a few thermal cycles, solder that won’t climb one lead while every other joint in the row wets cleanly, a resistor that turns up cracked at the body months after assembly. In each case the damage was done before the iron was switched on, while the leads were being bent to fit the footprint. Lead forming looks like the simplest operation in through-hole assembly, which is why it usually gets the least attention.
What actually damages a component lead?
Three things do most of the damage: bending too close to the body, gripping the lead at a single point, and bending the same lead more than once.
Bending too close to the body puts the strain into the seal where the lead enters the package. On an axial resistor that seal is the boundary between the ceramic or film body and the wire; on a glass diode it is the glass itself. Strain there cracks the seal, and a cracked seal lets moisture in long after the board has shipped.
Gripping the lead at one point does its damage more slowly. When the full closing force of a pair of pliers lands on one narrow contact patch, the lead deforms locally. The plating breaks at that point, and the lead is left with a notch that concentrates stress. Under vibration or repeated thermal expansion, that notch is where the lead eventually fractures.
Repeated bending work hardens the wire. Working a copper alloy lead raises its yield strength and uses up the ductility that lets it bend without cracking, so the second bend takes more force and the lead splits sooner.
The industry has put numbers on this. IPC-A-610, the acceptability standard for electronic assemblies, allows nicks, deformation and other damage of no more than 10 percent of the lead’s diameter, width or thickness, and applies that limit across all three quality classes (source). On a 0.6 mm resistor lead, 10 percent is 0.06 mm. A visible bite mark from a serrated jaw is deeper than that.
How far from the body should the bend start?
Leave at least one lead diameter of straight wire between the body and the start of the bend, and never less than 0.8 mm. The inside radius of the bend follows the same floor: one lead diameter minimum. Both figures exist to keep strain out of the seal.
| Requirement | IPC-A-610 |
| Clearance between the body and the start of the bend | At least 1 lead diameter, and never less than 0.8 mm (0.03 in) |
| Minimum inside bend radius | At least 1 lead diameter |
| Maximum allowable nick, deformation or damage | 10 percent of the lead’s diameter, width or thickness |
The radius is measured on the inside of the bend, not the outside, so a tight bend around a sharp jaw edge fails the requirement even when the lead looks square. The clearance figure is a floor and not a target: on hand-built boards there is rarely a reason to bend closer than 2 mm from the body, and the extra millimeter costs nothing.
Which jaws should you use for lead forming?
Use smooth jaws with a parallel action. The shape of the jaws decides where the closing force lands on the lead, which is the part of the operation you can control.
Conventional pliers pivot on a single joint, so the jaws close like scissors, meeting at their tips first and opening into a V behind the contact point. Gripping a thin lead in that V means the whole closing force arrives through one narrow line of contact. Parallel-action pliers replace that single pivot with a parallel linkage, so the jaws stay parallel to each other from fully open to fully closed: a lead sits against the flat of the jaw along its length, and the same hand pressure is spread over a much larger area.
Serrated jaws do the other kind of damage, and on a plated lead it is the worse kind. They are made to bite, which is what you want on a rusted fastener and precisely what you do not want on a component lead. Component leads are tin plated for a reason: tin takes solder readily, while the copper alloy underneath oxidizes quickly in air, and copper oxide resists wetting so the solder will not climb it (source). Cut through the plating and you expose base metal at exactly the point the joint has to form. Loss of plating is a recognized cause of poor wetting in production soldering (source).
So the jaw you want for lead forming is smooth and parallel, and narrow enough at the tip to reach a lead between two already-populated components without touching either.
Choosing pliers for lead forming
Most listings never state the jaw face, so judge it from the product photo: a smooth jaw shows a flat, unbroken surface where a serrated one shows cross-hatching or teeth. Precision ranges tend to say “smooth jaws” or “flat jaws” in the title, and they are usually sold for jewelry or electronics work rather than general assembly.
Tip width is the specification most listings omit, and it is the one that decides whether you can reach a lead between two seated components. The Maun snipe nose parallel pliers, from a UK hand-tool manufacturer that publishes the figure, are 2.0 x 1.8 mm at the tip, with smooth faces and a parallel jaw action.
Expect to pay more than a general-purpose pair from a hardware store. The extra buys jaw faces that will not print a pattern into the plating. If you are forming leads in any volume, a dedicated lead-forming tool or a simple bench jig will beat any hand pliers for speed and repeatability, and will give you identical spacing across a batch.
How do you form a lead step by step?
source – https://www.pexels.com/photo/technician-repairing-circuit-board-with-multimeter-38264253/
The sequence below assumes an axial part, formed by hand, with one 90 degree bend in each lead and a standard hole pitch.
- Measure the hole spacing first. Set your bend distance from the board, not by eye. Mismatched spacing forces the lead sideways as it seats, which reintroduces strain at the body.
- Grip the lead between the body and the bend point. Place the jaws so they hold the straight section, with at least one lead diameter of clearance to the body. The pliers, not the component, take the load.
- Bend the free end around the jaw face. Let the flat of the jaw set the radius. Do not lever against the tip or a corner, which is what produces a sharp inside radius.
- Bend to 90 degrees in one movement. Going past the angle and correcting back is a second bend in the same place, and that is where work hardening starts.
- Seat the component and check it sits flat. A body that stands proud on one side usually means the two bends are at different distances, not that the lead is too short.
- Inspect the bend before soldering. Look for a bright, unbroken surface. Any visible flat spot, notch or tooling mark on the plating is worth re-checking against the 10 percent limit.
Frequently asked questions
Can you bend component leads by hand?
For a one-off repair, yes, provided you support the lead close to the body with a fingernail or a smooth tool so the bend does not travel back into the seal. Hand bending gives inconsistent radii and spacing, so it is not suitable where several boards need to match.
Do you need a lead-forming tool, or will pliers do?
Pliers are fine for prototypes, repairs and small batches, and give you more control in awkward places. A forming tool is worth buying when you need identical spacing across many components, because it sets the bend distance mechanically.
Can a component be reused after its leads are straightened?
Sometimes, but treat it as suspect. Straightening adds a second working of the same wire, so the lead is more brittle than it was, and any plating damage from the original bend is still there. For anything going into a board that matters, fit a new component.
Does it matter which direction an axial component faces?
For function, no. For inspection and rework it does: most shops fit them so the markings all read the same way, because it makes value-checking and fault-finding much quicker on a populated board.
Final Words!
Keep the bend a clear lead diameter away from the body so the strain never reaches the seal, and hold the lead with something that spreads the load across a flat face. Smooth, parallel jaws do that second job, and they leave the tin plating intact so the joint wets the way it should. Both habits cost a few seconds each, and both remove failures that would otherwise show up long after the board leaves the bench.
