Through-Hole Lead Forming, Seating and Trimming

Through-hole parts are still used where the mechanical strength, the current or the connector family requires them. Their leads have to be formed to fit the hole spacing and then trimmed, and the way those two steps are done decides whether the part sits flush and whether the joint is reliable.

Why the Lead Is Formed

The lead is bent so that the spacing matches the holes and so that the body stands at the intended height above the board. Forming also provides a strain relief that keeps the solder joint from carrying the mechanical load.

A formed lead with a stress relief bend absorbs movement. A lead that goes straight into the hole transfers every movement into the joint. Our reliability notes describe how the load path is assessed.

Forming Tools and Damage

The lead should be formed with a tool that supports the lead where it leaves the body. Bending a lead by hand against the body damages the seal and, in a glass sealed part, cracks it.

The bend radius has a minimum, and the distance from the body to the start of the bend has a minimum as well. Both come from the component data rather than from convenience.

Lead Length and Trimming

The lead length below the board is set by the soldering method. Wave soldering needs enough lead to form a fillet, and the excess is trimmed afterwards; selective soldering and hand soldering allow a shorter protrusion.

Trimming after soldering loads the joint. The cutter should support the lead and the joint, and the operation should be done with a tool that shears rather than pulls. Our joint defects notes describe the damage that follows.

Seating and Flushness

Some parts must sit flush: a connector that mates with a panel, a part with a heatsink interface, or a standoff. Forcing a part flush against a formed lead stresses the joint before the board is even tested.

Where flush mounting is required, the lead spacing should match the hole spacing exactly and the lead should be formed with a short stress relief that still allows the body to sit down.

Hole Fit

The hole diameter must clear the lead with enough room for the plating and for the solder to rise. Too tight and the part cannot be inserted without force; too loose and the joint needs more solder than the fillet provides.

The annular ring must survive the drilling and the insertion, which is why the hole and pad sizes are chosen together. Our hole types notes describe how the two are specified.

Hand Assembly and Machine Assembly

Machine insertion requires the leads to be formed to a defined geometry so that the tool can grip and drive them. Hand assembly tolerates more variation and produces more of it.

Where both are used on the same board, the forming specification must serve both, or the parts inserted by machine will not match the ones inserted by hand. Our fabrication notes notes list the attributes that should be stated.

Verification

The check is the seating height, the lead protrusion and the condition of the lead where it leaves the body. All three are visible before soldering and none of them can be corrected afterwards.

A sample should be measured against the drawing rather than judged by eye, because the difference between a part that is seated and one that is nearly seated is small. Our quality notes describe how the result is recorded.

Process Control and Verification

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

Verification and Records

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

FAQ

Should leads be trimmed before or after soldering? After, in most processes, so that the fillet forms on a lead that is still supported by the fixture. The trimming method then has to avoid loading the joint.

Can a lead be bent more than once? No. A second bend on the same lead work hardens it and often cracks the seal, and the part should be replaced instead.

What does gopcb provide for lead preparation? We provide forming geometry with the stress relief and bend radius from the component data, form tooling that supports the lead at the body, lead length and trimming method by soldering process, hole fit and annular ring definition, seating height verification, and first article measurement against the drawing.