Wire and Cable Soldering Practice

A wire or a cable is soldered because the joint has to survive movement, and that requirement changes everything about the technique. The strand, the insulation and the strain relief all matter more than the appearance of the joint.

The failure modes are also different from those of a board joint: a wicking strand becomes rigid and breaks, while an overheated insulation provides no protection and may hide the damage.

Stranded Conductors

A stranded conductor has to be tinned or the solder has to flow between the strands, and the two approaches give different results. A fully tinned strand is easier to insert and stiffer, while a joint that is made without full tinning can leave strands that are not connected.

The number of strands and their gauge set the amount of alloy required, and a fine stranded cable draws alloy away from the joint by capillary action.

Wicking Under the Insulation

Alloy that travels under the insulation creates a rigid section where the cable will break when it flexes. The wicking is driven by heat and by capillary action along the strands.

The control is to limit the heat and the time, and to provide a mechanical stop such as a heat shrink sleeve or a clamp between the joint and the flexible section.

Insulation and Heat Damage

Insulation shrinks, chars or melts when it is overheated, and the damage may not be visible under a sleeve. The temperature limit of the insulation should be known before the iron is set.

A heat sink clip between the joint and the insulation limits the travel of heat, and it is the standard technique on a small wire. It should be used rather than relying on speed alone.

Strain Relief and Support

The joint should never be the point where the cable is restrained, because the alloy is the least ductile part of the assembly. A clamp, a tie or a sleeve should take the load.

Where a cable enters a connector or a housing, the entry point should be designed so that the movement is absorbed by the housing rather than by the joint.

Flux and Cleaning

An activated flux is used for a wire joint because the surface is often oxidised, and the residue should be removed where it could cause corrosion. The cleaning has to reach under the insulation, which is difficult.

A no clean flux that is qualified for the environment avoids the problem, and the choice should be recorded. This is the same decision described for flux residue risk.

Connectors and Terminals

A crimped terminal is mechanically superior to a soldered one where the crimp is correctly specified, and the two are often combined with a solder joint for a specific requirement. The crimp should be made first if both are used.

Where a wire is soldered into a connector cup, the cup should be filled and the wire inserted hot, so that the alloy surrounds the strands.

Inspection

Inspection looks for a shiny joint, full coverage of the strands, no wicking beyond the intended zone and no damage to the insulation. A pull test on a sample verifies the mechanical result.

The inspection standard should be written for the wire joint rather than borrowed from a board standard, because the requirements differ. Both belong to the same workmanship system described for joint workmanship.

Records and Qualification

The wire type, the alloy, the flux and the iron setting should be recorded for each assembly type, and the operators should be qualified on a representative joint. A pull test result is a useful part of the qualification record.

These records belong with the process evidence described for manufacturing processes.

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.

FAQ

Should a stranded wire be tinned before soldering? It is common practice, and it makes the joint easier to form while increasing the stiffness of the section.

Why does a wire break next to a joint? Because alloy wicked under the insulation created a rigid section that flexes at the boundary.

Is a crimp better than a solder joint? For a stranded wire in a terminal it usually is, provided the crimp tool and the terminal are correctly specified.

How is the joint verified? Visually for coverage and wicking, and by a pull test on a sample for the mechanical result.