Induction Soldering for Localised Heating: 7 Process Rules

Induction soldering heats the metal itself rather than a tool that then touches the metal. An alternating current in a coil produces a changing magnetic field, the field induces eddy currents in the workpiece, and the resistance of the workpiece converts those currents into heat. The joint comes up to temperature from the inside, and the surrounding assembly sees far less heat than it would from a conduction tool.

The process is used where heat has to be delivered to one place and kept away from everywhere else: a connector shell soldered to a board, a shield attached to a plane, a heavy terminal next to a plastic housing or an electrolytic capacitor. It is a controlled process, but it is only controlled if the coil, the power and the timing are treated as a system.

How the Heat Gets Into the Joint

The depth to which the induced current penetrates depends on the frequency and on the conductivity and permeability of the metal. At the frequencies used in hand and bench tools the current is concentrated near the surface, which is why the process heats the outside of a heavy part quickly and the inside slowly. That is convenient for a joint on the surface of a terminal and awkward for a joint buried in the middle of a thick metal body.

Coupling distance matters as much as frequency. The field strength falls rapidly with distance from the coil, so a coil held two millimetres further away may deliver a fraction of the energy. Position therefore has to be repeatable, which means a fixture or a stop rather than an operator judgement on every joint.

Where Localised Heating Wins

The advantage of the process is confinement. A conduction tool has to bring heat through the joint, and with a heavy part that means a long contact time and a large heat affected zone. Induction energy is generated within the joint, so the ramp is fast and the total heat delivered into the assembly can be much lower.

That is what makes the process suitable for a connector soldered to a ground plane, for a shield can joining a metal chassis to a board, and for a through hole terminal soldered next to temperature sensitive components. Where the neighbouring part is plastic, a solvent sensitive device or a component with a limited reflow rating, confinement is not a convenience, it is the requirement.

Coil Design and Coupling

The coil is the tool, and its shape decides the shape of the heated zone. A single turn around a terminal concentrates the field; a hairpin coil concentrates it along a line; a flat spiral spreads it over an area. Where the field has to be focused further, a magnetic concentrator can be added, and where the part is small the coil legs have to be brought close together without touching the work.

Coils are consumable to a degree. They deform, their insulation degrades and their geometry changes, and a coil that has been bent to reach around an obstacle will not couple the same way as a new one. The coil should be dedicated to the part, identified, and replaced when it is damaged rather than reshaped on the bench.

The Alloy and the Curie Point

Some induction systems use a self regulating alloy that becomes non magnetic above a characteristic temperature, so that it stops absorbing energy once it reaches the intended level. That behaviour gives a natural ceiling and removes the risk of an operator overheating a joint that heats very quickly.

The more common approach is a power setting and a timer, with the alloy selected for the temperature the joint needs. A tin based alloy with a modest melting range behaves predictably, while a higher temperature alloy shortens the window between wetting and damage. As with any soldering process, the alloy and the flux have to be paired, which is the point of the alloy and flux pairing review.

Dwell, Power and Ramp

Power controls how fast the joint heats, and dwell controls how long the heat is applied. A high power setting with a very short dwell heats the surface and leaves the interface cool, which produces a joint that looks soldered and behaves like a cold joint. A lower power setting with a slightly longer dwell lets the heat soak through the joint and is easier to repeat.

The dwell should be set from a trial on the real parts and then fixed. Where the joint fills slowly, the correct answer is usually more dwell or better coupling, not more power, because raising the power overheats the surface while the interface is still cold.

Fixtures and Repeatability

Because the process depends on coupling distance, a fixture that positions the coil relative to the joint is worth more than any amount of operator skill. The fixture must be non conductive and non magnetic, so that it does not heat itself and does not distort the field, and it must not be so close to the coil that it absorbs energy.

Work holding is also a safety matter. The field will heat any metal object near the coil, including a steel tweezer, a tool resting on the bench and a watch. A defined work area, a non metallic support and a clear rule about what may be placed near the coil prevent a burn as well as a defect.

Inspection and Verification

Visual inspection confirms the fillet and the wetting angle, and it will show an overheated joint by its colour and surface texture. It cannot confirm that the interface reached temperature, because induction heating is fast enough to produce a good looking surface over an unwetted joint.

For a critical joint, the process should be qualified by section, and the thermal path confirmed with a thermocouple or a temperature indicating material on the first article. Once the process is fixed, monitoring the power and dwell settings and checking the coil condition is enough to keep it inside the qualified window. Through hole joints with a heavy thermal path under load are worth reviewing against the thermal reliability data for the assembly.

Safety and Records

Induction tools are quiet and the field is invisible, which makes them easy to underestimate. The work area should be clear of metal, the coil should be inspected before use, and the power supply should be interlocked so that it cannot be energised without the coil connected. Where the process runs in a cell, the same lockout discipline applies as to any other machine.

The record should link the part to the coil, the power setting and the dwell, and it should be reviewed whenever the coil, the alloy or the connector supplier changes. Alternative localised methods exist and should be compared at design stage, including the approach described in the laser soldering notes, because the right choice depends on the joint rather than on the tool that happens to be available.

FAQ

Can induction soldering be used on any joint? No. It works best where the metal in the joint couples well with the coil, which means a metal terminal, a connector shell or a plated barrel. A joint between two small non magnetic parts is difficult to heat this way.

Why does the joint look bright but fail a pull test? Because the surface can reach soldering temperature while the interface is still below it. The heat is generated in the skin of the part, so the setting has to allow time for the heat to reach the interface, and the process should be qualified by section.

How close should the coil be to the work? Close enough to couple efficiently and far enough not to touch or arc. Because the field falls off quickly with distance, the gap should be set by a fixture stop and checked when the coil is changed.