Laser Soldering: Spot Size and Heat Input Control

Laser soldering delivers energy to a joint as a focused beam rather than by conduction from a hot tip. The heat is absorbed at the surface where the beam lands, and it spreads into the joint by conduction from there. That makes the process fast and highly local, and it also makes it sensitive to the optical properties of the surfaces it meets, because a reflecting pad absorbs far less energy than a dark one.

Where Laser Soldering Is Used

The process suits joints that cannot be reached by an iron or exposed to a wave: fine-pitch connectors on a densely populated board, joints beside heat-sensitive parts, and assemblies where the surrounding material must not be heated. It is also used where the joint count is low and the cost of a dedicated tooling set for selective soldering is not justified.

It is used on heat-sensitive substrates for the same reason. Where the laminate or the component cannot tolerate the thermal mass of an iron, a beam that heats only the joint and its immediate surroundings is the practical alternative to hand soldering.

Absorption and Spot Size

The fraction of the beam energy that is absorbed depends on the wavelength of the laser and on the surface. A shiny copper or gold surface reflects a large part of the energy, while an oxidised or flux-covered surface absorbs much more. This is why the flux application and the beam delivery are part of the same design decision.

Spot size sets the energy density and the area heated at once. A small spot concentrates energy on the joint and reaches temperature quickly but demands precise positioning; a larger spot is more tolerant of position but spreads heat into the laminate and the neighbouring parts. Spot sizes between 0.3 mm and 1.5 mm cover most work, with the smaller end used for fine-pitch joints.

Power Density and Heat Input

Power density is the laser power divided by the spot area, and it is the figure that determines whether the joint melts in the intended time. Too low and the process takes longer than the flux can survive, so the flux is consumed before wetting completes. Too high and the solder vaporises at the surface, producing spatter and a crater.

Heat input is the product of power and time, and the two are partly interchangeable. A lower power for a longer time gives a gentler ramp and a wider process window, at the cost of cycle time and of more heat conducted into the surroundings. Most programs use the shortest time that gives a consistent joint, because the surrounding material benefits from the shorter exposure.

Solder Wire Feed and Placement

Solder is usually supplied as wire fed into the joint, and the feed has to arrive at the right moment. Feeding before the joint has reached temperature pushes cold wire against the pad; feeding after the beam stops leaves unmelted wire resting on a solid joint. The delay between the beam starting and the wire advancing is a programmed parameter.

Wire placement matters as much as timing. The wire should be presented so that it meets the hot surface rather than the beam path, since a wire in the beam scatters and reflects energy and produces an inconsistent result. Fixed nozzles with a defined approach angle are more repeatable than freehand placement.

Thermal Damage Limits

The advantage of a laser is that the heat is local, but the limits are set by what the beam touches. Solder mask discolours and can lift if the spot overlaps the mask edge; plastic connector bodies soften; and the laminate under a pad can be damaged if the absorbed energy stays in one place too long.

The limits are managed by the spot size, the dwell and the position of the beam relative to the mask. Where the joint is close to a plastic body, the beam should be aimed away from it and the dwell shortened, accepting a slightly lower peak temperature in exchange for protecting the component. The standard controls for cold joints and for the thermal limits of nearby parts apply here as much as in iron soldering.

Joint Formation and Inspection

A laser joint forms quickly, and the fillet it produces is often smaller than one made with an iron because the heat is applied over a smaller area. The joint should still show wetting on both the pad and the pin, with a concave fillet and no evidence of spatter or of unmelted wire.

Inspection is visual with magnification, supplemented by a pull test on a sample. Because the process is programmed rather than operator-controlled, the results are consistent enough that the first article and the periodic sample carry most of the inspection load, provided the program is locked.

Process Window and Programming

The window is defined by the beam power, the spot size, the dwell, the wire feed delay and the wire length. Each is programmed per joint type, and joints with different thermal mass need different settings in the same way they do for robotic iron soldering.

Programming should start from a first article measured against the joint requirements rather than from a machine default. Where the process has been transferred from another product, the power and dwell have to be adjusted for the reflective properties of the finishes used, which is why a program is not portable between products without a trial.

Comparison With Iron and Selective Soldering

An iron is simple, cheap and insensitive to surface finish, but it applies heat through contact and needs a tip that can reach the joint. The selective soldering process applies heat through a small solder wave, which brings its own thermal mass and needs a pallet to protect the rest of the board. A laser applies neither contact nor mass.

The cost is in the optics and the programming, and in the sensitivity to surface condition. Where the joints are accessible and the thermal budget allows, the simpler processes are easier to control; where access is poor and the surroundings are sensitive, the laser is the process that can reach the joint without damaging everything else.

Verification and Records

The record should carry the laser power, the spot size, the dwell, the wire feed length and delay, and the measured joint result from the first article. Where a solderability problem is suspected on a particular board finish, a wetting test on a sample from the same lot is more informative than an adjustment to the program.

Process drift in a laser system usually comes from the optics or from the beam delivery rather than from the power supply. Window contamination, a damaged fibre or a misaligned focus change the spot size and therefore the power density, so those items should be checked whenever the joint result changes without a change in the program.

FAQ

What spot size should laser soldering use? Between roughly 0.3 mm and 1.5 mm for most work, with the smaller end for fine-pitch joints and the larger end where positioning tolerance is more important than locality of heating.

Why does the same program give different results on different boards? Because absorption depends on the surface. A shiny finish reflects more of the beam than an oxidised one, so the energy actually delivered to the joint changes with the finish.

Does laser soldering damage the laminate? It can, if the beam overlaps the mask or the dwell is too long. The damage is managed through spot size and beam position rather than through the power alone.