Laser Soldering for Sensitive Assemblies
The Idea Behind Laser Soldering
Laser soldering delivers energy to a single joint with a focused beam, so the heat is applied where it is needed and the rest of the assembly stays cool. The advantage is not speed, because the process is usually slower than a wave or a reflow, but thermal isolation: a joint can be made next to a component that cannot tolerate a board level thermal cycle, such as an already populated module, a battery, a connector with a plastic body or a sensor with a low temperature limit. Where the thermal budget has already been spent, the laser is sometimes the only way to make the joint.
How the Energy Is Absorbed
The beam’s energy is absorbed by the surfaces it hits, and the absorption depends on the wavelength and the material. A copper pad reflects much of a near infrared beam, while solder and a darkened surface absorb more. The practical consequence is that the process usually relies on the solder absorbing the energy rather than the copper, and that the paste or the preform’s condition affects the result. A shiny surface may need a higher power or a longer dwell, and a surface that has been contaminated may absorb differently from one that has not, which is one reason the process needs a stable surface condition.
Process Control
The parameters are the power, the spot size, the dwell and the beam’s position, and they interact with the joint’s geometry and the surrounding copper. A large copper plane conducts the heat away and needs more power or a longer dwell; a small pad reaches the temperature quickly and needs less. Because the thermal path varies with each joint, the parameters should be established per joint type rather than set once for the product. A pyrometer or a camera can monitor the joint’s temperature or its appearance during the pulse, which closes the loop and makes the process more repeatable than a fixed power and time.
The Joint and Its Materials
The joint is made with a paste, a preform or a solder coated pad, and its volume has to be controlled because the laser melts what is present without adding more. A joint with too little solder will not form a fillet, and one with too much will not reach temperature uniformly, since the energy has to melt the whole volume. The flux’s behaviour matters as well, because a flux that has to activate over a longer profile may not have time in a short laser pulse, and the residue left by an unactivated flux is a cleanliness risk. The process therefore suits a solder system whose flux activates quickly.
Where It Fits and Where It Does Not
Laser soldering fits a low volume, a high value assembly, a repair or a rework, and a product where the thermal budget is exhausted. It does not fit a high volume board with many joints, where a wave or a reflow is faster and cheaper by orders of magnitude, and it does not fit a joint whose solder volume cannot be controlled, because the process cannot add solder. It is also slow where a large area has to be heated, since the energy is delivered through a small spot. The decision should follow the thermal constraint and the volume rather than the technology’s novelty.
Verification and Reliability
A laser soldered joint looks different from a reflowed one, because the thermal profile is shorter and the grain structure is different, so the acceptance criteria should be developed for the process rather than inherited. The joint’s quality is verified by its appearance, by a cross section on a sample and by an electrical test. The reliability should be confirmed by thermal cycling where the product requires it, because a joint made with a short profile may have a different fatigue behaviour from one made in a reflow oven. Where the process is used for a repair, the verification is the same as for a production joint, since the joint does not know why it was made.
Safety and Equipment
The laser is a hazard, and the equipment must be enclosed, interlocked and operated by trained personnel with the appropriate eye protection. The beam’s reflection from a shiny surface is a hazard in itself, so the enclosure is part of the process rather than an accessory. The equipment’s calibration, including the power and the spot size, should be verified periodically, because a change in either changes the joint. The safety requirements are not optional and they affect the layout of the cell, which is worth considering before the process is installed rather than after.
Throughput and Integration
The laser process is usually integrated into a cell with a handling system, and the cycle time is dominated by the positioning and the dwell rather than by the beam itself. Where the joint count is low and the positions are accessible, the process can be automated to a reasonable rate; where the joints are scattered and the access is poor, the positioning time dominates and the process becomes impractical for anything but a sample or a repair. The integration should therefore be assessed from the joint count, the access and the required rate before the equipment is chosen, because a process that cannot reach the required throughput will not be used however good its joints are.
FAQ
Why use a laser rather than a reflow? To make a joint without heating the whole assembly, where the thermal budget or a temperature sensitive part prevents a reflow.
Is laser soldering fast? No, it is usually slower per joint; its advantage is thermal isolation rather than speed.
Why does the surface condition matter? The beam’s absorption depends on the surface, so a shiny or contaminated pad behaves differently.
Why set the parameters per joint type? The thermal path varies with the copper around each joint, so a single setting does not suit all of them.
What is the main safety issue? The beam and its reflection, which is why the equipment is enclosed and interlocked.
Conclusion
Laser soldering trades speed for thermal isolation, so use it where the thermal budget demands it and control the parameters per joint. Develop the acceptance criteria for the process. Selective heating belongs to SMT PCB assembly, the assembly it joins sits in PCB assembly, and the verification is part of PCBA testing. Laser processes are first qualified during prototype PCB assembly in 2026.
