Laser Soldering Guide for Selective Joints

Laser soldering uses a focused beam to heat a single joint. The energy is delivered to a spot a fraction of a millimetre across, and the heat reaches the joint through the solder and the pad rather than through the whole board. It is used where a joint has to be made without heating the surrounding assembly, and it suits applications where the process must be automated and repeatable without a wave or a reflow oven.

How the Energy Is Delivered

A diode laser or a fibre laser produces a beam that is focused by a lens onto the joint. The spot size is set by the optics, and it defines the area that receives the energy and therefore the resolution of the process.

Absorption of the beam depends on the wavelength and on the material. A surface that reflects the wavelength will not heat efficiently, while a surface that absorbs it converts the energy into heat within a shallow depth. Copper reflects much of the near infrared, so the initial heating is slow until the surface begins to oxidise or until a solder deposit is present to absorb the energy.

The beam can be delivered by a fixed head with a moving stage or by a scanner that steers the beam. The scanner is faster because it has no mass to move, but the working area is limited by the field of the lens.

Heating Rate and Thermal Damage

The heating rate is high because the energy is concentrated. A joint can reach the melting point in a fraction of a second, which is an advantage for productivity and a risk for the parts nearby.

Thermal damage is the main concern. The substrate under the pad, the component body and any plastic part close to the joint see a rising temperature that is not measured by the control system, and a joint that is soldered quickly can still be surrounded by material that has been degraded.

The control response is to measure the temperature rather than to set the power. A pyrometer or a thermocouple in a development run establishes the window, and the production setting is the power and the time that produce the target temperature. The two are then verified together.

Solder and Pad Preparation

The joint needs solder. It can be supplied as a paste deposit, as a wire fed into the beam or as a preform placed on the pad. The choice depends on the geometry and on the degree of automation.

Paste is convenient because it also provides flux, and the flux has to be activated by the heating itself, so the ramp and the dwell are set with the flux in mind. A deposit that is too large absorbs the energy at the surface and delays the heating of the pad beneath.

The pad finish and its condition matter more than in a mass process because there is little time for the flux to clean a contaminated surface. A finish that has oxidised in storage will reflect the beam and resist wetting, and the process will appear to be unstable when the real cause is the board.

Where It Fits in Production

Laser soldering suits a small number of joints on an expensive assembly, where a reflow or a wave would heat parts that cannot tolerate it. It also suits a joint in a location that no other process can reach, such as a pad beside a heat sensitive connector or under a shield.

It is used for a wire that has to be attached to a board, for a shield can that is soldered to a frame and for a repair on an assembly that cannot be reflowed again. In each case the value comes from being able to deliver heat locally.

It is not a replacement for a mass soldering process. The throughput per joint is low compared with a wave, and the cost per joint is high, so the decision is about capability rather than about speed.

Automation and Programming

The process is easy to automate because the parameters are electrical and the head is fixed or the beam is steered by a program. There is no consumable to wear in the same way as a soldering iron tip, and the result is repeatable once the window is established.

The program has to position the beam, control the power ramp and, where a wire is fed, synchronise the feed with the heating. Vision alignment is used to find the pad, because the spot is smaller than the placement tolerance of the assembly.

The parameters degrade if the optics are dirty. A lens with flux condensation or dust passes less energy, and the result is a joint that is under heated even though the program is unchanged. A cleaning schedule for the optics should be part of the maintenance plan.

Defects and their Causes

The characteristic defect is the cold joint, produced by insufficient total energy. It appears as a joint that has wetted but not formed a proper fillet, and it is often accompanied by a dull surface and a small contact area.

The opposite failure is the damaged joint, in which the energy has been sufficient to melt the solder but also to overheat the pad and the substrate. A lifted pad, a blister in the laminate and a damaged component body are all signs of too much power or too long a dwell.

Solder balls appear when the paste is heated too quickly, so the flux volatilises before the solder melts. The mechanism is the same as in a fast reflow ramp, and the remedy is the same: a slower initial heating so that the flux has time to work.

Verification and Records

The verification of a laser soldering process is done by measuring the joint. A pull test, a cross section and a temperature measurement in a development run establish the window, and a periodic check confirms that it has not moved.

The production record should contain the power, the dwell, the spot size, the solder and the flux, because those are the variables that a change of product would otherwise leave undocumented.

Where the process is used for a safety related joint, the record should also include the verification method and the frequency, so that the evidence trail exists for an audit as well as for an internal investigation.

Practical Rules

Establish the temperature window with a measurement, control the energy rather than the appearance, and keep the optics clean. Match the solder form to the geometry and check the pad finish before blaming the program.

Record the parameters and the verification with the build records and the defect history, and review the selective soldering data and the jetting process when a local joint has to be added after reflow.

FAQ

Why is copper hard to heat with a laser? Copper reflects much of the near infrared energy. Heating improves once the surface begins to oxidise or once a solder deposit absorbs the beam.

What is the main risk of laser soldering? Thermal damage to the material around the joint. The heating is fast and local, so the substrate and nearby parts can be degraded without the joint itself showing a fault.

Is laser soldering a replacement for wave soldering? No. It is used for a small number of joints that no mass process can reach or that cannot tolerate the heat, not for volume production.