Laser Solder Ball Bonding for Flex Circuit Assembly

Soldering a flex circuit with a conventional iron or with a hot air nozzle is a compromise. The iron contacts the joint and conducts heat into the polyimide, and the air nozzle heats everything within reach. Both methods put more energy into the substrate than the joint needs, and on a thin flex that energy shows up as a distorted coverlay, a delaminated pad or a cracked conductor next to the joint.

Laser solder ball bonding removes most of that excess. A small solder sphere is placed on the joint, a laser pulse delivers a defined amount of energy to the sphere alone, and the joint forms in milliseconds. The surrounding material sees far less heat, which is why the technique has become standard for flex assemblies that connect to cameras, displays and sensors.

Why the Heat Affected Zone Matters

Polyimide is a good thermal insulator, so heat applied to a pad does not spread away as quickly as it does on a ceramic or on a metal core. The coverlay that protects the conductor is an adhesive bonded film with a softening point well below the melting point of solder, and it is only tens of micrometres thick. Local heating above that softening point lets the coverlay lift and the adhesive flow, which changes the mechanical support of the conductor and can expose it to the environment.

The conductor itself is also at risk. Rolled annealed copper recrystallises when it is held above a certain temperature, and a recrystallised zone has lower fatigue resistance than the surrounding metal. A joint that is made hot and slowly develops a brittle band next to the pad, and that band is where a crack starts when the cable is bent in service. Controlling the heat affected zone is therefore about fatigue life as much as about appearance.

How the Process Works

The sequence begins with flux. A small amount of flux is applied to the pad, followed by a solder sphere that is positioned by a nozzle or by a pick and place head. A laser pulse, usually from a diode source at a wavelength that solder absorbs well, is delivered through the same head or through an optical fibre. The sphere melts, wets the pad and the component lead, and solidifies within a few milliseconds.

Because the energy is delivered to the sphere rather than to the assembly, the peak temperature of the substrate is far lower than in reflow. The pulse length, the power and the spot size determine how much energy reaches the joint, and they are set so that the solder reaches its melting point with enough time to wet but without superheating. A pyrometer or a thermal camera watching the joint closing the loop gives repeatable results from unit to unit.

Choosing the Solder Alloy

Tin lead alloys are the easiest to work with because they melt at a low temperature and wet readily, but they are restricted in most consumer products. Lead free alloys melt higher, which narrows the process window, and the common choices are near eutectic tin silver copper or a tin silver alloy with a small addition to improve wetting. The alloy has to be compatible with the pad finish on both sides of the joint.

Sphere diameter is selected from the pad size and the required joint volume. A sphere that is too small does not produce a fillet and leaves a joint that is vulnerable to thermal cycling, while one that is too large spreads beyond the pad and can bridge. A useful starting point is a sphere whose diameter is roughly the pad width, giving a joint height that is a fraction of the diameter after collapse.

Pad and Joint Design

The pad on a flex circuit should be as large as the pitch permits, because it has to carry the sphere and provide a wetting surface. An overhang beyond the polyimide edge is undesirable, since the unsupported copper conducts heat away poorly and can lift. Where the joint connects to a component, the two surfaces should be parallel so that the molten sphere bridges both without being pushed to one side by the placement force.

Thermal relief is not usually needed on a flex, because the copper is thin and the pad has little mass. What is needed is a defined pad geometry that appears in the same drawing as the rest of the pad design standards used by the fabricator, so that the stencil, the coverlay openings and the assembly process all agree with each other.

Process Control and Inspection

The parameters that need to be recorded are the laser power, the pulse duration, the spot size, the sphere diameter and the placement accuracy. A joint that has been made correctly has a bright, concave fillet and a seated component. Visual inspection under magnification catches gross problems, but the subtle ones, such as a joint that has wetted only one surface, require a cross section or a shear test.

Shear testing a sample of joints from each production lot gives a quantitative measure of joint strength, and the fracture surface shows whether the failure was in the solder or at the interface. Where the assembly will be thermally cycled, the resistance of a daisy chain through the joints is a more useful indicator than a single shear value, since it tracks the whole population of joints on the part.

Where the Process Fits Best

Laser solder ball bonding is most valuable where the substrate is heat sensitive, where the joint is close to a component that cannot be heated, or where the joints are so close together that a hot bar would touch several at once. It is less attractive for large joints that need a substantial volume of solder or for assemblies that can tolerate a conventional reflow.

In practice the technique is often used for a small number of critical joints on an assembly that is otherwise processed conventionally. That hybrid approach keeps the cost of the common joints low while protecting the few that would be damaged by a full reflow, and it is a good example of matching the process to the requirement rather than applying one method to the whole product. Cleaning after bonding still matters, so the residues left by the flux should be assessed against the requirements of the pcb cleaning process used for the rest of the assembly.

Comparing With Hot Bar and Reflow

A hot bar transfers heat through a tool that contacts the joint, so the tool temperature and the pressure both influence the result and both are applied to the surrounding area. Laser bonding delivers energy without contact, which removes the tool wear problem and the pressure variable, and it allows joints to be made on a pitch where a hot bar would touch its neighbours. The trade off is throughput: one joint at a time is slower than a bar that makes a row at once.

Reflow remains the cheapest way to make many joints, and it should be used wherever the substrate can survive it. The point at which it stops being suitable is usually the presence of a component that cannot be heated, such as a liquid crystal display, a microphone with a polymer membrane or a battery connector. Adding selective laser bonding for those joints avoids the cost of a low temperature alloy for the whole assembly and avoids a second reflow cycle, which is itself a reliability risk for the rest of the board.

Additional Considerations for This Build

Practical attention to solder joint pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating solder joint explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Does laser soldering damage the polyimide? It can if the energy is too high or the spot is misaligned, but the total heat input is far lower than with an iron or a hot air nozzle, which is the main reason the technique exists.

Can the process be used for through holes? Not directly, because the sphere sits on a surface. It is used for surface joints on flex and for connectors, where the joint can be formed on a flat pad.

What causes a joint to be weak? Usually insufficient energy so that the solder does not fully wet, or an oxidised pad finish. Both show up in shear testing before they show up in the field.