Solder Preforms and Step Soldering for Thermal Budgets

Solder preforms are pieces of solder formed into a defined shape and volume, used where the joint needs more material than a paste deposit can provide or where the alloy has to melt at a temperature the rest of the assembly cannot tolerate. They are common in power electronics, in hermetic packages and in assemblies that are soldered in more than one pass.

Why Preforms Exist

A paste deposit is limited by the stencil aperture and the print area, so a joint that has to fill a large gap or form a heavy fillet needs material that does not come from a print. A preform delivers a known volume of alloy exactly where it is needed.

Preforms also solve a thermal problem. Where an assembly contains a component that cannot be reheated to the melting point of the main solder, a preform with a lower melting alloy allows a second, cooler process to be run afterwards without disturbing the joints made in the first pass.

How a Preform Is Made

The material is cast or rolled into strip and then stamped, punched or cut into the required shape, and the forming operation leaves a defined edge quality. A preform can also be produced by a wire forming process for rings and washers, which is the usual route for a joint that has to surround a pin.

Preforms are supplied with or without flux, and the choice changes the placement process. A flux coated preform is easier to handle because it stays where it is put, while a bare preform needs a separate flux application and often a fixture to hold it during heating.

Alloy Choice and Melting Range

The alloy is selected for its melting range rather than for its strength, because the sequence of operations depends on the temperature at which it flows. A high melting alloy is used for the first pass and a lower melting one for the second, and the gap between them has to be wide enough to be practical.

Standard tin lead and lead free alloys cover most applications, and specialty alloys are used where the thermal window is very narrow. The alloy also determines the mechanical properties of the finished joint and its behaviour under thermal cycling, so the choice is not purely a process decision. Our solderability test notes describe how wetting is verified.

Step Soldering and Process Order

Step soldering means completing the assembly in a series of soldering operations, each with an alloy that melts below the previous one. The first operation uses the highest melting alloy, and each subsequent operation has to be cool enough that it does not remelt the joints already made.

The order is usually fixed by the components rather than by preference. A part that can only be soldered by hand is placed after the reflow stages, and a package that cannot be exposed to the peak temperature of the main process is placed in a later, cooler step.

Preform Size, Shape and Volume

The preform has to contain the right amount of solder for the finished joint, allowing for the material that will be consumed in forming the intermetallic layer and for any that flows away from the joint. The volume required is calculated from the joint geometry and then confirmed on a sample section.

Shape follows the joint. A washer suits a pin in a hole, a rectangle suits a lap joint between two flat surfaces, and a sphere suits a joint that has to fill a cavity. Wrongly sized preforms show up either as an incomplete fillet or as excess solder that bridges to a neighbouring pad.

Flux, Placement and Fixturing

The flux has to be active at the temperature the preform melts at, and it has to be capable of removing the oxide from both surfaces in the time available. A flux designed for a reflow profile may be exhausted before the preform reaches its melting point in a slower heating process.

Placement is a handling problem. Small preforms are difficult to place by hand at production rates, so they are often supplied on tape, in a pocketed tray or attached to the component. Where the preform has to stay in position while the assembly is moved, a small amount of adhesive or a mechanical feature is used to hold it.

Thermal Budget and Component Limits

The thermal budget is the total heat exposure the assembly can accept, and it is normally set by the most sensitive component rather than by the board. Every soldering step adds to that budget, so a design that needs three passes has to be planned around the part with the least tolerance.

Heating rate matters alongside peak temperature. A slow ramp gives the flux time to work and the assembly time to reach an even temperature, while a fast ramp risks thermal shock and a cold joint at the far end of a large part. Our thermal management notes describe how the heat spreads.

Inspection and Common Defects

The common defects are an incomplete fillet where the preform volume was too small, a cold joint where the heat did not reach the joint, and solder balls left where a coated preform shed its flux coating. Each has a visible signature under magnification.

Where the joint is hidden, x-ray or sectioning is required, and the acceptance criteria have to describe the void level and the fillet shape rather than simply stating that the joint is present. Our solder defect notes describe how those criteria are applied.

Specifying a Preform

The specification should state the alloy, the dimensions with tolerances, the flux type and coating weight, and the packaging format. It should also state the cleanliness requirement, because a preform with a heavy flux residue may need washing that the rest of the assembly does not.

The joint requirement itself should be written in terms of the finished fillet and the void allowance rather than only the preform. Our surface finish notes describe how the pads the preform wets onto are prepared.

Process Control and Verification

On a design of this kind, alloy is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

FAQ

Can a preform replace solder paste? For a single large joint it can, and it often gives a better controlled volume. For a fine pitch component the paste print is still the only practical method, and preforms are used only for the special joints.

How is the right preform size calculated? From the volume of the finished joint, including the fillet, minus the contribution of any paste already printed there. The calculation is then confirmed by sectioning a sample joint and measuring the result.

How does gopcb support preform assembly? We agree the alloy sequence and the thermal budget with the customer, confirm the preform volume against the joint geometry, control flux and placement, and verify the result by section and x-ray where the specification requires it.