Solder Preform Selection And Use

A solder preform is a piece of solid alloy shaped to fit a particular joint. It is used where the paste cannot supply enough metal, where the joint geometry cannot be stencilled, or where a sequence of soldering steps requires an alloy that will not remelt when the next step is performed. It is one of the oldest techniques in electronics assembly and it remains the practical answer for a set of problems that paste alone cannot solve.

This article explains when a preform is the right choice, what properties are specified, how the volume is calculated, and how the process is controlled.

When A Preform Is Used

The first reason is volume. A paste deposit loses about half its volume as the flux volatilises, so a joint that needs a large amount of metal requires a deposit that a stencil cannot print, particularly in a through hole where the barrel itself has to be filled. A preform supplies that metal as solid alloy and reduces the demand on the paste.

The second reason is geometry. A joint between a wire and a terminal, a shield attached to a castellation, a busbar bolted and soldered to a board, and a step in a housing that has to be sealed are all cases where no stencil aperture can be placed. The third reason is sequence: in a step soldering process a high melting point preform makes the first joint, and the later steps use lower melting alloys that do not disturb it.

Specifying A Preform

The alloy sets the melting point and therefore the position of the preform in the temperature hierarchy. The form is chosen from the joint: a disc or a washer for a cylindrical joint, a ring for a circular seam, a rectangle or a custom shape for a linear joint, and a sphere where a controlled volume is needed at a point. The dimensional tolerance matters because the volume is what the joint depends on, and both the thickness and the flatness of a flat preform are specified.

The flux is the second specification. A flux coated preform carries a defined quantity of flux on its surface, which simplifies the process and makes the joint repeatable, while a bare preform requires an external flux or a reducing atmosphere such as formic acid or hydrogen. The coating also affects the storage life and the handling, because the flux can absorb moisture and the preform can oxidise, so the packaging and the shelf life are part of the specification.

Calculating And Placing The Volume

The volume calculation starts from the joint. A cylindrical through hole joint requires the volume of the barrel plus the fillets on both sides, and an annular joint requires the volume of the ring that will be formed. The figure is then converted into a preform dimension using the density of the alloy, and the result is compared with what the paste can supply so that the two together meet the requirement.

Placement is the practical difficulty, because the preform has to stay where it is put until the solder melts. A tacky flux, a small dot of paste, a mechanical pocket, or a counterbore all provide the retention, and the pad or the hole is usually designed so that surface tension centres the preform as it melts. Where the preform is placed by a machine, a custom nozzle or a tape and reel format is used, and where it is placed by hand the operation is slower and less repeatable.

The Solder Process

The profile has to do three things at once: activate the flux, melt the preform completely, and allow the flux and the oxides to escape so that the joint is not voided. A preform that is only partly melted produces a joint with an unmelted core, which appears in a cross section as a distinct boundary within the solder and behaves as a crack. A preform that melts but cannot vent its flux produces a large void at the centre of the joint.

The atmosphere can replace the flux entirely. A bare preform processed in a reducing atmosphere, such as formic acid vapour or hydrogen, is cleaned by the gas rather than by a chemical flux, which removes the residue problem and suits a hermetic or a high reliability assembly. The trade is the equipment and the safety requirement, and the process window is narrower than with a flux.

Step Soldering

The hierarchy is built from the melting points. A first joint with a high melting point alloy, then a second with a lower one, then a third lower still, allows a complex assembly to be built without remelting what has already been made. The rule is that each subsequent process must stay below the solidus of the previous joint with a margin, and the margin has to allow for the tolerance on the oven, on the thermal mass, and on the alloy itself.

A typical ladder uses a gold tin or a high lead alloy for the first step, a tin antimony or a high melting tin silver alloy for the second, a standard tin silver copper alloy for the third, and a bismuth bearing alloy for the last. The components have to survive every step, and a component that is attached at the first step sees all the subsequent profiles, which is a constraint that is easy to overlook when the assembly is planned.

Defects And Verification

An incomplete melt, a void, a shifted preform, and an oxide inclusion are the defects to watch for. The first two are visible in a cross section and in an X-ray image, the third appears as an asymmetric fillet in a visual inspection, and the fourth appears as a dark line within the joint in a microsection. The joint strength is verified by a shear or a pull test on a sample, and the process is verified by measuring the temperature of the actual preform with a thermocouple during a profile run.

Where the joint has to be repeated many times, the value of the preform is the repeatability of its volume. A paste deposit varies with the stencil, the paste and the printer, while a preform is a manufactured part with a tolerance, so the volume is known before the process starts. The alloy context is described under lead free and leaded soldering, the case where a preform supplements a paste deposit under via in pad or plated through, and the fabrication context under PCB design and fabrication.

Additional Considerations for This Build

Practical attention to solder volume 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 volume explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

FAQ

Why use a preform instead of more paste? Because paste loses roughly half its volume as the flux is driven off, so the deposit needed to fill a large joint cannot be printed. A preform supplies the metal as solid alloy.

What holds the preform in place before reflow? A tacky flux, a dot of paste, a mechanical pocket, or a counterbore, and the pad is usually designed so that surface tension centres the part as it melts.

How is a step soldering ladder built? From the melting points, with each step performed below the solidus of the previous joint by a margin that covers the oven tolerance, the thermal mass, and the alloy tolerance.