Solder Preform: Preparation, Placement and Process Control

A solder preform is a piece of solder that has been cast or stamped into a shape and placed in a joint before heating. Its purpose is to deliver a defined volume of alloy to a joint that paste or wire cannot fill, either because the joint is large, because the geometry is awkward or because the alloy is one that cannot be made into a paste. The design of the preform is a volume calculation with mechanical constraints attached.

When a Preform Is the Right Tool

The first case is a joint that needs more solder than a paste deposit can supply. A large lug, a heavy terminal or a braided connection can require a volume that would need an impractical number of paste deposits.

The second case is an alloy that is not available as a paste. Some high temperature alloys, some gold containing alloys and some indium alloys are supplied as preforms rather than as powder, and the joint has to be designed around the available form.

The third case is a joint with an awkward geometry, such as a gap between two surfaces that must be filled from the side, or a joint that is made in a fixture where paste cannot be printed. In each case the preform is placed by hand or by a feeder rather than printed.

Sizing the Volume

The volume is calculated from the joint. The starting point is the volume of the gap between the two surfaces, plus the volume of the fillet that the design requires, minus any solder that will be displaced or wicked away.

The calculation should include the expansion of the parts during heating. A metal that expands more than the solder will open the gap and draw solder into it, and the preform that was sized at room temperature will be short at reflow temperature.

A useful rule is that the preform should provide about the volume of the joint plus a small surplus, and that the surplus should be visible as a fillet after reflow. A preform that leaves no fillet was sized at or below the minimum, and the joint will vary with the gap tolerance.

Shape and Placement

The shape is chosen so that the preform stays where it is placed. A flat washer suits a circular joint, a ring suits a pin, a rectangular strip suits a linear joint and a sphere suits a hole that has to be filled from above.

A preform that can move during heating will not be where the designer intended when the solder melts. The shape should include a feature that locates it, such as a hole that accepts a pin, a form that sits against a shoulder or a tapered section that wedges into the joint.

Placement can be manual, with a feeder or with a pick and place machine using a nozzle. Manual placement is slow and variable, so the preform should be designed so that it can only be placed one way, which removes an error mode entirely.

Flux and the Preform Coating

A preform can be supplied bare, with a flux coating or as a cored shape. The flux coating is convenient because it puts the flux exactly where the joint is, and it removes the need for a separate flux application.

A coated preform has a shelf life and it must be stored in a way that preserves the coating. A coating that has absorbed moisture or that has been touched with bare fingers will not perform as specified, and the defect appears as a joint that did not wet in one area.

Where a separate flux is used, the flux has to be applied before the preform is placed and it must remain active through the heating cycle. A flux that has evaporated by the time the solder melts leaves exactly the same result as no flux at all.

Heating Methods

A preform can be heated in a reflow oven, with a hot plate, with an induction coil, with a torch or with a resistance heater, and the choice depends on the assembly rather than on the preform.

Induction heating is common for a local joint because the heat is generated in the parts rather than conducted from outside. The heating is fast, and the preform melts when the parts reach temperature, which requires a control loop rather than a fixed time.

Whatever the method, the heating rate has to allow the flux to activate before the solder melts. A preform that is brought to the melting point in a second will not have had the flux working for long enough, and a cold or partial joint is the result.

Joint Design Around a Preform

The joint gap should be controlled. A gap that is too small restricts the flow and can trap flux gas, while a gap that is too large requires a large preform and produces a joint with a low mechanical strength. The usual target is a gap that places the joint between about 0.05 and 0.2 mm.

The metal surfaces have to be solderable. A preform cannot compensate for a surface that has oxidised, and a nickel plated terminal or a stainless steel part may need a specific flux and alloy combination.

The design should provide a path for the flux gas to escape. A closed joint with a preform inside will trap the gas and produce a void, and a small vent or a chamfer gives the gas somewhere to go.

Verification and Quality

Verification of a preform joint starts with the volume. A section of a sample joint shows whether the gap was filled and whether the fillet is present, and it shows the voids that a visual check cannot see.

A pull test or a shear test on a sample gives a number that can be compared with the design requirement and with previous builds. The value is less informative than the failure mode: a joint that pulls out of the bore is different from one that fractures in the solder.

In production, the checks are the preform placement, the presence of the flux and the temperature reached at the joint. The last of these is measured with a thermocouple on the first article rather than assumed from the equipment setting.

Practical Rules

Size the preform from the joint volume with a surplus for the fillet, choose a shape that locates itself, and control the gap and the surface condition. Let the flux activate before the solder melts.

Record the alloy, the shape and the heating parameters with the build records and the defect history, and review the alloy selection and the joint reliability data when a preform joint is qualified.

FAQ

When is a preform better than paste? When the joint needs more solder than a deposit can supply, when the alloy is not available as paste, or when the geometry makes printing impossible.

Why does the preform need a locating feature? A preform that can move before the solder melts will not be where it was placed. A hole, a shoulder or a taper holds it in position.

What gap should a preform joint have? Usually between about 0.05 and 0.2 mm. A smaller gap restricts the flow and traps gas, while a larger one needs a big preform and weakens the joint.