Solder Preform: Design Rules and Process Limits

A solder preform is a piece of solder manufactured to a specific shape and volume: a washer, a disc, a rectangle or a small formed pellet. It is placed in the joint before heating, and it melts to provide exactly the amount of alloy the joint needs. It is used where solder paste cannot deliver enough volume, where the joint is too large for paste to fill reliably, or where the alloy required is not available in paste form.

This article covers what a preform is for, how the alloy and the flux are chosen, how it is placed, and how voids are managed.

What A Preform Is For

A preform is a volume delivery device. A paste deposit is a thin layer whose volume depends on the print and on the reflow, while a preform is a solid object whose volume is known to a tight tolerance. Where a joint has to be filled completely, such as a large thermal pad under a power device or a hole that must be filled with solder, the preform is the only way to guarantee the amount of alloy.

The second reason to use one is the alloy. Some alloys are not manufacturable as paste, either because they cannot be made into powder or because the flux chemistry required is not available. Gold tin, some high lead alloys and several high temperature alloys fall into this category, and the only practical way to apply them is as a preform. Very high melting alloys are also used as a spacer or a stand-off rather than as a joining medium, holding a defined gap while a lower melting alloy does the joining. The metallurgy of the common alternatives is described under lead free versus leaded solder.

Choosing The Alloy

The alloy is chosen from the melting range, the mechanical properties required and the compatibility with the surfaces and with any existing solder on the board. A eutectic alloy melts at a single temperature, which makes the process easier to control, while a non eutectic alloy melts over a range and passes through a pasty stage during which the joint is not fully liquid. That pasty stage can be useful for a fillet that has to form gradually and it is a complication for a joint that must freeze quickly.

The mechanical requirement matters for a large joint. A joint that has to carry a thermal load or a mechanical load through a large area needs an alloy with adequate strength at the service temperature, and a joint that will see thermal cycling needs an alloy whose fatigue behaviour suits the applied strain. The current carrying requirement of the joint is set by the same geometry that carries it in copper, described under trace width and current calculation, because the solder itself is part of the conductor in a large joint.

Flux Coating And Placement

A preform is usually supplied with a flux coating, because a bare piece of solder oxidises and will not wet reliably. The coating is applied uniformly and its weight is specified, since the amount of flux determines how much oxide can be removed and how much residue is left. Where the flux is not sufficient, additional flux is applied to the joint; where the coating is heavy, the residue has to be cleaned.

Placement is normally by hand or by a pick and place machine with a custom nozzle. The preform has to sit flat on the surface it will wet, because a gap under it fills with flux and the flux has to escape before the alloy melts. Where the preform surrounds a hole or a pin, its internal and external dimensions are matched to the feature with a clearance that allows placement but does not allow it to move. The placement sequence and the way components and deposits are arranged around the feature follow the same logic as via in pad or plated through.

Voiding In A Preform Joint

Voiding is the main quality issue in a preform joint, and it comes from the flux rather than from the alloy. As the preform melts, the flux under it vaporises, and the vapour has to leave through the molten alloy. In a large joint the distance the vapour has to travel is long, so a significant fraction is trapped. The result is a joint with a large spread of voids rather than the small scattered voids typical of a paste joint.

Three measures reduce it. The first is to use a preform with a non volatile flux or with a controlled coating weight, so that there is less to vaporise. The second is to design the joint so that the vapour has a route out, which usually means not enclosing the preform completely and allowing a vent at one edge. The third is a slower ramp through the melting range, which gives the vapour time to escape before the alloy becomes fully liquid and closes the path. The acceptance criteria and the measurement are the same as for any other joint.

Joints That Need A Preform

The classic application is a large power device soldered to a thermal pad, where the joint has to fill an area of several square centimetres without voids that would block the thermal path. A second is a connector with a large pin that must be soldered into a hole whose volume is greater than the paste can fill. A third is a radio frequency or microwave assembly, where a controlled alloy is used to attach a component with a defined thermal expansion match.

Preforms are also used to repair. Where a joint has been removed and the pad is short of solder, a small preform provides the volume that the remaining solder lacks, and it does so without the flux load of a large paste deposit. The disadvantage of every preform application is the handling: each piece has to be placed individually, which limits the throughput and makes the process labour intensive unless the volume justifies a feeder.

Process And Inspection

The process specification for a preform joint states the alloy, the preform dimensions and tolerance, the flux coating weight, the placement accuracy, and the profile including the ramp through the melting range. The profile should be verified with a thermocouple on the joint itself, because a large joint heats more slowly than the surrounding board and a profile that is correct elsewhere may leave the preform unmelted at its centre.

Inspection is by X-ray for the void content and by section for the wetting and the fillet. A section shows whether the alloy has filled the joint and whether the fillet has formed at the edges; a joint that has melted but not filled is visible as a gap between the alloy and the pad. Where the joint carries a thermal load, the void distribution matters more than the total, because a void in the centre of the joint blocks the path more effectively than one at the edge.

FAQ

Can a preform be used with solder paste? It can, and the combination is common: the paste holds the preform in place and provides the flux, while the preform provides the volume. The paste volume is then reduced so that the total is correct.

Why does a preform joint have more voids than a paste joint? Because the flux under the preform has to escape through a longer path in a thicker joint. The measures are a low volatility flux coating, a vent path and a slower ramp through the melting range.

Can preforms be placed by machine? Yes, where the volume justifies a feeder and a nozzle, and where the preform is rigid enough to be handled. Small or fragile preforms are placed by hand, which limits the throughput.