Solder Preforms: Filling Joints That Paste Cannot
Some joints cannot receive enough solder from paste. The hole is too large, the board is too thick, the component cannot tolerate the reflow temperature, or the joint is being repaired rather than produced. A solder preform is the answer.
What a Preform Is
A preform is a precisely formed piece of solder, made as a washer, a disc, a rectangle or a shaped ribbon. It has a defined volume and a defined alloy, and it is placed by hand or by machine before the joint is heated.
Because the volume is fixed by the shape rather than by a printing process, a preform can deliver far more solder than a paste deposit of the same footprint.
The alloy is chosen independently of the paste, which allows a higher melting point alloy for a high temperature joint or a lower one for a temperature sensitive component.
When a Preform Is the Right Answer
When the volume required exceeds what an aperture can hold. The classical case is a heavy connector on a thick board where the barrel volume is large relative to the pad.
When the joint is inside a cavity or a recess where a stencil cannot reach. Preforms are common in hermetic packages and in assemblies with mechanical features that prevent printing.
When the temperature is limited. A preform of a lower melting point alloy can form a joint at a temperature that the rest of the assembly will tolerate, which is the reason preforms are used in step soldering.
Preform Shape and Placement
A washer sits around a pin and fills the joint from the top. A disc sits over a hole and melts into it. A shaped piece sits in a pocket machined into a component or a connector.
The placement must hold the preform in position until it melts. Tacky flux is the usual method, and the flux must be compatible with the alloy and must be applied in a controlled amount.
Where the assembly is placed upside down during reflow, the preform must be retained mechanically rather than by tack. This is a design constraint that is often discovered late. Our design release checklist notes where the placement method is recorded.
Flux and Atmosphere
A preform has more surface area per unit of volume than a paste deposit and therefore oxidises more. The flux must be adequate for the alloy and for the time at temperature.
Where the flux cannot reach the joint, as inside a closed cavity, a preform with a flux core or a flux coating is used. The coating delivers the flux where it is needed and leaves a residue that may not be cleanable.
In a difficult case, the joint is formed in a nitrogen or forming gas atmosphere so that oxidation is suppressed without relying on flux at all. Our solderability notes describe how the alloy and the flux interact.
Step Soldering
Step soldering uses alloys with different melting points so that later operations do not reflow earlier joints. A high melting point preform forms the first joints, and a lower melting point paste or preform forms the later ones.
The temperature difference between the steps must be large enough that the later process does not melt the earlier joint, typically at least thirty degrees below its solidus.
The sequence must be planned with the component temperature limits, since the first step is the hottest and it applies to everything already fitted. Our solder defects notes describe the reflowed joint appearance when the steps are too close together.
Verification of the Joint
The joint formed from a preform is verified in the same way as any other: by inspection of the fillet, by sectioning where the joint is hidden and by measuring the fill in a barrel.
The volume calculation is more reliable for a preform than for a paste deposit, because the volume is known exactly. This makes it possible to predict the fillet size rather than to discover it.
The remaining uncertainty is the loss of solder to the surfaces and to any wicking away from the joint, which is greater where the pad is large or where the flux is very active. Our quality notes describe the fill measurement.
Cost and Handling
A preform costs more than an equivalent volume of paste and requires a placement operation. Its use is justified by a requirement that paste cannot meet rather than by preference.
Preforms are supplied loose, in tape or in a cartridge for automated placement. Loose parts require a manual operation and a handling control, since a dropped preform on the floor of an assembly area is a stray piece of metal.
Storage is the same as for solder wire, with attention to oxidation and to the working life of any flux coating. A preform that has oxidised does not wet, and the failure appears as an incomplete joint rather than as a visible defect.
Process Control and Verification
On a design of this kind, step soldering is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.
Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.
The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
Process Control and Verification
On a design of this kind, step soldering is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.
Process Control and Verification
On a design of this kind, step soldering is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.
FAQ
Can a preform replace paste entirely? No. It supplies volume, while the paste supplies the flux and the fine deposit. Most joints that use a preform also use paste.
Can a preform be used in a pin in paste process? Yes, and it is the standard remedy where the printed volume is insufficient for the barrel.
What does gopcb provide for preform joints? We provide the volume calculation from the hole geometry, preform shape and alloy selection including step soldering sequences, flux and atmosphere recommendations, and sectioning to confirm the resulting fill.
