Solder Preforms and Pin-in-Paste Assembly
The Problem
A through-hole joint needs a fillet on both sides of the board to be mechanically sound, and the volume of solder that requires is far more than the paste that can be printed onto a pad at a normal stencil thickness. In a wave soldering process the volume comes from the wave. In a reflow-only process, where the board carries through-hole parts and is never waved, the volume has to be delivered some other way. Pin-in-paste, or paste in hole, is the technique of doing it in the reflow oven, and it uses either an oversized paste deposit, a solder preform, or both.
Pin in Paste
The principle is to print enough paste onto and into the hole and around the lead, so that when it melts it forms the fillets. The paste has to fill part of the barrel and cover the pad, which means the required volume is roughly the volume of the fillets plus the volume of the barrel that is not already occupied by the lead and the plating, less whatever is displaced. The arithmetic is not difficult but it is unforgiving, and a joint that is short of volume shows a concave fillet on the top side and often a visible gap on the bottom. The technique needs the holes to be sized for the process rather than for wave soldering, since a hole that is too large cannot be filled by the available paste.
Stencil Techniques for Extra Volume
Several techniques increase the printed volume. Overprinting extends the aperture beyond the pad, which puts paste on the solder mask and lets it flow back during reflow; it is simple but it risks leaving solder balls on the mask. A stepped-up stencil adds thickness locally around the through-hole pads, which prints a taller deposit without affecting the fine-pitch area. Aperture design can also place the deposit on the pad around the hole rather than into it, with a shape that directs the molten alloy into the barrel. Each technique has a limit set by the area ratio and by how much the solder can flow before it wicks away, and they are frequently combined.
Solder Preforms
A preform is a piece of solder manufactured to a specific shape and volume, placed either under the component or against the joint before reflow. The preform provides a known volume that the stencil cannot achieve, and it melts with the paste to form the fillet. Preforms are made in wires, washers, discs, squares and custom shapes, and the common choices for through-hole work are a washer around the lead or a ring placed on the pad. The preform has to be held in place during placement and reflow, which is usually done with the tackiness of the paste or with a small amount of adhesive, and it has to be placed by a machine for any volume production.
Preform Selection
The preform material should match the paste, since mixing alloys with different melting points produces a joint with an unpredictable composition and a melting range rather than a point. The volume is calculated from the joint requirement, minus what the paste contributes, and the shape is chosen so that it fits the available space without interfering with the component body or with a neighbouring part. The preform’s own surface has to be clean and its oxide content low, since it has to melt and coalesce with the paste; a preform that has been stored badly will leave a residue or fail to flow. Where the joint also has a thermal requirement, a preform with a higher melting point can be used deliberately to keep the joint solid during a subsequent reflow, though this is a specialised technique.
Reflow and Placement
A pin-in-paste joint needs more heat than a surface mount joint, because the volume of metal is larger and the barrel adds thermal mass. The profile therefore has a longer time above liquidus and sometimes a higher peak, which the surrounding surface mount components have to tolerate. The placement of the preform is a step that the placement machine has to be able to perform, which means a feeder and a nozzle, and the preform must not be disturbed by the placement of the component itself. Where the component is a connector with many pins, a preform for each pin makes the process slow, which is why preforms are used selectively on the joints that need the most volume rather than on all of them.
When to Choose Each
Use overprinting or a stepped stencil where the required volume is modest and the pad geometry allows it, since these need no additional component and no extra placement step. Use a preform where the volume is large, where the hole is large relative to the pad, or where the stencil technique has reached its limit. Use selective or wave soldering where the assembly has many through-hole joints, since delivering the volume from a wave is far simpler than printing it or placing preforms. The decision should follow from the number of joints and the volume each needs, not from a preference for a technology.
Pin in Paste and the Board Design
The board layout decides whether pin-in-paste is feasible before any process choice is made. A hole that is sized for a wave process is usually larger than the pin-in-paste process wants, because the barrel has to be at least partly filled by the printed paste, and a larger barrel needs more volume than the stencil can supply. The pad has to be large enough to hold the deposit and to allow an overprint, and the space around the pad has to be free of anything that would be contaminated by solder flowing over the mask. The interconnect geometry also matters: a pad connected to a heavy plane draws heat away from the joint, which makes the fill more difficult, so a thermal relief or a smaller plane connection helps. Where the design mixes pin-in-paste joints with fine-pitch surface mount parts, the stencil has to serve both, which usually means a stepped stencil and a careful aperture design rather than a single thickness.
FAQ
What is pin in paste? A reflow technique that delivers enough solder to a through-hole joint by printing a large paste deposit, using a preform, or both.
Why is extra volume needed? Because the fillets on both sides of the board require far more solder than a normal printed deposit provides.
What is a solder preform? A piece of solder made to a specific shape and volume, placed at the joint to supply a known amount of alloy.
Should the preform match the paste? Yes. Mixing alloys produces an unpredictable joint with a melting range rather than a melting point.
Why not use preforms on every joint? Because each one has to be placed, which is slow and adds a step that can go wrong.
Conclusion
Pin-in-paste is a volume problem, and the solution is a stencil technique, a preform, or a combination of both chosen from the amount of solder the joint needs. Match the preform to the paste, size the hole for the process, and allow more heat in the profile. Through-hole assembly is described under SMT PCB assembly, the joint quality that results is verified by PCBA testing, and the hole and pad geometry belongs to PCB design and layout. Mixed assemblies are normally proven through prototype PCB assembly in 2026.
