Pin In Paste Reflow For Through Hole Components

Through hole components have traditionally been soldered by bringing the board into contact with a wave of molten solder, or by pointing a small wave at the joints in a selective process. Both work, but both require a second thermal excursion after the surface mount reflow. Pin in paste is the alternative: solder paste is printed into the through hole, the component is placed into the paste, and the whole assembly is reflowed once, with the through hole joints forming at the same time as the surface mount joints.

This article explains how the process works, why the paste volume is the central problem, how the stencil and the pad are designed for it, and what has to be verified.

The Principle

The paste is printed into the plated barrel and, in most designs, onto an area around the hole as well, so that there is a reservoir of metal to fill the joint. The component is then placed with enough force to push its pins through the paste, and the assembly passes through a single reflow profile. During reflow the paste inside the hole melts, wets the pin and the barrel, and forms a fillet on both sides of the board.

For the joint to form correctly, two conditions have to hold. The barrel has to be filled with solder rather than with flux residue, because a partly filled barrel produces a joint that looks acceptable from the visible side and is weak inside. And the paste must not be ejected from the hole before it melts, which happens when the volatiles in the flux expand faster than they can escape through the paste above them.

The Volume Problem

A printed deposit loses part of its volume as the flux evaporates and the metal consolidates. Paste contains metal and flux in roughly equal volumes, so the deposit has to be about twice the volume of the solder that the joint requires. The joint requires the volume of the barrel plus the volume of the two fillets, and on a thick board with a large hole that total is considerably more than a normal surface mount deposit.

The stencil alone often cannot supply that volume, which is why the standard solutions are all about adding a reservoir. The most common is an overprint: the aperture is larger than the pad so that a pool of paste sits around the hole and feeds it as the solder melts. A stepped stencil, in which the area around the hole is thinner than the rest, allows a larger volume to be printed without changing the rest of the board, and where even that is not enough, paste can be dispensed into the hole before placement or a solder preform can be added.

Stencil And Pad Design

The aperture design is a balance between volume and clearance. Enlarging the aperture around the hole increases the reservoir, but the overprint must not reach an adjacent pad and must not bridge to a neighbouring hole on a fine pitch connector. Where the pitch is tight, the available area between holes may be too small for the required volume, and the design then needs a different approach such as a stepped stencil, a countersunk hole, or a dispensing step.

The pad and the hole geometry follow from the component. The hole has to accept the pin with a small clearance so that the paste can surround it, the pad has to be large enough for the overprint, and the pin has to be long enough to pass through the board and be visible on the other side. A pin that is too short, or a component that stands off the board by a large amount, changes the volume calculation and may make the process impractical.

Process Considerations

The placement force has to be enough to push the pins through a printed layer of paste without deforming the component or the board. A connector with many pins requires a placement force that the machine can supply evenly, and the pins have to be aligned with the holes before the force is applied, which is why a pin in paste connector is usually specified with a compliant or chamfered lead.

The profile is the second variable. A ramp that is too fast heats the flux volatiles before the paste can vent them, and the pressure pushes paste out of the hole, which appears as a solder ball or a spatter on the board. A slower preheat allows the volatiles to escape while the paste is still solid, and a soak that keeps the paste near its melting point for a short time helps the reservoir to flow into the barrel rather than out of it. Where a component has a large thermal mass, the profile has to deliver enough heat to the joint without overheating the surface mount parts nearby.

Design Rules

The overprint area has to respect the clearance to other features. On a fine pitch connector the paste printed around one hole can bridge to the next, so the aperture has to be reduced and the volume made up elsewhere. The hole size and the pin diameter have to leave a gap that the paste can enter, and the board thickness sets the barrel volume that has to be filled.

The thermal mass of the component is also a design consideration. A large connector mounted in paste acts as a heat sink and takes longer to reach the reflow temperature, which affects both the profile and the surrounding joints. The paste volume calculation, the stencil design, and the profile are therefore developed together, and the result is verified on the real assembly rather than on a coupon with a different thermal mass.

Verification

The fill of the barrel is the parameter that matters and it is invisible from the top. X-ray inspection shows the solder inside the hole on a sample basis, and a cross section on a first article confirms the fill and the shape of the fillet. A visual inspection of the solder side shows whether a fillet has formed and whether paste has been ejected, and a weighing check of the printed deposit confirms that the stencil is delivering the volume that the design assumes.

The failure modes are worth watching for. A partly filled barrel, a solder ball thrown from a hole, a pin with no fillet on the solder side, and a component lifted by the expansion of the paste are all signs that the volume or the profile is wrong. The assembly consequences of a shifted or disturbed joint are described under component shift during reflow, the alloy context under lead free and leaded soldering, and the pad geometry under pad design standards.

FAQ

How much paste does a pin in paste joint need? Roughly twice the volume of solder that the joint requires, because the paste loses its flux during reflow. The solder volume is the barrel plus the two fillets.

Why does paste get blown out of the hole? Because the flux volatiles expand faster than they can escape through the paste above them. A slower preheat lets them vent while the paste is still solid.

Can pin in paste be used on a fine pitch connector? It can, but the overprint area between the holes limits the available volume, so a stepped stencil, a preform, or a dispensing step is often needed to reach the required fill.