Pin-In-Paste: Preparation, Placement and Process Control

Through-hole connectors are mechanically robust and electrically reliable, and they are also the reason many assemblies need a second, slower soldering process. Pin-in-paste, sometimes called intrusive reflow, removes that step by printing enough solder paste into and around the holes so that the joints form during the same reflow cycle as the surface mount parts. It works well when the geometry and the stencil are designed for it.

Why Reflow Through-Hole Parts

Every additional soldering process adds cost, tooling, and a thermal excursion that the board and its components must survive. A wave solder step requires a pallet or selective nozzle, exposes the whole assembly to a second thermal cycle, and applies solder to joints that were already finished. Reflowing the through-hole parts eliminates that step and produces a joint with a fillet on the top and bottom side much like a wave soldered joint.

The process also removes the need for masking and for the flux residues that wave soldering leaves behind, which simplifies cleaning. Against that, it demands a higher solder volume than the printed circuit board would otherwise require, and the extra volume has to come from somewhere without disturbing the surface mount deposits nearby.

Paste Volume and the Aperture Overprint

The volume needed to fill a hole and form both fillets can be calculated. The paste required equals the volume of the barrel plus the volume of the two fillets, minus the volume displaced by the lead, and the result is typically two to four times the volume of a normal surface mount deposit for the same pad area. Because a stencil of normal thickness cannot release that solder paste volume, the aperture is overprinted beyond the pad boundary.

Overprinting means the aperture extends onto the solder mask around the pad. The mask surface must be smooth enough to release the paste, and the clearance to neighbouring apertures must still be adequate to avoid bridging. Where the overprint would collide with an adjacent pad, the design has to move the pads further apart or accept a smaller fillet, and that trade-off should be made deliberately rather than discovered on the first build.

Stencil Design for Pin-in-Paste

Stencil thickness is the first decision. A thicker stencil releases more paste, which helps the through-hole joints but degrades the fine pitch surface mount deposits. Many designs solve this with a stepped stencil, thinner over fine pitch areas and thicker over the connector, or with a two print process. A step of 0.05 to 0.10 mm is common and is well within the capability of a laser cut stencil.

Aperture shape matters as much as size. A rounded rectangular opening releases paste more reliably than a sharp cornered one, and a slight reduction in area ratio improves release at the cost of volume. The area ratio, defined as the aperture area divided by the aperture wall area, should stay above 0.6 to release the paste cleanly, which is what limits how small a thick stencil aperture can be.

Hole Fill and Barrel Wetting

The printed paste has to fill the barrel during reflow, and capillary action alone is not enough on a plated hole of 1 mm diameter. The paste in the overprint melts and flows into the hole, and the lead acts as a wick. Whether the barrel fills completely depends on the hole diameter, the lead diameter, the paste volume, and the profile, and the relevant geometry is the annular gap between the two.

A gap that is too small restricts the flow of flux gases and can produce voids or blowholes, while one that is too large requires an impractical volume of paste. Practical designs keep the annular gap in the range of 0.2 to 0.3 mm and the hole diameter below about 1.2 mm. Above that size, the barrel is difficult to fill and a selective soldering process is usually the better answer.

Where a joint must be filled to a defined level, such as for a press-fit or a high current connection, the requirement should be stated on the drawing with a measured criterion. Visual inspection of the bottom fillet alone does not prove that the barrel is full, and a microsection on a sample is the only reliable check. The pad and hole geometry that makes this practical follows the same pad design rules used elsewhere, with the annular gap treated as the controlling dimension.

Component and Connector Limits

Not every through-hole part suits the process. Components with a long body and a single row of pins are ideal, because the pins are accessible and the paste volumes are similar. Parts with many rows of pins, with a large thermal mass, or with a body that sits flush with the board are harder, because the paste cannot be printed where the body would sit and the heat needed to reflow the joints is greater.

Connector materials also matter. A plastic housing that softens at reflow temperature will distort, and a part with a compliant pin design may not tolerate the reflow excursion. The component datasheet should be checked for a reflow rating before the process is committed, and a sample should be pushed through a full profile with the joints inspected afterwards.

Inspection and Defect Modes

The characteristic defect of pin-in-paste is an incomplete fill, which appears as a joint with a fillet on one side only and a void inside the barrel. Bridging across adjacent pins on a fine pitch connector is the second, and it usually indicates excess paste or insufficient spacing between apertures. Solder balls around the joint occur when paste is deposited on the mask and does not coalesce.

Inspection therefore combines visual checks of the fillets with an X-ray view of the barrel for critical joints. Where the connector carries power, a microsection of a sample joint provides the definitive evidence, and the result should be recorded with the paste volume and the profile used so that a later build can reproduce it.

Process Window and Profile

Pin-in-paste usually needs a slightly longer soak than a surface mount only assembly, because the extra paste volume needs time to reach temperature and to allow the flux to work. A peak temperature at the upper end of the alloy window helps the paste flow into the barrel, but it must stay within the rating of the components on the board.

The process window should be established with a test vehicle that includes the connector and a thermocouple in the barrel of one joint. Measuring the barrel temperature rather than the board surface shows how much of the profile is available for the fill, and it is the measurement that turns a marginal process into a controlled one. The same vehicle is useful for prototype qualification, because it validates the design and the process at the same time.

FAQ

What is the maximum hole size for pin-in-paste? About 1.2 mm in practice for a reliable fill, and a 0.2 to 0.3 mm annular gap helps the paste flow. Larger holes generally need selective or wave soldering instead.

Can pin-in-paste joints be inspected visually? The fillets can be, and the top fillet is usually a good indication. For critical joints such as power connections, X-ray or a microsection of a sample is needed to confirm barrel fill.

Does the process work with lead-free alloys? Yes, and the higher melting point makes the thermal balance more demanding. The profile should be validated with a thermocouple inside a barrel rather than on the board surface.