Pin in Paste Through Hole Reflow
Pin in paste allows a through hole connector to be soldered in the same reflow pass as the surface mount parts, which removes a wave or selective soldering step from the line. The idea is simple: print enough paste into and around the hole, place the component, and let the reflow oven form the joint. The difficulty is that a through hole joint needs far more alloy than a surface mount pad, and the stencil has to deliver it.
How the Joint Fills
Paste printed over the hole sits above a void, and during reflow the alloy flows down the barrel and wets the lead and the plated wall. To form a proper fillet on both sides, the paste has to contain enough metal to fill the barrel and leave a fillet, plus the amount that is lost to the stencil geometry and to any wicking.
The required volume is therefore the barrel volume plus two fillets, and it is much larger than the volume of a surface mount deposit of the same pad area. This is the central constraint of the process and the reason most pin in paste problems are volume problems.
Calculating the Paste Volume
The calculation begins with the barrel volume, which is the cross sectional area of the finished hole minus the lead, multiplied by the board thickness. The fillets are estimated from the geometry, typically as a fraction of the hole diameter, and the total is the metal volume required.
Paste is roughly half metal by volume, so the printed deposit has to be about twice the required metal volume. That figure is what drives the stencil design, and it is usually the point at which a designer discovers that the aperture cannot be squeezed onto the pad. Our article on paste volume and stencil design covers the geometry limits.
Stencil Design for Through Holes
The usual approach is to print a larger aperture over the pad and to accept that some paste will be pushed into the hole by the squeegee. Overprinting beyond the pad is common, because the surrounding area is often free of other features and the extra volume is needed.
A stepped stencil can add further volume by thickening the foil over the through hole area. Where even that is not enough, the design may need a second print or a preform, and the decision should be made from the volume calculation rather than from a trial at the printer.
Component and Connector Requirements
Not every through hole component tolerates reflow. The body has to withstand the peak temperature, and the leads have to be long enough to protrude below the board so that a fillet can form on the underside. A lead that is too short produces an unreliable joint that cannot be inspected.
Plastic connectors are the usual concern, and the datasheet peak temperature is the limit that governs the profile. Our article on paste qualification describes how the profile window for such an assembly is established.
Defects and Their Causes
Insufficient fill is the most common defect and appears as a joint that is short of alloy, with no fillet on the top side. It follows directly from a volume shortfall, so the first corrective action is to measure the deposit rather than adjust the profile.
Excess paste produces the opposite problem: solder balls on the top side, bridging between adjacent pins, and paste left on the stencil face that transfers to the next board. Where a connector has a fine pitch, the bridging risk becomes the limiting factor and may rule the process out. The general diagnostic sequence is described in our article on reflow defect troubleshooting.
Thermal Considerations
A connector body is a large thermal mass, and it sits above the paste during reflow. The leads act as heat paths into the board, so the joints nearest the body may be the last to reach temperature. The profile has to allow for that, which usually means a longer soak rather than a higher peak.
The board thickness also matters. A thick board needs more paste to fill the barrel, and it also takes longer to heat, so the two constraints push in the same direction and make the process window narrower.
Inspection
Inspection has to confirm fill on both sides. The top side can be inspected optically, and the bottom side after the board is turned over, but a barrel that is only partly filled is not visible from either. X-ray is the only method that shows the barrel, and it should be used on a sample until the process is proven.
Once the process is stable, the inspection can be reduced to a deposit volume check at the printer, which is the variable that actually controls the outcome. That shift from inspecting the joint to controlling the deposit is what makes the process viable in volume production.
Checks Before Release
The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold.
A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.
FAQ
Can pin in paste replace wave soldering entirely? For boards with a few through hole parts, often yes. For a board dominated by through hole connectors, wave or selective soldering is usually more practical.
Does the hole need to be filled completely? A fillet on both sides plus a sound barrel is the requirement. Complete fill to the top of the barrel is not necessary if the fillets are correct.
Why does the paste volume need to be twice the metal volume? Because paste is approximately half metal by volume once the flux is excluded. The deposit must therefore be about double the alloy the joint needs.
