Pin in Paste and Through Hole Reflow Soldering
Pin in paste prints solder into the through hole pads and relies on the reflow oven to fill the barrels, which removes a wave soldering step. The saving is real, and the process is less forgiving than wave soldering because there is no second chance to add solder.
How Much Solder the Barrel Needs
The volume required is the annulus between the pin and the barrel wall, times the board thickness, plus a fillet at each end. That figure is usually far larger than the volume of paste an ordinary pad aperture holds.
The arithmetic should be done per hole rather than estimated. Our hole copper notes describe the plating that shares the barrel.
Adapting the Stencil
The extra volume comes from an overprint, a stepped stencil, or a combination. The overprint extends the aperture onto the mask beyond the pad, and the printed paste is then drawn into the barrel during reflow.
The overprint area has to be clear of other features. Our paste volume notes describe the measurement that confirms the volume.
Component and Pin Considerations
Not every connector tolerates the reflow temperature, and a plastic body that deforms or a pin that oxidises will not fill. The pin length and the hole clearance also decide how easily the paste is drawn up.
A long pin in a tight hole resists filling. Our connector notes describe the selection that follows.
Profile for Barrel Filling
The paste has to be molten long enough to flow down the barrel and up the other side. A profile that is adequate for surface mount pads can be too short for a thick board.
The soak and the time above liquidus both contribute. Our profile notes describe the measurement.
Bridging and Incomplete Fill
The two failure modes pull in opposite directions: too much volume bridges adjacent pins, and too little leaves the barrel partly empty. The window between them is narrower than for wave soldering.
The fill is confirmed by X-ray, which shows the solder level in the barrel. Our X-ray notes describe the technique.
When the Process Is Not Suitable
A board with many connectors, a thick stack up, a thermally demanding plane or a high pin count is often better served by selective soldering. The decision should be made against the board rather than by preference.
Our thermal mass notes describe the constraint that most often decides it.
Verification
The verification is a paste volume measured at the first article, a profile recorded at the barrel, and an X-ray fill check on a sample that shows the solder level reached.
Our first pass yield notes describe how the records are used.
Process Control and Verification
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. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
Checks Before Release
The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.
A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. 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.
Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.
FAQ
Can pin in paste replace wave soldering entirely? On a board with a modest number of accessible through hole parts it can. Where a connector has a long pin or a high thermal mass, selective soldering remains the more reliable route.
Does the hole need a larger diameter? A slightly larger hole helps the paste flow, but it changes the barrel fill requirement, so the two have to be considered together.
What does gopcb provide for pin in paste work? We provide a volume calculated per hole from the annulus and the board thickness, an overprint or stepped stencil designed to supply it, a profile verified at the barrel rather than at a test point, X-ray confirmation of the fill level, and a check for bridging on the same sample.
