Pin in Paste Reflow for Through-Hole Components

Pin in paste places the solder for a through hole joint as paste in the stencil, so that the joint is formed in the reflow oven rather than by a wave or by hand. It removes a process step and it puts a demanding requirement on the deposit volume.

What the Joint Needs

The joint needs enough alloy to fill the barrel and to form a fillet on both sides. The barrel volume is fixed by the hole, and the fillet adds a requirement on top of it, so the paste has to deliver more metal than a surface mount joint of the same footprint.

That volume is the central problem of the process. A deposit that is too small leaves a partial fill and one that is too large leaves solder balls on the top side. Our paste volume notes describe how the deposit is measured.

Stencil Design for the Volume

The aperture is usually oversized and the stencil is thicker than the surface mount requirement. The extra area has to be placed where it does not bridge to a neighbouring pad, which usually means a shape that extends away from the pin rather than around it.

A stepped or a two level stencil is common, because the through hole sites need more paste than the fine pitch sites on the same board. The step is a design decision that belongs in the stencil data rather than at the printer.

The paste cannot enter the barrel before reflow, so the alloy has to flow into it as it melts. A gap in the middle of the barrel after reflow is normal, and the acceptance is written for the fillet rather than for a full barrel.

Where the joint has to carry current, the fill matters more, and the design moves to a wave or to a selective process rather than to pin in paste. Our joint criteria notes describe how the acceptance is written for each case.

The pin and the hole have a clearance, and the paste has to bridge it as it melts. A clearance that is too large leaves a gap that the alloy cannot fill by capillary action alone at the profile the rest of the board needs.

The pin is also a thermal path, and a pin connected to a plane draws heat away from the deposit. The profile is developed with the pin in place rather than on a bare board. Our profile notes describe how that measurement is arranged.

The characteristic defects are an incomplete fill, a solder ball on the top side from excess paste, a bridged pin and a joint that is sound and short of alloy. Each points at a different variable and the deposit volume is the first one to check.

The top side ball is the one that reaches the customer as a loose object, which is why it is treated as a defect rather than as a cosmetic issue.

The process suits a board with a small number of through hole parts, a fine pitch surface mount population and a reflow oven that is already required. It suits less well a board with many connectors and a thick barrel requirement.

The alternative is selective soldering, which gives a better fill and a longer cycle. The choice is usually made on the number of joints and on the fill requirement rather than on cost alone. Our board quality notes describe how the finished joints are judged either way.

Process Control and Verification

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.

Does the barrel have to be filled? Not completely. The acceptance covers the fillet and the fill percentage for the joints that carry current.

Can pin in paste be used with a connector? It can where the housing tolerates the reflow temperature and the pins have room for the paste beside them.

What does gopcb provide for pin in paste? We provide aperture and stencil thickness design for the volume, stepped stencils where the board needs two volumes, profile development with the pins fitted, deposit volume measured against the joint requirement, and acceptance written for the fillet and for the current carrying joints separately.

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.