Designing Paste Volume for Pin in Paste Reflow Joints

Pin in paste is the technique of printing solder paste into the through holes of a mixed technology board so that the through hole parts are soldered in the same reflow cycle as the surface mount parts. It removes a wave soldering or selective soldering step from the flow, which saves a handling operation and avoids exposing the surface mount joints to a second thermal cycle. The technique is most useful on boards with a small number of through hole parts, where a dedicated wave or selective soldering step would dominate the cycle time.

What Pin in Paste Solves

On a board that carries both surface mount and through hole components, the traditional flow is to reflow the surface mount side first and then to solder the through hole parts from below. That second operation requires masking, fluxing and a thermal cycle that the surface mount joints have to survive. Wave soldering also exposes the whole board to flux and to a solder wave, which puts the surface mount joints and the mask at risk for the benefit of a few through hole pins.

Pin in paste removes that operation by printing enough paste into the hole to form a proper fillet during the reflow cycle. The benefit is a shorter flow, less handling damage and fewer thermal cycles, but the technique only works when the paste volume is designed rather than estimated. Pin in paste is unforgiving of a short deposit, because there is no second chance to add solder once the board has left the oven.

Paste Volume and Solder Demand

The volume of solder needed to fill a through hole joint is calculated from the hole diameter, the pin diameter, the board thickness and the desired fillet. The resulting figure is far larger than a typical surface mount deposit, which is why the stencil and the aperture design have to be treated differently. The calculation should be done for the worst case combination of maximum hole size and minimum pin diameter, because that is where the joint will be short.

If the printed volume is short, the joint fills only partly and forms a concave fillet that fails the acceptance criteria. If it is excessive, the paste is pushed out onto the surface during insertion and forms solder balls that have to be removed. Our land pattern guide covers the pad geometry that supports the deposit. A paste volume that is outside the practical range is a signal that the design should change rather than that the printer should be pushed harder.

Stencil Design for Paste in Hole

The stencil has to deliver the extra volume, which normally means a thicker foil and an overprinted area around the hole. A stepped or a cavity stencil is often used so that the fine pitch surface mount apertures keep their own area ratio while the through hole sites receive a much larger deposit. The step or cavity is produced during stencil manufacture, so the requirement has to be stated in the stencil data rather than added later.

Aperture shape matters as much as area. Both the stencil aperture area and its shape have to be chosen together, because a larger aperture on a thin foil cannot deliver the volume that a smaller aperture on a thick foil can. A circular aperture over a hole is often replaced by a shape that spreads the paste across the pad and over the hole mouth, which helps the paste to be pushed into the hole by the insertion of the pin. Our paste inspection guide describes how the printed volume is verified. Paste inspection before insertion is the only chance to catch a short deposit, and it is far cheaper than reworking a board after reflow.

Placement and Component Retention

Through hole parts printed with paste have nothing to hold them in place before reflow, unless the leads are clinched or the part sits in the hole by interference. Components that stand above the board can be displaced by the conveyor, by vibration or by the airflow in the oven.

Retention options include a longer lead that is bent over, a small adhesive dot, or a carrier that holds the parts until the paste melts. The choice depends on the component mass and on the orientation of the board through the oven, and it should be tested on a real assembly rather than assumed. A component that moves before the paste melts will often still solder, which makes the defect intermittent and hard to trace.

Reflow Profile Considerations

The reflow profile has to do two jobs at once: form the surface mount joints and drive enough heat into the through hole joint to melt the paste in the barrel. The hole is surrounded by laminate and by the component body, so it heats more slowly than an exposed surface mount pad. A thermocouple placed inside the barrel, or on the pad next to the hole, gives a much better picture than one placed on a surface mount pad.

The soak time is usually extended so that the barrel reaches temperature before the peak. A profile that works for a thin board may be short for a thick one, and the difference shows up as an incomplete fillet on the through hole joints rather than as a visible surface mount defect. The profile should therefore be developed on the thickest and most heavily copper loaded board in the family.

Voiding in Through Hole Joints

Voids form in pin in paste joints when flux volatiles cannot escape from the barrel before the solder solidifies. A large paste volume produces more volatiles, and the barrel gives them a long path to the surface, which is why voiding is more common here than in a surface mount joint. Voids that are small and dispersed are normally acceptable, while a single large void across the joint is a reliability concern.

Paste chemistry, the profile and the hole geometry all influence the result. A longer soak, a slower ramp through the melting range and a paste with a lower volatile content all reduce voids, and the acceptance limit should be agreed with the customer. Our solder defects guide describes how these conditions are classified. Recording the void percentage with the X-ray image makes it possible to compare suppliers and profiles on a consistent basis.

Inspection and Acceptance

Through hole joints are inspected from the secondary side for a fillet and from the primary side for evidence that the paste reached the top of the pad. A joint that looks good from below can still be hollow inside if the paste did not fill the barrel. A microsection on the first article is the definitive check, and it should be repeated whenever the stencil or the paste changes.

X-ray inspection is the practical way to check fill and voiding, while a microsection is used for qualification and for failure analysis. Visual criteria alone are not sufficient for a joint that has to carry current or withstand thermal cycling. For a small through hole part, a simple pull test on a sample can add useful evidence at very little cost.

Design Rules for Success

Design rules start with the hole and the pad. A hole that is too large relative to the pin requires an impractical paste volume, and a pad that is too small cannot support the deposit. The usual guidance is to keep the annular ring generous and the hole clearance modest. A plated hole that is well within the drilling tolerance also helps, because the paste volume calculation assumes a known barrel diameter.

Thermal relief design matters as well, because a pad connected to a heavy plane will sink heat away and delay the melting of the paste. A web style relief, rather than a solid connection, keeps the joint reachable by the profile without weakening the electrical path. Where the plane is heavy, a slightly longer soak is often needed to bring the joint up to temperature without overheating the rest of the board.

Process Control Points

The controls are the printed paste volume, the stencil condition and thickness, the insertion depth and clinch angle, the profile measured on a real assembly, and the inspection result for fillet and voiding on a sample basis. Every one of those items is measurable, and together they make a technique that is often described as difficult into a routine process.

A first article check should be sectioned to confirm that the barrel is filled, because that is the one condition that cannot be judged from the outside. Our production flow guide places these checks in the wider assembly sequence. Designing the paste volume at the layout stage is what makes the difference between a reliable pin in paste process and a permanent rework burden.

FAQ

How much paste should be printed for a through hole joint? Enough to fill the annular space plus the desired fillet, which is normally several times a surface mount deposit. The volume is calculated from the hole, the pin and the board thickness rather than estimated.

Why do pin in paste joints contain voids? Flux volatiles are trapped in the barrel while the solder solidifies. A longer soak, a slower ramp and a lower volatile paste reduce the effect, and the acceptance limit should be agreed with the customer.

Do through hole parts need to be glued down? Only if they are heavy enough to move before the paste melts. Clinched leads, an adhesive dot or a carrier can each hold the part through the oven.