Pin-in-Paste Printing: Getting Volume into Through Holes

Pin-in-paste printing places solder paste into the through holes of a mixed-technology board so that the through-hole joints are formed in the same reflow cycle as the surface mount joints. The wave soldering step and the masking that goes with it both disappear, and the stencil volume becomes the variable that decides whether the joints are sound. The saving in process steps is real, and it is paid for with a narrower print window.

The method is attractive on boards that carry only a few through-hole parts, and it becomes difficult as the ratio of hole volume to printable area rises. A design with a connector of many pins on a narrow pitch is a harder proposition than a design with two isolated terminals.

How the Process Works

The stencil carries extra apertures over the through holes in addition to the surface mount apertures. Paste is printed into those apertures and enough of it is pushed into the hole that the barrel is partly filled before reflow. The component is placed, and the reflow cycle then forms both the surface mount joints and the through-hole joints. The sequence matters, because paste pushed into the hole during printing behaves differently from paste that is only sitting on the pad.

The paste that fills the hole comes partly from the printed deposit and partly from paste drawn in by capillary action as the alloy melts. Both contributions depend on the geometry, and neither is large enough on its own for a hole of any depth.

Volume Balance for a Through Hole

The volume needed to fill a barrel is the area of the annulus between pin and hole multiplied by the board thickness, and a fillet at each end is required on top of that. The arithmetic is quick and it usually shows that a standard aperture cannot supply the volume in one print.

Two approaches follow. The aperture over the hole can be enlarged beyond the pad to print more paste than a normal deposit, or the hole can be designed smaller so that less volume is needed. The second is a design decision and the first is a stencil decision, and both are usually needed together on the same product.

Stencil Design and Paste Overprint

Overprinting extends the aperture beyond the pad, and the paste that sits on the solder mask has to be pulled into the joint during reflow. The extension is limited by the clearance to the neighbouring pad, because paste that bridges to a neighbour before reflow produces a short.

A common arrangement is a shape covering the pad with a defined margin around it, with the aperture area stated as a multiple of the hole area. The multiplier is calibrated from the first article rather than from a formula, because wetting, component geometry and hole tolerance all shift it. The two adjustments are made together on the stencil drawing rather than one after the other at the machine.

Aperture Size and Paste Release

Area ratio still applies. A large aperture releases paste more completely than a small one, which helps here, because apertures over through holes are usually larger than fine pitch surface mount apertures. The limiting feature becomes the neighbouring apertures on the same stencil. The margin between the overprint and its neighbour is the practical limit on the multiplier.

Release also depends on the wall condition and on the print speed. A slow print fills the aperture more completely and may push paste through into the hole, which is desirable, while a print that is too slow leaves paste on the mask around the aperture. The balance is found by measuring the deposit volume, described in the notes on paste volume measurement.

Hole and Pin Tolerance

The hole diameter has a tolerance and so does the pin, and the difference between the two sets the annulus area that has to be filled. A hole at the upper limit with a pin at the lower limit needs appreciably more paste than the nominal case does.

That variation is why a pin-in-paste process is qualified on the worst case rather than on the nominal. A process that works on a nominal sample and fails on a large hole will fail in production at a rate that follows the tolerance distribution of both parts. A process qualified on the nominal case has been qualified on a case that occurs only occasionally in production. The hole side of the same tolerance stack is described in the notes on board thickness and stack-up.

Reflow Considerations

The thermal profile has to heat the through-hole volume as well as the surface mount joints. A barrel inside a thick board takes longer to reach liquidus than a small surface mount joint, and that difference becomes a requirement for a longer time above liquidus than a surface mount only board needs.

A longer molten time increases the opportunity for paste on the mask to draw into the joint, which helps, and it also increases oxidation on the surface of the deposit. The practical limit is set by the components on the board rather than by the paste. A profile written for the surface mount joints alone will under-heat the through-hole volume.

Inspection Limits

Inspection of a pin-in-paste joint is harder than inspection of a wave soldered joint. The barrel fill cannot be seen from the top, and the fillet on the top side is not a reliable indicator of what happened inside the hole.

X-ray inspection shows the fill on the board and does not show whether the alloy has wetted the barrel wall. The acceptance criteria for fill are described in the notes on barrel fill.

Common Defects and Their Causes

Insufficient fill is the characteristic defect and it usually traces to a volume shortfall: an aperture that is too small, a hole at the upper tolerance limit, or a print that left paste on the stencil instead of on the board. Partial fill at the top of the barrel with a sound joint at the bottom is the signature of the first two.

Small solder balls around the hole come from paste printed on the mask that never reached the joint. Bridging between adjacent pins follows from an overprint that is too generous, and it appears at the placement stage rather than after reflow if the paste is inspected before the component is placed.

Qualification and First Article

Qualification starts with a volume calculation and continues with a cross section. The first article is printed, placed and reflowed, and a cross section shows the actual fill and the wetting on the barrel wall, which no external inspection can reveal. The cross section should be taken through the worst case hole rather than through the most convenient one.

The conditions of the profile that produced a good first article become the production recipe. The flux side of the same profile is described in the notes on preheat and flux activation.

FAQ

Can pin-in-paste replace wave soldering completely? On boards with a small number of through-hole parts it can. A board with a large connector usually needs both processes, or a redesign of the connector.

How much paste is needed for a through hole? Enough to fill the annulus over the board thickness plus a fillet at each end. The calculation is quick and it usually shows that an overprint is required.

Is X-ray enough to accept the joint? It shows fill and not wetting. A cross section on the first article is what confirms the barrel wall.