Paste in Hole Solder Filling for Through Hole Joints
Paste in hole is the technique of printing solder paste into a through hole so that the joint can be formed during the same reflow step that solders the surface mount parts on the board. It removes a wave soldering or selective soldering operation, which is attractive on a mixed technology assembly, but it also puts the whole burden of solder volume onto the stencil and the print. This article explains how the technique works, how the volume is calculated, where the voids come from and how the process is controlled.
What Paste in Hole Is and Why It Is Used
In a conventional mixed technology process, the surface mount parts are reflowed and the through hole parts are wave soldered afterwards. Paste in hole eliminates that second soldering step by filling the barrel with paste during printing, so that the reflow oven forms both types of joint in one pass.
The benefits are a shorter process, less thermal exposure for the assembly and the removal of a wave or selective soldering operation with its own consumables. The cost is a demanding print: the paste has to fill a barrel rather than sit on a pad, and the volume needed is several times the volume of a normal surface mount deposit. The process also puts the entire solder volume under the control of the printer, so a print fault becomes a joint fault with no second chance.
How Much Solder a Joint Needs
The solder volume for a through hole joint is the volume of the barrel plus the volume needed to form a fillet on each side. The barrel volume follows from the hole diameter and the board thickness, and it is far larger than the deposit for a surface mount pad. A hole of a few tenths of a millimetre in a normal board demands a deposit that a conventional aperture cannot deliver.
The calculation also has to allow for the pin that will occupy the barrel. A close fitting pin displaces much of the volume, while a loose fitting pin leaves more to fill. The percentage fill that the specification requires should be stated, because a joint that is 75 percent filled and one that is 100 percent filled are different products.
Stencil Design for Through Hole Apertures
The aperture for a through hole is usually a large opening that overlaps the pad and the hole, and it is often fed by a wider area to hold enough paste to fill the barrel. The stencil thickness is a major variable: a thicker stencil delivers more volume, but it also affects the print quality of the fine pitch surface mount apertures elsewhere on the same board.
A stepped stencil solves that conflict by thinning the stencil over the fine pitch area and thickening it over the through hole area. The step is an extra fabrication step and it introduces a transition zone where the squeegee can leave an uneven deposit, so the layout should keep the two regions apart. Where the two areas must sit close together, a single intermediate thickness with a wider aperture is often a better compromise than a step.
Overprinting and Reflow Behaviour
The aperture is deliberately overprinted onto the pad and into the hole, which means the deposit has no defined edge and its shape depends on the paste rheology. A paste that holds its shape will bridge over the hole and leave a void underneath, while a paste that slumps will drain into the barrel and leave too little on the pad.
During reflow the paste melts, wets the barrel and the pin, and then contracts as it forms the fillet. The contraction is what draws solder down into the barrel, so the print has to leave enough material on the surface for the fillet to form after the barrel has been filled. That interaction is the reason the process window is narrower than a normal surface mount print.
Voiding in Through Hole Fills
Voids in a through hole joint come from flux volatiles that cannot escape, from air trapped in the barrel when the paste is printed, and from incomplete filling. A tall barrel with a close fitting pin traps volatiles at the bottom, and the void ends up where it does the most harm to the current path.
The paste chemistry, the preheat profile and the print parameters all influence the result. A profile that drives the volatiles off before the paste melts produces fewer voids, while a fast ramp traps them. Vacuum assisted reflow is used in the most demanding applications where the void limit is tight, and it is most effective when it is applied while the alloy is still fully molten.
Pin in Paste and Mixed Technology Assemblies
The technique is most often used for connectors and for a small number of through hole parts on an otherwise surface mount board. A connector with many pins needs a uniform fill across all of them, and a single starved aperture leaves one pin with an incomplete joint that is difficult to detect.
Where the assembly has both a large through hole content and fine pitch surface mount parts, the trade-offs start to favour keeping a second soldering step. The decision should be made from the volume calculation and the print capability rather than from a preference for a shorter route.
Process Control and Print Verification
The print is the process. Paste viscosity, stencil condition, squeegee pressure and speed, separation speed and the support under the board all change the volume that ends up in the barrel. Our solder paste inspection notes describe how the deposit is measured before reflow, which is the only way to detect a starved aperture early.
Board support deserves particular attention, because a board that flexes under the squeegee will have an uneven deposit across the panel, and the through hole apertures are usually the largest and therefore the most sensitive to that variation.
Inspection of Filled Joints
After reflow, the top side fillet is visible and can be inspected optically, while the barrel and the bottom side need X-ray or a section. A joint that looks correct from above can hide a barrel that is only partly filled, particularly where the paste bridged the aperture instead of draining into the hole.
Our solder defect notes group the failures that appear in these joints, and our land pattern guide explains how the pad geometry around the hole affects the print. Our quality guide describes how a partially filled joint is classified at gopcb, and our hole types notes cover the barrel geometry that has to be filled.
When Not to Use Paste in Hole
Paste in hole is not the right answer for a board with a very thick stack, a large number of connectors or a requirement for a fully filled barrel with tight void limits. In those cases a wave or selective soldering process delivers the volume far more reliably, and the extra step is the cheaper of the two options.
The judgement should be made with the volume calculation in hand and with a print trial on a real panel, because the theoretical window and the practical window are rarely the same size.
FAQ
Can paste in hole produce a fully filled barrel? It can for a thin board with a reasonable paste volume, but the margin narrows as the board gets thicker and the pin gets tighter. Where the specification demands full fill, a second soldering operation is usually the more reliable route.
Does a stepped stencil always help? It helps when the fine pitch and through hole requirements conflict, but it adds a step and a transition zone. A single thickness is often acceptable when the fine pitch features are forgiving.
How is the fill percentage measured? By X-ray for a non destructive estimate and by sectioning for the definitive figure. The measurement location should be stated, because a joint can be full at the top and open at the bottom.
