Paste in Hole: Reflowing Through-Hole Connectors With Printed Paste
Paste in hole prints solder into the barrel of a through-hole part and reflows it in the same pass as the surface mount devices, which removes a wave soldering step and the handling that goes with it. It works only when the printed solder volume is calculated rather than guessed, because a barrel that is short of alloy cannot be topped up after the connector is seated.
How Paste in Hole Differs From Wave Soldering
In wave soldering the barrel is filled from below by capillary action, and the process can be adjusted until the fill is right. In paste in hole the alloy is placed before the connector is seated and the only chance to correct the volume is at the printing step, so the process is controlled by the stencil and the paste rather than by the wave.
The advantage is that the connector sees one thermal cycle instead of two, and the assembly no longer passes over a wave with the surface mount side exposed to it. The cost is that every through-hole joint now depends on a printing operation that is designed for pads 0.3 mm wide rather than for barrels 1 mm across. A product that uses through-hole reflow also has to place the connectors before the oven rather than after it, so the placement sequence and the stencil have to be planned together.
Solder Volume Needed for a Full Barrel
The volume required is the annular space between the pin and the hole wall, and it is computed rather than estimated. A 1.0 mm hole through a 1.6 mm board with a 0.6 mm pin holds about 0.8 cubic millimetres of alloy, and the printed paste volume has to be roughly twice that, because solder paste is only about half metal by volume once the flux has volatilised.
That doubling is where most short fills originate. A deposit sized to the hole volume looks generous on paper and produces a barrel that is half full, and the shortfall is not visible until the assembly is X-rayed or sectioned.
Stencil Aperture Design and Overprint
The stencil aperture is normally the hole diameter plus 0.2 mm to 0.30 mm of overprint on each side, so that the aperture also feeds the fillet on the solder side. The extra area is what supplies the alloy that forms the toe fillet, which the barrel volume alone does not provide.
Where the connector sits in a region of thin paste, the stencil is stepped down around the body so that the connector housing does not lift the stencil off the pads nearby. A step of 0.10 mm to 0.15 mm is typical, and the print is then checked for paste smearing at the step boundary.
Pin to Hole Clearance
The clearance between pin and hole sets how easily paste enters and how easily flux gases leave. A diametral clearance of 0.20 mm to 0.25 mm fills reliably; below 0.15 mm the paste bridges at the entry and the barrel fills only part way, and above 0.30 mm the alloy has to span a gap it cannot bridge and the joint wicks unevenly.
Clearance is a design decision taken with the connector datasheet in hand, because the pin tolerance belongs to the component rather than to the board. Both the maximum pin diameter and the minimum hole diameter have to be checked, not the nominal figures.
Reflow Profile for Through-Hole Mass
A connector is a large thermal mass compared with a 0402 chip, and the profile has to bring it to temperature without overheating the small parts on the same panel. A soak of 90 to 120 seconds between 150 and 180 degrees lets the connector body catch up, and the time above liquidus is usually extended to 60 to 90 seconds to complete the fill.
Bottom-side preheat helps on thick connectors, and it is set from a thermocouple attached to the barrel rather than to the board surface. The barrel lags the surface by 20 to 30 degrees on a 1.6 mm board, so a profile that looks correct at the surface can be 15 degrees short inside the hole; our profile verification notes describe how the couple is attached. Where several connector sizes share one panel, the largest body sets the soak time and the smallest is protected by keeping the peak below its own rating.
Voiding and How It Is Reduced
Voids in a paste in hole joint come from flux volatiles that cannot escape while the alloy is liquid, and they are largest when the preheat is fast and the clearance is tight. Slowing the ramp through the flux activation range gives the volatiles a path out before the alloy solidifies, and a vent path through the clearance does the rest.
Where voiding matters electrically, a vacuum assisted reflow or a longer soak reduces it further, but neither is a substitute for the correct paste volume. A joint with 30 percent void and a full fillet is usually acceptable, while a joint with 5 percent void and a half filled barrel is not.
Connectors That Cannot Take Reflow
A connector body rated only for wave soldering will not survive a 245 degree peak, and the plastic softens enough to lose pin position even when it does not melt. The temperature rating is in the datasheet, and it should be checked before the part is designed onto the reflow side.
Tape and cover films on connectors are another limit, because they are applied for wave soldering and can lift during reflow. Where a part has to be reflowed and is not rated for it, the usual answer is a selective soldering step after reflow rather than a change to the profile.
Inspection and Acceptance Criteria
Barrel fill is measured by X-ray at an angle, which shows the alloy level inside the hole without opening the joint, and it is confirmed by microsection at first article. The figure normally applied is 75 percent vertical fill on a supported hole, with the class differences applied to fillet shape and wetting rather than to the barrel.
Visual inspection confirms the toe fillet and the absence of bridging, and it is the only check that can be made at full production speed. X-ray then replaces the section for routine sampling, and the viewing angle matters because a vertical image of a barrel shows the pin and hides the alloy level underneath it. Our X-ray and AOI notes set out where each method is used.
Process Control and Documentation
The parameters that have to be recorded for each run are the paste lot and its open time, the stencil thickness and aperture set, the print speed and pressure, and the profile with the thermocouple position. The paste volume deposited is verified by inspection on the first panel and periodically after that.
The drawing should also carry the fill requirement and the clearance band, because both are design inputs rather than shop options. A note that states only the hole size leaves the fill to the process, and the process cannot change a volume it was never given.
FAQ
Can paste in hole be used with a connector that has a plastic locating peg? Only if the peg clears the hole it enters and the body is rated for the reflow peak. The peg is a location feature rather than a solderable one, so it must not be relied on to carry alloy into the barrel.
Why does the printed volume have to be twice the solder volume? Because paste is a suspension of metal powder in flux, and roughly half of the printed volume is flux that burns off or volatilises during the profile. Sizing the deposit to the finished solder volume leaves the barrel about half full.
Is a second reflow pass enough to correct a short fill? No. The alloy is already solid and the connector is seated, so a second pass adds no material. A short fill is corrected at printing, by apertures sized from the calculated volume rather than from the hole diameter.
