Pin-in-Paste: Through-Hole Reflow for Mixed Assembly

Pin-in-paste is the process of printing solder paste into a plated through-hole, inserting the component lead into that paste, and soldering it in the same reflow oven pass that solders the surface-mount parts — no separate wave or hand-solder step. Also called paste-in-hole (PIH), through-hole reflow (THR), or intrusive reflow, it folds the few leaded connectors and headers on an otherwise SMT board back into one thermal process. The catch: a flat stencil aperture can’t fill a hole, so the whole game is depositing enough paste. Get the volume wrong and the joint starves or the connector melts.

This guide covers the mechanics, the paste-volume math that actually sizes your stencil aperture, the hole and lead design rules, the reflow-profile traps with high-mass connectors, and an honest comparison of where pin-in-paste beats wave and selective soldering — and where it loses.

Pin-in-Paste at a Glance

• One print, one place, one reflow — through-hole and SMT parts solder together, eliminating a separate wave or selective step and a second thermal cycle.
• Solder paste is only ~50% metal by volume, so you must overprint, step the stencil up, or add solder preforms to get enough metal into the barrel.
• Hole diameter should be ~0.25–0.3 mm (10–12 mil) larger than the lead — big enough to insert and fill, small enough not to demand absurd paste volume.
• Only use parts rated to survive the full reflow peak (~245–260°C); connector plastics and electrolytics are the usual failures.
• IPC-A-610 Class 2 needs ~50% vertical hole fill; Class 3 needs ≥75%. That target drives the whole paste-volume calculation.

What Is Pin-in-Paste (Through-Hole Reflow)?

In a normal SMT line, a stencil prints paste onto surface pads, parts are placed, and the board reflows. Pin-in-paste extends that exact oven pass to leaded parts. During the same stencil print that pastes the SMT pads, paste is also forced into the plated through-holes. The leaded component is placed so its pins sit inside the paste-filled barrels. In the oven, the paste in the holes melts alongside the paste under the chips, wicks through the barrel, and forms a fillet on both sides of the board.

The motivation is almost always to delete a process step. Picture a board that’s 95% surface-mount with three or four through-hole connectors. Wave soldering forces the whole panel through a molten-solder wave just to make a dozen joints — an extra machine, a pallet to mask the SMT side, extra flux and cleaning, and a second thermal cycle for parts already reflowed once. Pin-in-paste makes those joints in the pass you’re already running. It’s the natural through-hole strategy for the kind of board covered in our guide to mixed assembly, where a handful of leaded parts ride along with a mostly-SMT design.

Through-hole parts haven’t disappeared, and they won’t — power connectors, board-to-board headers, large electrolytic capacitors, relays, RJ45 jacks, and DC barrel jacks all need a leg through the board for mechanical strength. Pin-in-paste is how you keep those parts without keeping a wave machine.

How the Pin-in-Paste Process Works, Step by Step

A pin-in-paste line looks almost identical to a standard SMT line, with through-hole placement added before reflow rather than after.

1. Confirm the parts are PiP-ready. Verify each through-hole component survives the full reflow peak (~260°C for several seconds). Connector plastics and electrolytic caps are the parts that fail this.
2. Design holes and stencil for volume. Size the hole to the lead, then calculate the paste volume and design the aperture (overprint, step-up, or preform) to deliver it — the math is below.
3. Print paste. The squeegee forces paste through the stencil into the plated barrels and onto the SMT pads in one stroke. A center bar in the aperture can reduce push-through on large holes.
4. Place components. SMT parts go down by pick-and-place. PiP-rated through-hole parts in tape-and-reel can be machine-placed; otherwise an operator inserts them after SMT placement without disturbing the paste-printed SMT parts.
5. Reflow. The board runs one thermal profile. Paste in the holes melts, wicks through the barrel, and forms top and bottom fillets simultaneously with the SMT joints.
6. Inspect. AOI for the SMT side; X-ray or microsection to confirm barrel fill and voiding against IPC-A-610. You can’t judge hole fill by eye alone.

Pin-in-Paste Solder Volume: The Math That Sizes Your Stencil

This is the part competitors skip and the part that actually decides whether your joints fill. The solder needed in the finished joint is the volume of the plated hole, minus the volume of the pin sitting in it, plus the volume of the fillets you want on each side:

Solder joint volume = V(hole) − V(pin) + V(fillets)

But you don’t print solder — you print paste, and paste is roughly half flux by volume. So the paste you actually deposit has to be scaled up by the paste reduction factor (the fraction that’s metal), typically 0.45–0.55:

Paste volume needed = Solder joint volume ÷ paste reduction factor (≈0.50)

Then you size the aperture so its open area times stencil thickness equals that paste volume. A worked reference makes the scale clear: for a 0.9 mm (≈36 mil) lead, a 1.5 mm (≈58 mil) hole, and an 80-mil pad, free volume-calculator tools land on a round aperture near 4.9 mm (≈194 mil) diameter — or a square roughly 4.4 mm (≈172 mil) on a side — to fully fill the barrel and form a fillet. That’s a large aperture, which is exactly why hole size matters so much.

Counterintuitive insight #1: oversizing the hole is what makes pin-in-paste fail, not undersizing it. A hole 1 mm larger than the lead leaves a huge empty barrel that needs a flood of paste to fill — paste a flat aperture physically can’t supply without sprawling onto neighbors. The industry rule from people who do this daily: keep the hole only 0.25–0.3 mm (10–12 mil) larger than the lead. Tight holes need less solder, fill more reliably, and resist push-through. Loose holes are the root cause of most starved PiP joints.

Pin-in-Paste Design Rules for Holes, Leads, and Stencils

Three things have to be engineered together — the hole, the lead, and the stencil. Get them aligned and PiP is boringly repeatable; get one wrong and you chase defects.

Hole and Pad Geometry

• Hole-to-lead clearance: 0.25–0.3 mm (10–12 mil) over the lead diameter. Tighter starves on insertion; looser demands unachievable paste volume.
• Pad (annular ring): keep it as small as mechanically acceptable — often about 2× the barrel diameter. A smaller pad reduces the solder volume the print has to supply.
• Surface finish: ENIG promotes wetting and improves hole fill versus a duller finish; solder mask with higher surface energy keeps overprinted paste flat instead of beading into the component.

Lead Length and Tip

• Protrusion: keep it short. Long pins displace paste on insertion and wick solder away from the barrel; X-ray studies show 75–100 mil protrusions push significant paste out of the hole. Aim for near-flush, with about 5 mil minimum so AOI can still confirm the pin is present.
• Tip shape: rounded, not blunt. Blunt or chisel ends shove paste straight through the barrel on insertion.

Getting Enough Paste Into the Barrel

Stencils have trended thinner — 100–120 µm today versus a 150 µm norm a decade ago — to serve fine-pitch SMT, which makes barrel fill harder and overprinting more necessary. Three techniques, in escalating order:

1. Overprint. Extend the aperture into a larger ring around the hole; on reflow the extra paste coalesces and wicks down the barrel. If real estate is tight on one side, offset the overprint toward open board area.
2. Step-up stencil. Add foil thickness locally (0.15–0.2 mm is common for THT) to raise volume only where the connector sits. Mind the keep-out: per IPC-7525, a step needs roughly 1.8 mm clearance from nearby apertures for a 2-mil step, so dense areas may not allow it.
3. Solder preforms. Place a small solder washer or shaped preform at the hole; it supplies pure metal that wicks into the barrel — the answer for connectors a print alone can’t fill. The paste flux cleans the preform, so no extra flux is needed.

Counterintuitive insight #2: lead-free hurts pin-in-paste more than it hurts ordinary SMT. SAC-type lead-free pastes wet and coalesce less aggressively than the old SnPb pastes, so overprinted paste that used to flow and wick cleanly into a barrel now tends to stay put or form stray solder balls. On a lead-free PiP build, lean harder on tight holes, ENIG, and preforms — the wicking generosity you remember from leaded paste isn’t coming to save a marginal design.

Reflow Profile for Pin-in-Paste and High-Mass Connectors

The same single profile has to fully wet a tiny 0402 and a chunky metal connector on the same board. The connector is the problem: it’s a thermal sink that shadows the barrel from convection, so the holes can lag well behind the SMT joints in temperature. A profile that’s perfect for the chips can leave the connector barrels below liquidus.

Practical targets and fixes for the through-hole side:

• Lead-free peak around 235–245°C with 45–90 seconds time above liquidus (TAL) — enough to wet the heavy barrel without cooking heat-sensitive SMT parts.
• A gradual preheat/soak (often 150–200°C for 60–120 s) lets flux outgas before the metal coalesces, which suppresses voids in the barrel.
• Put thermal reliefs on any plated hole tied to a large copper pour. Without them the pour sinks heat away and the barrel never reaches wetting temperature — the single most common PiP profile failure on power boards.
• Place tall, high-mass through-hole parts away from delicate SMT zones to limit thermal shadowing, and confirm with thermocouples on the actual barrels, not just the board surface.

Counterintuitive note on voids and ramp: a faster ramp doesn’t reduce voiding — it can trap flux volatiles that haven’t had time to escape the deep barrel, leaving gas voids the X-ray will catch later. Give the preheat time to do its job.

Pin-in-Paste vs Wave vs Selective Soldering

Pin-in-paste isn’t always the answer. The decision turns on how many through-hole joints you have, whether the parts survive reflow, and whether the holes can be filled by a print. Here’s the honest comparison.

Factor	Pin-in-Paste (PiP)	Wave Soldering	Selective Soldering
Best fit	Few THT parts on a mostly-SMT board	Many THT joints, simpler boards	A moderate count of THT parts, mixed boards
Process steps	Folds into the single SMT reflow	Separate step after reflow + masking pallet	Separate step, no full immersion
Thermal cycles	One	Two (reflow + wave)	Two, but localized heat
Typical defect rate	Starved/voided if paste volume wrong	First-pass yields can dip near 10% on pallet-masked mixed boards	Often <1% with tuned parameters
Throughput	Fast (no extra line)	Highest for bulk THT	Slower, joint-by-joint
Main limit	Paste volume + part heat tolerance	Bridging/icicles; hard on fine-pitch SMT	Speed and equipment cost

Honest trade-off: pin-in-paste removes a machine, a pallet, and a thermal cycle — real savings on a board with a few connectors and batches under roughly 10,000 units. But it’s unforgiving: it only works if the parts tolerate reflow and the holes can be filled by a print or preform. The moment you have many through-hole joints, oversized barrels, or heat-fragile connectors, wave or selective soldering is the right call. Match the method to the board, not the other way around.

Common Pin-in-Paste Mistakes (DFM Checklist)

Hand this to anyone laying out a board for through-hole reflow:

1. Oversized holes. A barrel much larger than the lead needs more paste than a print can supply — keep it 10–12 mil over the lead.
2. Sizing the aperture without the volume math, forgetting paste is ~50% flux, and printing far too little metal.
3. Specifying a connector that can’t survive ~260°C reflow — molten plastic housings and degraded platings.
4. Long or blunt leads that displace and push paste out of the barrel on insertion.
5. No thermal relief on holes tied to copper pours, so the barrel never reaches liquidus while the rest of the board is done.
6. Overprinting toward a neighboring part instead of toward open real estate, causing bridging or beading onto the component.
7. Ignoring the step keep-out and putting a step-up edge too close to other apertures, wrecking paste transfer nearby.
8. Judging hole fill by the visible top fillet only — without X-ray you miss bottom-side and barrel voids that fail Class 3.

Five Things to Do Monday

• Pull every through-hole part’s datasheet and confirm a reflow-rated max temperature before committing to PiP.
• Run the V(hole) − V(pin) + V(fillet) calculation (or a free StencilCoach-style tool) for each PiP part and size the aperture from it.
• Re-check drill sizes: tighten any hole more than 0.3 mm over its lead.
• Add thermal reliefs to all PiP holes connected to planes, and flag tall connectors near fine-pitch zones.
• Plan X-ray or microsection sampling to verify barrel fill against your IPC-A-610 class.

Frequently Asked Questions About Pin-in-Paste

What is pin-in-paste soldering?

Pin-in-paste, also called through-hole reflow or intrusive reflow, prints solder paste into plated through-holes, inserts the component leads into the paste, and solders the joints during the normal SMT reflow pass. It lets a few through-hole parts be soldered alongside surface-mount components without a separate wave or hand-solder step.

How is pin-in-paste different from regular reflow?

Regular reflow solders surface-mount parts whose paste sits on flat pads. Pin-in-paste extends that same oven pass to leaded parts by filling the hole barrels with paste. The hard part is volume — a flat aperture can’t fill a hole, so you overprint, step the stencil up, or add solder preforms.

Can any through-hole connector be used with pin-in-paste?

No. The part must survive the full reflow peak, typically 245–260°C for several seconds — many connector plastics and electrolytic capacitors can’t. The geometry also has to allow barrel fill and short leads. Always check the datasheet’s reflow rating and review the part with a DFM engineer first.

Why are my pin-in-paste joints starved or missing a bottom fillet?

Almost always insufficient paste. A flat, pad-sized aperture can’t fill a barrel. Fix it by overprinting, stepping the stencil up locally, or adding a preform — and size the aperture from a volume calculation that accounts for paste being roughly half metal by volume. Oversized holes make this worse.

What hole size do I need for pin-in-paste?

Keep the plated hole about 0.25–0.3 mm (10–12 mil) larger than the lead diameter. That’s enough clearance for insertion and barrel fill but small enough that a printable amount of paste fills the hole. Larger holes need more solder than a stencil print can realistically supply.

What hole fill does IPC require for through-hole reflow?

IPC-A-610 generally calls for about 50% vertical fill for Class 2 and at least 75% for Class 3 high-reliability assemblies, with voiding kept low. That target sets your required solder volume. Verify it with X-ray or microsection, since the top fillet alone doesn’t prove the barrel is filled.

Does pin-in-paste save money versus wave soldering?

For a mostly-SMT board with only a few through-hole parts and batches under roughly 10,000 units, yes — it removes a wave or selective machine, a masking pallet, extra flux and cleaning, and a second thermal cycle. With many through-hole joints or heat-fragile parts, wave or selective soldering is more economical.

Making Pin-in-Paste Work on Your Board

Pin-in-paste rewards engineering and punishes guesswork. Confirm the parts survive reflow, keep holes only 10–12 mil over the lead, size the aperture from the volume math with overprint or preforms, add thermal reliefs on planed holes, and verify barrel fill against your IPC-A-610 class. Do that and you delete an entire soldering step from a mixed-technology board with no loss of joint strength — the same single-pass thinking behind a well-tuned reflow profile.

Not sure a connector will fill — or survive — in a single reflow pass? Send us your Gerber and BOM for a free DFM review and we’ll flag hole sizes, paste volume, and any part that can’t take the heat before the stencil is cut.