How to design a PCB for the automatic selective welding process
Selective soldering is a machine-driven process. Once the nozzle path is fixed, the board has to work with it. This guide walks through the layout decisions that decide whether a joint forms in 2 seconds or scorches the laminate: pad geometry, thermal mass, mask openings, pallet support and panel edges. Written for hardware engineers and process engineers who need to release a board that runs on a selective solder machine without hand rework.
In this article
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Key takeaways
What the automatic selective welding process actually does to your board
A selective solder machine moves a small solder wave or a hollow nozzle along a programmed path. It only touches the joints you list in the program. Everything else on the board stays cold. That is the whole point: you can wave-solder a connector after SMT reflow without reheating the 0402 parts next to it.
The nozzle is the constraint. Typical openings run 3–8 mm, and the body behind the opening is wider than the opening itself. A 6 mm nozzle needs roughly 3 mm of free space on every side before the shroud clears the tallest neighbor part. If your layout puts a 5 mm tall electrolytic cap 2 mm from a through-hole pin, the machine will either skip the joint or drag solder across the cap.
Heat comes from two places. The top-side preheat raises the local area to 90–110 °C, and the nozzle adds the rest to reach 245–260 °C at the joint. The board has to let that heat build up at the pin without pulling it into a copper plane. That is a layout decision, not a machine setting.
Cycle time follows the number of joints and the dwell per joint. A 1.6 mm thick board with a 5 mm nozzle and a 20 mm wide pad typically needs 2–4 seconds of dwell. Double the copper and you double the dwell, or you raise the preheat and risk damage to nearby plastic parts.
- 1Machine touches only programmed jointsEverything outside the path stays below reflow temperature.
- 2Nozzle geometry sets the clearance rule3 mm free ring around each joint is a safe starting point.
- 3Heat has to stay localLarge copper pours are the main reason joints fail on the first pass.
Pad geometry and copper balance around each through-hole joint
Size the pad for the nozzle, not for the drill. A through-hole pin in a 1.0 mm hole wants a 1.8–2.2 mm annular pad for selective soldering. Smaller than that and the nozzle cannot deliver enough solder volume before it moves on. Larger than 2.5 mm and you start bridging to the next pin at 2.54 mm pitch.
Connect ground pins with thermal spokes, not a solid pour. Four spokes of 0.3–0.5 mm width, with a 0.3 mm gap, cut the heat sink effect enough that a normal dwell time works. A solid connection to a 50 × 50 mm plane can add 2–3 seconds per joint and still leave a cold fillet.
Keep the copper on the solder side as even as possible. A board with 70 percent copper on one half and 20 percent on the other will need two different dwell settings. That means two programs, two fixtures and more room for error. Balance the pour where you can, and group the selective-solder joints into one region of the board.
Trace width matters less than people expect. Once the pad is large enough, a 0.25 mm trace versus a 0.5 mm trace changes dwell by a few tenths of a second. The plane connection is the number that moves.
- 1Annular pad 1.8–2.2 mmFits a 1.0 mm hole and a 3–5 mm nozzle.
- 2Four thermal spokes0.3–0.5 mm wide, 0.3 mm gap, cuts dwell time.
- 3Group the jointsOne region means one program and one fixture.
Solder mask openings, component height and nozzle clearance
Open the mask 0.1–0.15 mm beyond the pad on every side. That dam is what separates a clean joint from a bridge at 2.54 mm pitch. If you open the mask all the way between two pins, the nozzle wave will carry solder across and you will see it at final inspection.
Height is the harder limit. Measure the tallest part within 5 mm of a selective-solder joint. Over 8 mm and the nozzle body may not clear it during the approach move. Over 12 mm and you usually need a longer nozzle, which changes the flow characteristics and the dwell setting.
Keep connectors and tall headers on the far side of the joint row, not between two rows. A double row of through-hole pins with a 10 mm part in the middle forces the machine to approach from one direction only. That is workable, but it doubles the program length and the risk of a missed joint.
If the layout cannot avoid a tall neighbor, tell the process engineer before the board is released. A different nozzle or a different approach angle is cheaper to plan than to discover on the first article.
- 1Mask dam 0.1–0.15 mmSlightly larger than the pad on all sides.
- 2Keep parts under 8 mm near jointsTaller parts need a longer nozzle and a new program.
- 3One approach direction is fineTwo directions cost cycle time and add missed-joint risk.
Panel edges, pallets and board support for automatic selective welding
The pallet holds the board flat and carries it through the machine. It needs a bare strip on the solder side, usually 5 mm minimum, with no components and no tall vias. If the strip is narrower, the clamp fingers sit on a part and the board tilts. A tilted board changes the gap between nozzle and pad, and the joint goes cold on one side.
Fiducials help the machine find the board, but selective soldering mostly relies on mechanical location. Two 3 mm tooling holes on opposite corners, 0.05 mm tolerance, are more useful than a dozen fiducials. Put them outside the component area so the pallet pins can reach them.
Board thickness drives support. Below 1.0 mm, the preheat will bow the panel unless the pallet supports the middle. Above 2.0 mm, the dwell time climbs and the nozzle wears faster. The common range for selective solder work is 1.2–1.8 mm.
Leave the panel rails intact until after soldering. Depaneling before the selective process means handling loose boards into a pallet, which costs time and adds edge damage. Route the breakaway tabs so the tab keeps the board rigid through the machine.
- 15 mm bare strip on the solder sideGives the pallet clamp a flat surface.
- 2Two 3 mm tooling holes±0.05 mm, on opposite corners, outside the component area.
- 31.2–1.8 mm board thicknessA practical range for preheat and dwell control.
Step by step: designing a board for the automatic selective welding process
- 11. List every joint the machine will touchMark them on the assembly drawing before you start routing. Group them into one or two clusters, not scattered across the board. Count the joints; a machine program costs time per joint, and 40 joints in one area is a different job than 40 joints in six areas.
- 22. Set the annular pad sizeFor a 1.0 mm finished hole, use a 1.8–2.2 mm pad. For 1.3 mm holes, use 2.2–2.6 mm. Keep at least 0.4 mm of mask dam between adjacent pads at 2.54 mm pitch. If the pitch is 2.0 mm, drop the pad to 1.6 mm and expect a slower program.
- 33. Break the plane connectionsReplace solid copper connections on ground and power pins with four thermal spokes, 0.3–0.5 mm wide, 0.3 mm gap. Simulate or measure the dwell on a test coupon. A joint that needs more than 5 seconds is usually a plane problem, not a nozzle problem.
- 44. Clear the nozzle envelopeDraw a 3 mm ring around every selective-solder joint on the layout. Nothing taller than 3 mm goes inside that ring. Nothing taller than 8 mm goes within 5 mm of it. Move tall caps, inductors and connectors away from the joint row before routing, not after.
- 55. Check the mask layerMask openings 0.1–0.15 mm larger than the pad. No mask slivers under 0.1 mm wide. Remove mask between pads only where the machine needs a shared solder reservoir, and confirm that choice with the process engineer.
- 66. Design the pallet interfaceKeep a 5 mm bare strip on the solder side at both long edges. Add two 3 mm tooling holes, 0.05 mm tolerance, on opposite corners. Keep all tall parts off the strip. Check that the strip has no exposed vias that could short against the pallet.
- 77. Set up the panel and thicknessUse 1.2–1.8 mm FR-4 unless the design needs otherwise. Keep the rails attached through soldering. Put the breakaway tabs on the short edges so the board stays rigid. Confirm the panel fits the machine conveyor width before you release the artwork.
- 88. Run a first-article checkBuild one panel and inspect the fillets. Look for cold joints, bridges and scorched mask. Measure the dwell that produced a good fillet and write it into the process sheet. If any joint needed a second pass, fix the layout rather than adding a rework step.
Design choices and what they cost you
Use this to pick a direction before you release artwork.
| Design choice | Good for | Watch out for |
|---|---|---|
| Solid plane connection | High-current pins, few joints | Dwell over 5 s, cold fillets |
| Four thermal spokes | Ground pins on a plane | Spoke width below 0.3 mm |
| 1.8 mm annular pad | 1.0 mm hole at 2.54 mm pitch | Bridges at 2.0 mm pitch |
| 2.4 mm annular pad | 1.3 mm hole, hand access | Bridges to the next pin |
| 0.1 mm mask dam | Fine-pitch through-hole rows | Solder wicking under the mask |
| 0.15 mm mask dam | Most selective-solder boards | Mask slivers on tight pitch |
| 1.2 mm board | Low thermal mass, fast cycle | Panel bow under preheat |
| 1.8 mm board | Stiff panels, many joints | Longer dwell, nozzle wear |
| Rails left on | Machine handling and rigidity | Depaneling after soldering |
| Joints in one cluster | One program, one fixture | Congested routing near the cluster |
Fix the layout, not the rework station
A board designed for the automatic selective welding process runs one program, one fixture and one dwell setting. If a joint needs a second pass, the copper or the clearance is wrong. Change the layout while it is still a file.
Frequently asked questions
Can selective soldering handle both through-hole and SMT parts on the same side?
Yes, and that is where the process earns its place. The SMT parts are already reflowed and stay below their reflow temperature during selective soldering. The limit is height and distance: keep SMT parts under 8 mm tall and at least 5 mm from any joint the nozzle will touch.
If an SMT part sits inside the 3 mm nozzle ring, the machine cannot approach that joint from that side. You can move the part, move the joint, or accept a one-direction approach. Decide during layout, not after the first panel is built.
How much clearance does a selective solder nozzle really need?
Plan on 3 mm of free space around the joint for the nozzle body and nitrogen shroud. Some machines with a small 3 mm nozzle get by with 2 mm, but that is machine-specific and worth confirming before you lock the layout.
Height is the second number. A part taller than 8 mm within 5 mm of a joint usually forces a longer nozzle. Longer nozzles change the flow, so the dwell setting from your test coupon may not transfer.
Does thermal relief always help?
It helps when the joint connects to a large plane. Four spokes of 0.3–0.5 mm cut the dwell time noticeably and give a more repeatable fillet.
It does not help on signal pins with no plane connection, and it can hurt if you use it on a pin that needs high current. For power pins, keep a solid connection and accept the longer dwell, or move the pin off the plane and run a wide trace instead.
What board thickness should I specify?
Most selective-solder work sits between 1.2 mm and 1.8 mm. Thinner boards bow under preheat unless the pallet supports the middle. Thicker boards need more dwell and wear the nozzle faster.
If the design must be 0.8 mm, add pallet support and confirm the preheat profile with the machine operator. If it must be 2.4 mm, budget for longer cycle time and check nozzle life.
Should I depaneling before or after selective soldering?
After. Loose boards are harder to load into a pallet, and the edges are exposed during handling. Keep the rails attached, run the panel through the machine, then depanel.
Route the breakaway tabs on the short edges so the panel stays rigid. Tabs on the long edges can flex when the pallet clamps down, which changes the nozzle-to-pad gap.
How do I know the layout is ready for production?
Run a first-article panel. Inspect every fillet, measure the dwell that produced a good joint, and check for mask scorching near the nozzle path. If one joint needed a second pass, treat it as a layout problem first.
Write the confirmed dwell times and approach directions into the process sheet. That sheet is what makes the second run match the first.
Send us the board and the joint list
Upload your Gerber files and assembly drawing. We review pad geometry, plane connections and nozzle clearance, and we send back a DFM note with the selective-solder joints flagged. Quotation and DFM analysis within 12 hours.
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