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1.1 THT-Friendly Design

Through-hole technology remains essential for connectors, power components, and high-reliability joints, but it only works smoothly when the board is designed with soldering in mind. A THT-friendly layout removes hidden obstacles for wave and selective soldering, ensuring molten solder flows naturally without constant adjustments. By controlling lead clearance, pad geometry, thermal relief, and pallet compatibility, design choices upstream determine whether assembly runs effortlessly or becomes a fight against defects.

1.1.1 What “solders well” means (in factory words)

  • Leads enter easily, sit straight, and don’t wobble.
  • Holes wet through: a neat fillet on the bottom and visible wetting on top, with solid barrel fill (aim high; don’t design for “barely enough”).
  • No bridges across pin rows, even on thick boards.
  • Techs don’t need to slow the wave or babysit selective nozzles. That’s all design.



1.1.2 Hole size & pad stack (the #1 success lever)

Give solder a fair path up the barrel and leave copper to grab it.

Clearance rule of thumb (finished hole vs max lead Ø)

  • General THT (wave/selective): lead Ø + 0.20–0.30 mm (8–12 mil).
  • Thick boards (≥2.0 mm), heavy copper (≥2 oz), or thermally hungry pins: lead Ø + 0.30–0.45 mm.
  • Square leads: size to the diagonal (≈ 1.414 × side) + clearance.

Annular ring (radial copper beyond hole edge)

  • Comfortable: 0.25–0.30 mm per side.
  • Minimum you’ll regret: 0.15–0.20 mm (use only if space is brutal, then protect with good fab capability).

Quick chooser (finished sizes, round leads)

Max lead Ø

Finished hole Ø

Pad Ø (with 0.30 mm annular ring)

0.50 mm

0.70–0.80 mm

1.30–1.40 mm

0.80 mm

1.00–1.20 mm

1.60–1.80 mm

1.00 mm

1.30–1.50 mm

1.90–2.10 mm

1.30 mm

1.60–1.90 mm

2.20–2.50 mm

1.60 mm

2.00–2.30 mm

2.60–2.90 mm

Design notes

  • Specify finished hole (not drill) and let fab account for plating.
  • Use teardrops on pads tied to thin traces—saves pads from peel-outs.
  • Avoid mask in the hole (no tenting): open mask 0.10–0.15 mm larger than pad.



1.1.3 Lead preparation & protrusion

Leads that are too long bridge; too short don’t show a clean fillet.

  • After solder protrusion target: 0.5–1.5 mm above the top pad (domed, wetted).
  • Before solder cut length: plan BOM/forming so trimmed leads land near that window.
  • Clinching?
    • Selective/wave: no clinch (straight leads) unless called out; clinches block nozzle flow and trap flux.
    • Hand-solder/repair: a 5–15° light clinch can help hold parts—keep within land so the iron can actually reach.
  • Avoid “spears”: specify de-burred, plated lead ends on big posts and tabs.



1.1.4 Thermal reliefs & copper balance (heat is a design choice)

Through-hole to big planes is a soldering tax unless you add relief.

  • Thermal relief spokes:
    • 4 spokes per pad (default), 2–3 on fine pitch.
    • Spoke width: 0.25–0.40 mm (10–16 mil).
    • Use both layers if both connect to planes; keep symmetry so heat is even.
  • Avoid solid pours to planes on THT pins—unless you want hand-solder only.
  • Balance copper around dense headers to limit local heat sinks; a little hatch or thieving copper helps.
  • On very hungry pins (chassis lugs, big inductors), give extra clearance and wider spokes or plan preheat at selective.


1.1.5 Layout for wave/selective: orientation & thieves

Small geometry nudges pay back with fewer bridges.

  • With wave direction: run rows perpendicular to the wave if possible; long parallel troughs love to bridge.
  • Solder thieves (“robber pads”): add small tail pads at the end of fine-pitch pin rows (downstream side) to pull bridges off the last pin.
  • Staggered pins: if you control the connector, stagger or alternate pad sizes to break straight solder dams.
  • Pin spacing: ≥ 2.54 mm rows are easy; ≤ 2.00 mm rows demand perfect hole sizing + thieves.
  • Keep SMT away from miniwave splash zones: 3–4 mm radius clear is a good start unless pallet masks them.



1.1.6 Pallets & keepouts (make the fixture your friend)

Wave/selective pallets are glass-filled composites that expose only what you want soldered.

  • Window edges: keep pads ≥ 2.5–3.0 mm from pallet cutouts so dams seal.
  • Component height inside windows: confirm tallest part < pallet pocket depth; add soft standoffs where needed.
  • Tooling & support: add tooling holes and panel rails (Ch. 2.5) so pallets carry the panel without flex.
  • Gasketing surfaces: avoid vias/mask steps under pallet seals; keep them flat copper/mask.
  • Selective nozzle access: reserve a 3–4 mm component keepout around each THT pad group for the miniwave; turn tight clusters into small islands the nozzle can circle.
  • Flux shadowing: large shields/heatsinks? Give a flux access slot or plan a second pass with different attack angle.



1.1.7 Board thickness & copper weight (adjust the holes)

Thick + heavy copper = more drag on molten solder.

  • If board ≥ 2.0 mm or internal planes ≥ 2 oz, push the hole clearance toward the upper end (see 12.1.2).
  • Give more annular ring (≥ 0.30 mm) so the pad doesn’t crater during rework.
  • Expect longer dwell at selective; design spacing so heat doesn’t cook nearby plastics.



1.1.8 Mixed technology (SMT + THT without tears)

  • SMT near THT? Either pallet-mask the SMT during wave or run selective solder. Don’t count on glue to survive forever.
  • Sequencing: typical flow is SMT → reflow → selective/wave for THT → AOI/test. Design access for that miniwave after tall SMT goes down.
  • Heat-sensitive parts: keep them out of splash and away from high-dwell THT pins; add shields or move them.


1.1.9 Materials & finishes (solderability starts here)

  • Lead finishes on parts: matte Sn or SnPb (if allowed) wet best; Ag/Ni finishes age—watch date codes.
  • PCB finish: ENIG and ImmAg are THT-friendly; OSP is fine but don’t age it—incoming checks matter.
  • Mask clearance: relieve mask 0.10–0.15 mm around pads so flux and solder can flow; avoid “donuts” that trap flux.


1.1.10 Top 7 design mistakes (and the quick fix)

  1. Holes too tight → random non-fills. Fix: +0.1–0.2 mm hole.
  2. Tiny annular rings → pads lift during rework. Fix: +0.1 mm radial copper; add teardrops.
  3. No thermal relief to planes → cold joints. Fix: 4 spokes, 0.25–0.40 mm.
  4. Rows parallel to wave → bridges. Fix: rotate or add thieves.
  5. SMT crowding miniwave → splash defects. Fix: 3–4 mm keepout or pallet mask.
  6. Lead spears → icicles & shorts. Fix: pre-form/cut; target 0.5–1.5 mm final protrusion.
  7. Pallet window too tight → leaks and mess. Fix: ≥2.5–3.0 mm dam; flatten under seals.

1.1.11 Pocket DFM checklist (print with your drawing)

Holes & pads

  • Finished hole = lead max + 0.20–0.30 mm (thick boards: +0.30–0.45)
  • Annular ring ≥ 0.25–0.30 mm radial; mask relieved; teardrops on fine traces
  • Square leads sized to diagonal + clearance

Thermals & copper

  • 4-spoke thermals to planes (0.25–0.40 mm)
  • Copper balance near dense headers; thieves on row ends

Wave/selective access

  • Rows perpendicular to wave where possible
  • 3–4 mm miniwave keepout from SMT; pallet dams ≥ 2.5–3.0 mm
  • Tooling holes & rails present; flat sealing surfaces for pallet

Leads & parts

  • Post-solder protrusion 0.5–1.5 mm; no clinch for selective
  • Heat-sensitive parts away from long-dwell pins

Notes

  • Call out finished hole sizes and thermal relief style
  • Identify wave direction or selective path on the assy drawing
  • Include acceptance targets (barrel wetting, fillet form) on the fab/ass’y notes


Done well, THT design turns soldering into a routine, low-risk process—parts seat cleanly, joints fill fully, and bridges disappear. These practices not only improve yield and reliability but also reduce rework and inspection overhead, keeping production fast and predictable.


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