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1.1 THT-Friendly Design

Through-holeHole technologyTechnology (THT) remains essentialmandatory for mechanical stability (connectors, relays), high-power components, and high-reliabilityjoints joints,requiring butsuperior itmechanical strength. However, THT only worksfunctions smoothly when the boardPCB layout is designed withfor the specific dynamics of molten solder flow (wave or selective). Disciplined THT design—controlling hole clearance, thermal balance, and orientation—directly reduces bridging, non-fills, and rework, protecting the entire assembly process yield.

1.1.1 The DFM Mandate: Solder Flow and Barrel Fill

Successful automated THT soldering inrequires mind. A THT-friendly layout removes hidden obstacles for wave and selective soldering, ensuringthe molten solder flowsto naturallyclimb withoutthe constantpin adjustments.and Byfill controllingthe entire plated barrel via capillary action. This requires precise control over hole geometry and local thermal management.

The Success Levers (Hole Size and Pad Stack)

Hole Clearance: The space between the plated hole wall and the component lead clearance,is padcritical geometry,for thermalflux relief,to escape 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 needsolder to slow the wave or babysit selective nozzles. That’s all design.enter.



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)

MaxBoard/Copper lead ØCondition

Finished holeHole Ø∅ (vs. Max Lead ∅)

Pad Ø (with 0.30 mm annular ring)Rationale

Standard Board0.50 mm(< 2.0 mm, < 2 oz Cu)

+0.70–20 – 0.8030 mm clearance

1.30–1.40Provides mmoptimal space for capillary rise and venting.

Thick/Heavy Copper0.80 (≥ 2.0 mm or ≥ 2 oz Cu)

+0.30 – 0.45 mm clearance

Increased volume and thermal mass require greater flow space.

Annular Ring: The radial copper surrounding the hole must be robust to withstand mechanical stress and multiple rework cycles. Target 0.25 – 0.30 mm radial copper (0.5 mm total copper beyond the finished hole edge).

1.00–1.2 Thermal Management: Controlling Heat Sinks

A major cause of non-fill and cold joints in THT is a lack of localized heat. Pins connected directly to large copper planes or heavy ground pours act as heat sinks, starving the joint of thermal energy.

  • Thermal Relief Spokes: Pins connecting to internal planes must use thermal relief pads with defined spokes. This limits the cross-sectional area of copper drawing heat away from the joint while maintaining the electrical connection.
    • Mandate: Use 4 spokes (default) with a 0.25 – 0.40 mm width. If both top and bottom layers connect to planes, apply symmetrical relief on both.
  • Copper Balance: Design copper pours to be symmetrical near dense pin fields (e.g., connectors) to prevent localized cold spots that cause inconsistent barrel fill.

1.1.3 Layout and Orientation for Automated Soldering

The physical orientation of pins relative to the solder wave or selective nozzle movement dictates the risk of bridging.

  • Wave Soldering (Bulk): Run long pin rows perpendicular to the wave direction where possible. Parallel rows create channels that trap solder, leading to bridging.
  • Solder Thieves (Robber Pads): Add small, non-functional copper pads downstream of fine-pitch THT pin rows to intentionally pull excess solder off the last pin, preventing bridging icicles.
  • Mixed Technology Keepouts: SMT components must be kept ≥ 3 – 4 mm away from the THT pins to avoid being hit by solder splash during wave/selective processes. SMT components near THT must be secured with glue/epoxy or placed on the top side to avoid being knocked off during THT processing.

1.1.4 Lead Preparation and Fixturing

The preparation of the component lead and the final fixturing method influence the mechanical success of the joint.

  • Lead Protrusion: The final, cut length of the component lead after soldering should be 0.5 – 1.5 mm above the top pad. Too short prevents complete barrel fill; too long wastes material and increases the risk of damage during handling.
  • Clinching: For automated soldering (wave/selective), leads must remain straight (no clinch) unless specified otherwise. Clinched leads block the flow of solder and trap flux.
  • Pallets/Fixtures: If a wave or selective pallet (fixture) is required, the PCB must include tooling holes and panel rails for stable, repeatable indexing. Pallet window edges must be ≥ 2.5 – 3.0 mm away from the solder joint to maintain a clean solder dam.

1.1.5 Top Design Mistakes and Quick Fixes

Design Mistake

Symptom

First Fix (DFM Tweak)

Hole clearance too tight

Random non-fills (solder starvation).

Increase hole ø by +0.1 mm and verify the annular ring remains adequate.

Solid pours to THT pins

Cold joints or excessive hand-solder time.

Apply 4-spoke thermal relief (0.25–0.40 mm wide spokes).

Rows parallel to wave

Solder bridging between pins.

Rotate the component if possible, or add solder thief pads at the end of the row.

Small annular ring

Pad lift/crater during manual rework.

Increase copper area (≥ 0.25 mm radial) and use teardrops on thin traces.

Final Checklist: THT DFM Audit

Category

DFM Mandate

Verification Requirement

Hole Geometry

Finished hole ø must be ≥ lead ø + 0.20 mm

1.60–1.80Annular ring ≥ 0.25 mm radial copper; mask must be relieved.

Thermal Balance

1.00Pins connected to planes must use 4-spoke thermal relief (0.25 mm spoke width minimum).

1.30–1.50Verify mm

1.90–2.10thermal mmspokes on both top and bottom connection layers.

Layout

1.30Pin mmrows must be perpendicular to the wave direction where possible; solder thieves added to fine-pitch rows.

1.60–1.903 – 4 mm

2.20–2.50 mmkeepout maintained around THT pins for SMT components.

Lead Prep

Target post-solder lead protrusion must be 0.5 – 1.605 mmmm.

No clinching unless specified for mechanical retention.

Fixturing

Tooling holes and flat sealing rails must be included for stable palletization.

Pallet window edges ≥ 2.00–2.305 mm

2.60–2.90 mmfrom nearest solder pad.

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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