3.1 THT-Friendly Design

Through-Hole Technology (THT) remains mandatory for mechanical stability (connectors, relays), high-power components, and joints requiring superior mechanical strength. However, THT only functions smoothly when the PCB layout is designed for 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.

1.3.1.1 The DFM Mandate: Solder Flow and Barrel Fill

Successful automated THT soldering requires the molten solder to climb the pin and fill the 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 is critical for flux to escape and solder to enter.

Board/Copper Condition	Finished Hole ∅ (vs. Max Lead ∅)	Rationale
Standard Board (< 2.0 mm, < 2 oz Cu)	+0.20 – 0.30 mm clearance	Provides optimal space for capillary rise and venting.
Thick/Heavy Copper (≥ 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).

1.3.1.2 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.

1.3.1.3 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.

1.3.1.4 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.

1.3.1.5 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	Annular ring ≥ 0.25 mm radial copper; mask must be relieved.
Thermal Balance	Pins connected to planes must use 4-spoke thermal relief (0.25 mm spoke width minimum).	Verify thermal spokes on both top and bottom connection layers.
Layout	Pin rows must be perpendicular to the wave direction where possible; solder thieves added to fine-pitch rows.	3 – 4 mm keepout maintained around THT pins for SMT components.
Lead Prep	Target post-solder lead protrusion must be 0.5 – 1.5 mm.	No clinching unless specified for mechanical retention.
Fixturing	Tooling holes and flat sealing rails must be included for stable palletization.	Pallet window edges ≥ 2.5 mm from nearest solder pad.