Through-Hole Construction

Through-hole (PTH — Plated Through-Hole) construction is the technique of populating a PCB by inserting component leads through drilled holes and soldering them on the opposite side. It predates surface-mount technology and is still the dominant method for kits, repair work, power components, connectors, and any component that must be socketed or replaced. Virtually every ham radio kit — from QRP transceivers to antenna tuners — uses through-hole assembly. This lesson takes you from bare board to populated, soldered, and inspected assembly.

Complete through-hole assembly sequence: empty board, lead forming, component insertion, DIP socket alignment, electrolytic capacitor polarity, lead bending to hold parts, soldering sequence, and lead trimming.

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Board Anatomy

A through-hole PCB has two sides: the component side (top) where parts are inserted, and the solder side (bottom) where solder joints are made. Most hobby and kit PCBs use FR4 (fiberglass-reinforced epoxy laminate) substrate, which is mechanically strong and handles soldering temperatures without delaminating. Older or cheaper boards use phenolic resin — brown and more brittle, adequate for audio frequencies but not recommended for RF above a few MHz.

Each hole is surrounded by a circular copper land called a pad. The pad connects to copper traces that route signals between components. The hole itself is plated — the barrel of the hole is coated with copper to electrically connect the top and bottom pad. The outer ring of each pad is called the annular ring. Silkscreen printing on the component side shows component outlines, reference designators (R1, C1, U1), and polarity markings.

Board Preparation

Before inserting a single component, prepare the board:

1. Visual inspection: Check for manufacturing defects — cracked traces, missing plating in holes, missing pads. On kit boards, check that the silk screen markings match the BOM (bill of materials) for the kit.
2. Clean the pads: If the board has been in storage and shows any oxidation (dull or dark pads), clean it with IPA (isopropyl alcohol) on a lint-free cloth. Do not use acetone — it attacks the silkscreen ink and some solder mask materials.
3. Sort components: Organize all components into labeled groups before starting. Resistors bend and fall; capacitors are often unmarked; ICs are ESD-sensitive. Pre-sort into a parts organizer or tape them to a piece of paper with their reference designators labeled.
4. Verify values: For resistors, measure or read the color code for each value and confirm it matches the BOM before inserting. An incorrectly placed resistor that is already soldered takes 10 minutes to remove; confirming the value takes 5 seconds.

Lead Forming

Component leads must be bent to match the hole pitch before insertion. Incorrect lead forming results in components that do not sit flat, leads that miss holes, and stress on the component body.

Lead forming: axial resistors and diodes bent 90° on a forming jig to match pad pitch; radial capacitors with straight leads at correct hole spacing; correct 1–2 mm body standoff clearance.

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Axial Components (Resistors, Diodes, Small Capacitors)

Axial components have leads emerging from each end. Use a lead-forming jig — a small tool that bends both leads simultaneously at the correct pitch. Bend at the body shoulder, not at the lead-body junction (which can crack the glass or ceramic body). The bend must be 90° and clean. A common mistake is making a gradual curve rather than a sharp right-angle bend — gradual bends cause the component to sit at an angle and allow the body to move during soldering.

For axial resistors: the standard pitch on kit boards is 10 mm (0.4"), 12.5 mm (0.5"), or 15 mm (0.6"). Confirm the hole pitch before forming. For axial diodes: note which end is cathode (the end with the stripe) before forming, because the diode will look the same from either end once inserted.

Radial Components (Most Capacitors, LEDs, Small Transistors)

Radial components have both leads emerging from the same end. Do not bend radial leads — insert them straight. The hole pitch is fixed by the component; if the holes are too close or too far apart, the component can be gently squeezed or spread at the body (not at the lead-body junction), but never force it. A 2 mm gap between the component body and the board surface (achieved by the natural lead length after insertion) is the standard standoff.

Jumper Wires

If the kit requires bare wire jumpers (short connections on a single-layer board), cut tinned copper wire to exact length, strip both ends 5 mm, bend to fit, and insert. Tin the bare ends before inserting if the wire is not already tinned.

Population Order: Shortest First

The most important rule of through-hole assembly: always install the shortest components first and the tallest last.

The reason is mechanical. To solder components, the board must be turned upside down (or held inverted) so the solder side faces up. Gravity then holds the shorter components against the board while the board is inverted. Tall components fall out when the board is inverted, requiring tape or clamps — much more complex than simply installing them last.

The correct population sequence from first to last is:

1. Jumper wires — bare wire links, lowest profile
2. Resistors — standard ¼W or ½W, typically 6–7 mm tall
3. Small signal diodes — 1N4148, 1N914 etc. — similar height to resistors
4. Ceramic disc capacitors — typically 5–8 mm tall depending on value
5. IC sockets — flat, align flush with the board
6. Film capacitors (polyester, polypropylene) — taller than ceramics
7. Transistors (TO-92) — small plastic body, leads at 2.54 mm pitch
8. Electrolytic capacitors — tall cylindrical body; polarity critical
9. Trim potentiometers, crystals, larger diodes (1N4001 etc.)
10. Large connectors, transformer, heat-sinked power components
11. ICs — inserted into sockets last of all

After installing each group, invert the board (lean it against a box or use an inverted PCB vise), solder all leads from that group, and trim them before proceeding to the next group. Do not insert all components and then solder — components inserted but not yet soldered can shift and fall when the board is moved.

IC Sockets

For DIP (Dual In-line Package) ICs, always use sockets. A socket allows the IC to be removed and replaced without desoldering. It also protects the IC from heat during soldering — CMOS ICs and microcontrollers can be damaged by soldering temperatures if soldered directly.

Socket Types

Alignment

Every DIP socket and IC has a notch or dot marking pin 1. The silkscreen on the PCB shows a matching notch or dot. Align the socket notch with the silkscreen marking before soldering. Tack two diagonal corner pins first, verify the socket is flush and correctly oriented, then solder the remaining pins.

For dual sockets (using one 14-pin socket in place of a 28-pin): this is a common cost-saving approach but introduces a seam at the center. Be aware that pins in the center row where the two sockets meet may have slightly less reliable contact. For critical circuits, use a single correct-length socket.

Polarity-Sensitive Components

Reversing a polarized component can cause circuit failure or component damage. Before soldering each polarized component, verify its orientation against the silkscreen marking.

Insertion Heights and Standoffs

Components should sit at specific heights above the board:

• Resistors, small ceramic caps: Flush to the board (1–2 mm clearance). The lead bends hold the component down during soldering.
• Electrolytic capacitors: 3–5 mm standoff. The cylindrical body must have clearance to allow the safety vent (top or bottom) to operate. Never press an electrolytic flat against the board.
• Power diodes, rectifiers: If dissipating significant power, leave 5 mm clearance for airflow.
• Power transistors (TO-220): Mount vertically with a heatsink, or horizontally against the chassis with a heatsink tab. The mounting screw or clip applies pressure; do not rely on the lead solder joints for mechanical support of the transistor body.
• Crystals: Mount per manufacturer recommendation — some have specified air-gap requirements between the can and the board.

Soldering Sequence

Once components of a given height group are inserted and leads bent slightly outward (about 30°) to hold them against the board, invert the board and solder:

1. Solder the two corner pins of each component first (tack). This locks the component in position.
2. Verify orientation and height while you can still reheat the tack joints easily.
3. Solder remaining pins in any order, working from one end of the board to the other to prevent heat buildup in one area.
4. Allow each joint to cool naturally — do not blow on it.
5. After completing a row, inspect the joints before moving on.

Lead Trimming

After soldering, component leads extending below the solder side must be trimmed. Use flush-cut wire cutters (also called flush cutters or diagonal cutters) — the flat face of the cutter must face toward the board so the cut leaves the minimum stub.

Trim to 1–1.5 mm above the solder fillet. Cutting shorter risks cutting into the solder joint itself. Cutting longer leaves protruding stubs that can short against nearby connections.

Safety: Lead clippings fly with considerable force and are sharp. Always trim with the board oriented so clippings fly away from your face. A piece of tape placed over the joint before cutting will catch the clipping — peel the tape off after.

After trimming, inspect each trimmed joint under a loupe. Trimming can disturb a marginal joint. Any joint that looks dull or irregular after trimming should be re-flowed with fresh flux.

Quality Check

After all components are installed and soldered, perform this three-step check:

1. Solder side: Inspect each joint under magnification — all should be smooth, bright, and concave. No bridges between adjacent leads. No cold or dry joints.
2. Component side: Verify component orientation (stripe on diodes, + on caps, notch on IC sockets). Check that no components sit at an angle. Check heights are appropriate.
3. Electrical: Before applying power, probe with a continuity meter between adjacent pads on the solder side to catch any bridges. Then check supply to ground — should be open circuit (the circuit draw will not be visible without power, but a short will show immediately).