Through‑Hole Assembly

1. What Is Through-Hole Assembly?

Through‑Hole assembly (often abbreviated as THT) is a circuit board manufacturing process where axial, radial, or multi-pin component leads are inserted into pre-drilled through holes on the printed circuit board and soldered onto pads on the opposite side.

Typical Through‑Hole components include leaded resistors, capacitors, relays, connectors, and certain power-device packages. The specific component package and application determine how it should be mounted and supported.

Three terms need distinction:

• Through‑Hole technology, or THT: The mounting approach.
• Plated Through‑Hole, or PTH: A hole with conductive plating along its wall.
• Through-hole assembly: The process of installing and joining the components.

2. The Through-Hole PCB Assembly Process

Executing a high-yield through hole PCB assembly run requires controlled lead preparation, precise insertion, and thermal management.

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| 1. Component Lead Bending & Pre-forming               |
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| 2. Insertion (Manual Line or Automated AI Machinery)  |
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                           v
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| 3. Soldering Stage (Wave, Selective, or Manual Soldering)|
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                           |
                           v
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| 4. Post-Solder Aqueous / Solvent Cleaning             |
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                           |
                           v
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| 5. Inspection (AOI, X-Ray Barrel Fill, Ionic Testing) |
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• Lead Preparation: Components arrive in bulk, tape-and-reel, or ammo boxes. Leads must be trimmed and pre-formed to fit pad spacings without placing stress on the component body.
• Insertion Methodologies:
  • Manual Assembly: A skilled through hole assembly operator manually places complex, irregular, or heat-sensitive connectors onto the board.
  • Automated Through Hole Assembly: High-speed axial and radial insertion machines automatically punch leads into the PCB at high UPH (Units Per Hour) speeds. (Data needed: specify specific UPH throughput limits comparing high-speed AI equipment against manual assembly lines).
• Securing Leads: Leads are slightly clinched on the solder side to prevent component floating or displacement during transit to the soldering stage.

3. Through‑Hole vs. SMT: Choosing the Assembly Method

Through-hole and surface-mount technology should be compared against the actual product requirements rather than ranked by a universal reliability claim.

4. DFM Guidelines for Through‑Hole Design and Lead Preparation

Implementing strict Design for Manufacturability (DFM) rules reduces manufacturing defect rates and avoids costly manual rework.

• Pin-to-Hole Ratio: The plated through-hole (PTH) diameter must be sized correctly relative to the lead diameter. If the hole is too tight, solder cannot flow up the barrel; if it is too loose, capillary action fails, leaving insufficient solder volume. (Data needed: specify recommended drill-to-lead diameter clearance ratios, typically 0.15mm–0.25 mm allowance).
• Thermal Relief Design: Pins connected to internal ground or power planes require thermal relief pads with spoke patterns. Without thermal relief, the copper plane acts as a heat sink, drawing energy away during through hole soldering and causing cold solder joints.
• Lead Protrusion & Component Pitch: Adequate clearance around through-hole pads allows automatic insertion heads and selective solder nozzles to navigate without colliding with adjacent SMT parts.

5. Quality Control and Cleanliness Verification in THT Manufacturing

High-density through-hole joints accumulate flux residues inside barrel cavities and under heavy component bodies, making cleaning and verification vital.

• Flux Residue Trapping: Wave and selective fluxing apply active organic or rosin-based compounds that can trap active halides inside plated barrels if not flushed thoroughly.
• Ionic Contamination Control: Uncleaned flux residues absorb atmospheric moisture, triggering electrochemical migration (ECM) and dendritic growth beneath connectors.
• Process Integration with Welleltron Instruments: Contract manufacturers use Welleltron ionic contamination test equipment to execute accurate ROSE (Resistivity of Solvent Extract) testing on post-cleaned boards. Measuring extract conductivity validates that residual ionic salts fall within acceptable limits before boards move to final enclosure assembly.

6. Common THT Soldering Defects and IPC-A-610 Inspection Criteria

Visual and X-ray inspection protocols evaluate solder joints against international acceptability standards like IPC-A-610.

• Insufficient Vertical Barrel Fill: Solder fails to wick up the plated through-hole. IPC-A-610 Class 2 and Class 3 require a minimum of 75% vertical solder fill inside the plated barrel.
• Solder Bridging (Shorts): Excess solder creates unwanted conductive pathways between adjacent pins, common when lead protrusion lengths are too long or wave chain speed is miscalibrated.
• Solder Icicles and Voiding: Caused by incorrect flux activation, degraded solder purity, or improper temperature profiling during wave exit.
• Blowholes and Pinholes: Trapped moisture inside the PCB laminate vaporizes during thermal exposure, outgassing through the molten solder joint and leaving cavities.

       ACCEPTABLE (Class 2/3)                    DEFECT (Insufficient Fill)
      
    |  |  | Solder |  |  |                   |  |  |          |  |  |
    |  +--+--------+--+  |                   |  +--+          +--+  |
    |  |  |  Lead  |  |  |                   |  |  |  Lead    |  |  |
====+==+==+========+==+==+====           ====+==+==+==========+==+==+====
    |  |  |  >>>>  |  |  |                   |  |  |  Air     |  |  |
    |  |  | >= 75% |  |  |                   |  |  |  Gap     |  |  |
    |  |  | Barrel |  |  |                   |  |  | < 75%    |  |  |
    |  |  |  Fill  |  |  |                   |  |  | Fill     |  |  |
====+==+==+========+==+==+====           ====+==+==+==========+==+==+====
    |  +--+--------+--+  |                   |  +--+--------+--+  |
    |  |  | Solder |  |  |                   |  |  | Solder |  |  |

7. Frequently Asked Questions on Through-Hole Assembly

• Q1: When should a hardware design specify THT over SMT?
  • A: THT is preferred when components experience strong mechanical forces (e.g., switches, heavy connectors), carry high current/voltage loads, or operate in high-vibration environments like automotive or aerospace systems.
• Q2: What is the minimum acceptable vertical barrel fill for IPC Class 3 THT joints?
  • A: Under IPC-A-610 standards, Class 3 high-reliability electronics require a minimum of 75% vertical solder fill inside the plated through-hole barrel.
• Q3: How does selective soldering protect nearby surface-mount devices?
  • A: Selective soldering uses targeted miniature solder nozzles and drop-jet fluxers to apply heat strictly to specific through-hole pins, keeping thermal exposure away from neighboring SMT parts on the board bottom.
• Q4: What Should Be Checked After Through-Hole Rework?
  • A: Inspect the affected connection, hole and pad condition, nearby components, and cleanliness. Complete the required electrical or functional retesting and document the release decision.