THT Assembly Process – Step-by-Step Detailed Explanation

1. Component Preparation

In this initial stage, all required THT components are picked based on the Bill of Materials (BOM). The leads of the components are trimmed and shaped to match the PCB layout using manual tools or automatic lead forming machines. This step ensures that:

• Leads will fit precisely into the drilled holes.

• The height of the components on the PCB remains consistent.

• Insertion becomes smoother and faster.

Proper labeling and sorting also prevent mix-ups during insertion.

2. Manual or Automated Component Insertion

Once prepared, components are inserted into the PCB by aligning their leads with the appropriate holes. This is done in two ways:

• Manual Insertion: Done by trained operators referring to assembly drawings. Care is taken to insert components in the correct orientation (especially for polarized parts like diodes, electrolytic capacitors).

• Automatic Insertion: For high-volume production, axial and radial component insertion machines are used to speed up the process and improve accuracy.

This step requires ESD safety measures to protect sensitive components.

3. Lead Clipping or Bending

After insertion, the component leads are clipped or bent on the solder side of the PCB:

• Clipping: Excess length of leads is trimmed to ensure they don’t touch other circuits or cause shorts.

• Bending: Leads are slightly bent outward to hold the components in place during the soldering process.

This step stabilizes the component position and prepares the assembly for soldering.

4. Wave Soldering or Selective Soldering

In this crucial step, all the inserted components are soldered to the PCB:

• Wave Soldering: The PCB passes over a wave of molten solder, which bonds the component leads to the copper pads on the board.

• Selective Soldering: Used when mixed technology (SMD + THT) is involved. Solder is applied only to specific areas using a nozzle or mini-wave.

Prior to soldering, flux is applied and the board is preheated to activate flux and avoid thermal stress.

5. Post-Solder Cleaning (If Required)

If rosin-based or water-soluble flux was used, cleaning is necessary to remove flux residues:

• Aqueous Cleaning: Involves deionized water and detergents.

• Solvent Cleaning: Uses specialized chemicals to dissolve flux residues.

This step is important to avoid long-term corrosion, improve reliability, and pass ionic cleanliness standards.

6. Visual Inspection / AOI

The assembly is visually inspected or passed through an Automated Optical Inspection (AOI) system:

• Checks for solder joint quality (no voids, cold joints, or bridges).

• Verifies component presence, orientation, and alignment.

• Identifies any missing or incorrect components.

This step helps ensure early detection of defects before moving to functional testing.

7. Rework and Touch-Up (If Required)

If any defects are found during inspection, rework is performed:

• Defective components are desoldered and replaced.

• Cold or cracked solder joints are retouched with soldering irons.

• Any solder bridges or extra solder is removed.

All rework should follow IPC standards and be documented for traceability.

8. Final QC Inspection

In the final step, the board undergoes a quality control check to ensure it meets all workmanship and electrical standards. This includes:

• Verifying repairs were properly done.

• Confirming no visible defects remain.

• Tagging the board as approved for testing or packaging.

Only boards that pass this inspection proceed to functional testing or the next manufacturing phase.