Standard Process for PCB Reflow Soldering

Standard Process and Key Control Points for PCB Reflow Soldering:

I. Pre-soldering Preparation

Solder Paste Printing

Use a steel mesh to precisely print solder paste onto the PCB pads. The openings in the steel mesh must be strictly aligned with the pad positions.

→ Printing quality directly affects soldering yield rate

Component Placement

The pick-and-place machine uses a vision positioning system to precisely place SMD components on the solder paste.

Special components require manual re-inspection of position accuracy

2. Soldering Process (Four-Stage Temperature Control)

Preheating Zone → Holding Zone → Reflow Zone → Cooling Zone

Preheating Zone (60–150°C)

Temperature rise rate: 1–3°C/second

Function: Evaporate solvents, activate flux, prevent thermal shock

PCB heats uniformly until solder paste softens and collapses

Holding Zone (120–165°C)

Constant temperature time: 60–120 seconds

Function: Evaporate residual moisture, remove oxide from pads

Ensure uniform temperature across all PCB areas

Solder paste fully melts, liquid tin wets pads to form alloy solder joints

Peak temperature duration: 30–60 seconds

‌Cooling Zone‌

Forced air cooling rate: 2–4°C/second

Function: Rapidly solidify solder joints to form stable metal connections

III. Special Process Treatment

‌Double-sided PCB Welding‌

Prioritise welding the side with fewer components (red glue process), then use solder paste process for the second side; or adopt layered temperature control (lowering temperature in the lower zone)

‌Nitrogen Protection‌

Inject nitrogen into the furnace to reduce oxidation and enhance solder joint reliability

‌Through-hole components‌

Customised needle tubes and steel screens are required to increase solder paste volume; components must withstand temperatures of 235-250°C‌

4. Post-soldering inspection

‌Cool to room temperature‌ before removing the PCB to avoid burns or thermal deformation‌

Inspect solder joints:

Smooth surface with no voids

Wetting angle ≤90°

No component displacement or cold solder joints