Soldering procedure of flexible circuit board

Before soldering the flexible Circuit board, the pad is coated with flux and treated with soldering iron to avoid improper tin plating or oxidation of the pad, resulting in lousy welding. The chip does not need to be processed.

 
Carefully place the PQFP chip on the PCB board with tweezers, and be careful not to damage the pin. Align the chip with the pad and ensure that the chip is placed in the right direction. Set the temperature of the soldering iron to more than 300 degrees Celsius, dip the tip of the metal with a small amount of solder, use the tool to hold down the chip in its position, add a small amount of flux to the two diagonal pins, and still hold the chip down. Weld the two diagonal pins so that the chip is fixed and immobile. Recheck the position of the chip after the diagonal welding. If necessary, adjust or remove and realign the position on the PCB board.

 
When starting to weld all pins, the solder should be added to the tip of the soldering iron, and all pins should be coated with flux to keep them moist. Use the soldering iron tip to contact the end of each chip pin until the solder flows into the pin. Keep the tip of the soldering iron parallel to the welded pin during welding to prevent the solder from overlapping.

 

After welding all pins, soak all pins with flux to clean the solder. 

Remove excess solder where necessary to eliminate any short-circuiting and lapping. Finally, use tweezers to check for virtual soldering. After checking, remove the flux from the circuit board, dip the harrowing brush with alcohol in the direction of the pin and wipe it carefully until the flux disappears. The resistive and capacitive components of the patch are relatively easy to weld. First, the tin can be placed on a solder joint, then one end of the element can be placed, and then the part is clamped with tweezers. If you have put it straight, weld the other end.


In the layout of flexible circuit board welding, When the circuit board size is too large, the welding is easy to control, but the printed line is long, the impedance is increased, the ability of anti-noise is decreased, and the cost is increased. If it is too small, the heat dissipation will drop, the welding is difficult to control, and the adjacent lines will interfere with each other, such as the electromagnetic interference of the circuit board. 

Therefore, the Victory PCB board design must be optimised:

Shorten the connection between high-frequency components and reduce EMI interference.
Heavy (if more than 20 g) components shall be fixed with a bracket and welded.
Heat dissipation should be considered to prevent the defect and rework of Δ T on the surface of the heating element, and the heat-sensitive element should be far away from the heat source.

 
The arrangement of components is as parallel as possible, so it is not only beautiful and easy to weld but also suitable for mass production. The circuit board is designed as a 4:3 rectangle (preferred). Do not change the width of the wire to avoid discontinuity in the wiring. The copper foil will expand and fall off easily when the circuit board is heated for a long time. Therefore, a large area of copper foil should be avoided. If you want to learn more about flexible PCB, Pls visit Rayming Technology.

Soldering a Flexible Circuit Board (FPC) requires special attention due to its delicate and heat-sensitive nature. Below is a step-by-step soldering procedure for FPC:

1. Preparation

a. Gather Necessary Tools and Materials

• Soldering iron (Temperature controlled, 280–320°C)
• Fine-tipped soldering iron tip (preferably conical or chisel tip)
• Lead-free solder (SnAgCu or SnPb)
• Flux (No-clean flux recommended)
• Tweezer (Anti-static)
• Kapton tape (for heat resistance)
• Magnifying glass or microscope (for fine-pitch components)
• Heat-resistant mat

b. Pre-clean the FPC

• Clean the soldering pads with isopropyl alcohol (IPA) to remove dirt and oxidation.

c. Secure the Flexible PCB

• Use Kapton tape or a fixture to hold the PCB in place and prevent movement.

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2. Soldering Process

a. Tinning the Solder Pads

• Apply a small amount of flux on the soldering pads.
• Lightly tin the pads with a thin layer of solder to improve adhesion.

b. Component Placement

• Use tweezers to carefully place the component on the solder pads.

c. Hand Soldering Method

1. Apply Flux - Helps in solder flow and prevents oxidation.
2. Touch the soldering iron to the pad & component lead – Make sure it heats evenly.
3. Apply solder wire – Let it flow smoothly over the connection.
4. Remove the iron – Avoid excessive heat to prevent damaging the FPC.
5. Cool Down – Let the joint cool naturally; avoid air blowing to prevent thermal shock.

d. Reflow Soldering (Optional)

• For fine-pitch components, use a hot air rework station set to 220–250°C.
• Apply solder paste and reflow under controlled temperature.

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3. Inspection and Cleaning

a. Inspect Solder Joints

• Use a microscope to check for:
  • Cold solder joints
  • Solder bridges
  • Insufficient solder
• Rework if necessary.

b. Clean Flux Residue

• Use isopropyl alcohol and a lint-free cloth or brush.

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4. Final Testing

• Continuity Test: Use a multimeter to check for shorts or open circuits.
• Functionality Check: If the PCB is part of a system, test its functionality.

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Tips for Soldering Flexible PCBs

• Use lower temperatures (280–320°C) to prevent melting.
• Minimize heat exposure to avoid damaging the flexible substrate.
• Avoid excessive solder to prevent bridging and poor connections.
• Use a fine-tip soldering iron for precision.
• Use Kapton tape to protect sensitive areas from heat.

Would you like additional guidance on troubleshooting soldering defects?