How to Solder Flex Circuit Boards by Hand?

Most engineers should have known a thing or two about soldering a rigid circuit board by hand, but when it comes to the flex circuit board? Flex circuit boards are now more and more mentioned in electronics, what should be paid attention to when we need to do FPC soldering manually?

Solder Flex Circuit Boards Step by Step

1. Apply flux to the pads before soldering, and pre-treat using the soldering iron to prevent the pads from being oxidized, resulting in poor soldering. Generally, no need to do this when soldering chips to the flex circuit board.
2. Carefully place the PQFP chip on the flex circuit board with tweezers, taking care not to damage the pins. Align it with the pad to ensure that the chip is placed in the correct orientation. Heat the soldering iron to more than 300℃, apply a small amount of solder to the tip, press the chip on the FPC by tools, apply slight solder to the two pins at the opposite corners then get them soldered, thereby to fix the chip. Double-check the position of the chip after soldering, adjust or unmount for re-solder the chip if any position offset.
3. When soldering all the leads, the solder should be applied to the tip of the soldering iron and all pins to keep them wet. Pre-heat the end of each pin of the chip with a soldering iron tip until the solder attaches to the pins. Keep the tip of the soldering iron in parallel with the pins during soldering to the flex circuit board to prevent solder-bridge.
4. Inspect and remove excess solder after soldering all the pins to avoid bridges. Use a tweezer to check if there is a rosin joint. Wet the pins with flux, then dip the hard brush with alcohol to carefully wipe it along with the pins until the remove the flux from the flex circuit board.
5. The SMD resistances or capacitors are relatively easy to solder. You can apply tin to the solder joint first, then put one end of the component on it, clamp the component with a tweezer, solder this end and check if it gets welded correctly. If alright, just put it down and solder another end.

In addition, when the FPC board size is too large, although the soldering is easier to control, the lines are along long, the impedance is increased, the noise resistance is reduced, and the cost is increased.

When the flex circuit board is too small, the heat dissipation is lowered, difficult to control the soldering, the adjacent lines interfere with each other, such as EMI. Therefore, the FPC board design must be optimized:

• Shorten the traces between high-frequency components and reduce EMI interference.
• Components with a large weight (eg over 20g) shall be fixed by brackets then welded.
• The heat dissipation should be considered to prevent a large temperature difference on the surface of the component which will destroy the parts. The heat-sensitive component should be away from the heat source.
• The layout of the components is as parallel as possible, which is not only beautiful but also easy to weld, good for PCBA mass production. The dimension ratio 4:3 (rectangle) is better.
• Large area copper foil should be avoided because when the flex circuit board is heated for a long time, the copper foil might be liable to expand and fall off.