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Flexible Circuit Board Assembly - How to Solder Flex Printed Circuit?


Fri, Jul 08, 2022

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In one article "Flexible PCB Manufacturing Process: How to Fabricate FPC Step by Step", we reveal how we fabricate from copper foil to laminates and finished flexible circuit boards. There's still a question: how is the flexible PCB assembly process?

All know that flexible PCBs are light and bendable. Any vibration or wind can cause flexible PCBs to move, and flex PCBs are easy to roll up. So the biggest challenge of flexible circuit board assembly is to hold the flex boards during SMT assembly.

This article will clarify what is unique about flexible PCB assembly.

Part 1. How to Solder Flex Printed Circuit: Flexible Circuit Assembly Process

flexible PCB assembly

Flex circuit assembly refers to SMT assembly to flexible printed circuit boards(FPC). Electronic components can only surface mount on FPC.

Below is the flexible circuit assembly process from PCBONLINE's PCB assembly factory.

Step 1. Baking flexible PCB

The polyimide or pester substrate of flexible PCBs may absorb moisture. So we bake the boards. While rigid PCBs don't need baking for assembly.

Step 2. Solder paste printing

solder paste printing

We place an SMT stencil in an automatic solder paste printing machine. The machine works according to the program designed by our CAM engineer.

In the machine, a scraper presses on the SMT stencil to release the solder paste through openings. In this way, solder paste is "printed" on the PCB pads of the flexible circuit board.

Step 3. Solder paste inspection (SPI)

An infrared camera scans the component mounting side, and the SPI machine compares the solder paste with the reference specs to ensure the solder paste size, shape, and thickness are qualified.

Step 4. Electronic components mounting

On the SMT line, the high-speed and functional mounting machines pick and place surface-mount devices (SMDs) on PCB pads according to the program designed by our CAM engineer.

Step 5. X-ray inspection for BGA

x-ray inspection

If PCB components include BGAs, an x-ray inspection is necessary to ensure the balls in the BGA bottom are okay for reflow soldering.

Step 6. Reflow Soldering

flex circuit assembly reflow

In the reflow oven, solder paste and balls melt and cure so that SMDs bond to the flexible PCB tightly.

Step 7. Automatic optical inspection (AOI)

The AOI can implement before or after the reflow soldering. At PCBONLINE, we set it at the end of the SMT assembly line.

An infrared camera scans the surface of the flexible PCBA, and the machine compares it with the reference specs to ensure the assemblies have no catastrophic issues.

After assembly, we implement the functional test to ensure the PCBA works well.

Is the difference between flexible circuit assembly and regular PCBA manufacturing only in baking? No, also in assembly jigs.

Part 2. What is Special about Flex PCB Assembly? - Jigs

Regular PCB assembly doesn't need jigs, but flexible PCBs need jigs to fix them during SMT assembly.

flexible PCB assembly jigs

The jigs used for flexible circuit assembly include a base, a padded board (magnetic or non-magnetic), and a steel sheet for the magnetic pad. All these jigs are customaccording to the flexible PCB size, positioning holes, and SMDs.

Please check the jigs one by one and how we use them below.

FPC Assembly Jig #1 - The Base

base for flex circuit assembly

The base carries the pad board, flexible circuit board, and steel sheet. By the pin snags, it fixes the things on it.

FPC Assembly Jig #2 - The Pad Board

pad board for flex circuit assembly

The padded board is used for solder paste printing, SMD mounting, and reflow soldering in the flex circuit assembly. It can be magnetic or non-magnetic. It carries the flexible PCB during assembly.

FPC Assembly Jig #3 - The Steel Sheet

steel sheet for flex circuit assembly

The steel sheet is needed when the pad board is magnetic. It lays on the flexible PCB on the pad board to fix the circuit board. The openings of the steel sheet are for mounting SMDs on the flexible PCB.

So how do we use these jigs for flexible circuit assembly?

First, we put the base on a table and trap the pad board on the base through the pin snags.

Second, we place the flexible PCB on the pad board through the pin snags.

The third step includes two situations:

If the pad board is magnetic, we place the steel sheet on the flexible PCB through the pin snags. Then we take the pad board, FPC, and the steel sheet together from the base for flexible circuit assembly.

If the pad board is non-magnetic, we manually attach high-temperature tape on the flexible PCB to fix it on the pad board. Then we take the pad board and FPC together for flex circuit assembly.

After the flex circuit assembly, the flexible circuit board assembly will get rid of the jigs.

The flex circuit assembly using the magnetic pad board is a little more expensive than that using a non-magnetic pad. But for bulky PCBA manufacturing, the assembly using the magnetic padded board has higher efficiency and a lower unit price.

Both the above flexible circuit assembly methods are available from PCBONLINE. Our engineer can help choose the appropriate flex circuit assembly method for your project.

Part 3. Why PCBONLINE for Flexible Circuit Assembly?

If you are looking for flexible PCB fabrication and assembly, you can work with the one-stop manufacturer PCBONLINE.

flexible PCB assembly

Our flexible PCB fabrication includes FPC laminates and finished flex circuit boards, and we reduce manufacturing costs.

Here are reasons for you to collaborate with PCBONLINE for flexible circuit assembly:

Here are reasons to program ICs for PCB assemblies from PCBONLINE:

  • We provide one-stop flex circuit design, manufacturing, and assembly services.
  • Besides flex PCB assembly, we also provide value-added services such as conformal coating, IC programming, mechanical boxes, and box-built assembly.
  • We can custom fabricate jigs for flexible circuit assembly.
  • PCBONLINE fabricates and assembles 1 to 10-layer single-sided, double-sided, with and without adhesive, yellow, black, white, and transparent flexible PCBs.
  • For bulky flex circuit assembly orders, we offer a free complete flex PCBA sample and functional testing.

From PCBONLLINE, the minimum finished flex PCB thickness is 0.08mm, and the minimum width of the FPC is 2.36mm. We can also add stiffeners to the flex PCBs. If you have any flex circuit fabrication and assembly needs, please feel free to send your inquiry to PCBONLINE by email at info@pcbonline.com.


The characteristics of lightness and flexibility make flexible PCBs easy to move or roll up, so we have to fix them during flex circuit assembly. Flex circuit assembly is different from regular PCB assembly, and it requires baking FPC and assembly jigs. If the FPC quantity for flex circuit assembly is large, it's better to use the magnetic pad board and steel sheet to fix the board; if the quantity is small, it's better to use the non-magnetic pad board and high-temperature tape to fix it. For flex circuit assembly, don't miss the one-stop manufacturer PCBONLINE.

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