Complete Introduction to Flexible PCB Assembly Process

JHYPCB is a professional PCB manufacturer, providing prototype manufacturing and assembly services for single-sided, double-sided, and multilayer Flexible PCB.

Overview of Flexible Printed Cricuit

Flexible PCB, also known as a flexible printed circuit, sometimes we call it flexible electronics, and FPC is its abbreviation.

Flexible PCB is different from rigid PCB. Rigid PCB is made of FR4 as a base material, while FPC is made of flexible insulating base material such as PI. The PCBA assembly process of FPC is also very different from rigid PCB assembly. Because FPC is very soft, it needs a dedicated carrier to complete a specific process.

Flexible PCB

Introduction To Flexible PCB Assembly Process

1. FPC pretreatment

Due to the flexible nature of flexible PCBs, they cannot be vacuum packaged like rigid PCBs. The flexible PCB will absorb moisture in the air in transportation and storage. Therefore, a pre-baking process is required before SMT to discharge the moisture in the board. The pre-baking process prevents the FPC from delamination and blistering due to the rapid evaporation of water absorbed by the FPC into water vapor to protrude from the FPC under the high-temperature impact of the reflow soldering.

The pre-baking time for FPC is 4-8 hours, and the baking temperature is set at 80-100°. In exceptional cases, the temperature can be set above 125°, but at the same time, the baking time should be shortened accordingly.

It should be noted that before baking, we need to conduct a baking test to determine whether the FPC can withstand the set baking temperature.

Secondly, FPC stacks should not be too much during baking.

The state of the FPC after baking should have no noticeable color change, deformation, and warping.

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2. Preparation of the carrier board

Read the positioning hole data according to the Gerber file of the FPC to make a high-precision FPC dedicated carrier board.

A flexible PCB dedicated carrier board requires light enough, high strength, low heat absorption, fast heat dissipation, and low warpage after repeated thermal shocks.

Commonly used carrier materials include aluminum plates, silica gel plates, and special heat-resistant magnetized steel plates.

3. Formal assembly of flexible circuit boards

3.1 Fix the FPC to the cardboard

Before the flexible PCB surface mount, the flexible PCB needs to be accurately fixed on the carrier board. The carrier board’s purpose is to facilitate subsequent solder paste printing and surface mounting.

Fix the FPC to the Cardboard
Fix the Flexible PCB to the Cardboard

There are two ways to fix the Flexible PCB on the carrier board:

a. Single-sided tape fixation: Fix the four corners of the flexible PCB on the carrier board with high-temperature single-sided tape, and no FPC offset, or warping is allowed. The viscosity of the tape must be moderate, it must be easily peeled off in the subsequent process, and there must be no residual glue left on the FPC.

b. Fixing with double-sided tape: first, stick the high-temperature resistant double-sided tape on the carrier board, and then paste the FPC on the carrier board. In particular, it should be noted that the viscosity of the tape also needs to be moderate; otherwise, it will easily cause the FPC to tear when it is peeled off later.

3.2 Mixing the Solder Paste

Solder paste is used to solder surface mount components. Take the solder paste out of the refrigerator, and after it is warmed up, we also need to use an auto mixer to stir the solder paste for 10 minutes before using it.

FPC Assembly-Mixing The Solder Paste
FPC Assembly-Mixing The Solder Paste
FPC Assembly-Mixing The Solder Paste

FPC has no special requirements for the solder paste composition, but it has higher requirements for the printing performance of the solder paste. The solder paste should have good thixotropy, can be easily demolded, and can be firmly attached to the surface of the FPC pad. There should be no problems such as poor demolding, blockage of the stencil opening, and collapse after printing.

3.3 Solder Paste Printing

First, fix the FPC on the table.
Secondly, with the help of the SMT stencil, the solder paste is transferred to the pads of the flexible circuit board.

FPC Assembly-Fix the FPC on the table
FPC assembly-Solder paste printing use stencil
FPC assembly-solder paste printing finished

3.4 Feeder Installation

Feeder facilitates the surface mount components pick and place.

Flexible PCB assembly-feeder installation
Flex PCB assembly-feeder installation

3.5 SMT Assembly

We use medium and high-speed placement machines for placement according to the product’s characteristics, the number of components, and the improvement of placement efficiency. Since there is an optical MARK positioning mark on each flexible PCB, there is almost no difference between SMD mounting on FPC and mounting on Rigid PCB.

It should be noted that although the flexible PCB is fixed on the carrier board, the surface of the flexible PCB is still not as flat as the rigid PCB. Therefore, the nozzle drop height and blowing pressure of the placement machine need to be accurately set, and the nozzle movement speed also needs to be reduced. This requires the placement machine to have the BAD MARK recognition function.

3.6 Manually Components Placement

After the SMT assembly is completed, the placement of the manual components starts. Visual checking is required before the start of manual components placement. After the Visual checking is confirmed to be correct, the electronic components that cannot be completed by the automatic placement machine can be manually placed on the pads.

Flexible PCB Assembly - Manually Components Placement
Flexible PCB Assembly - Manually Components Placement
Flexible PCB Assembly - Manually Components Placement
Flexible PCB Assembly - Manually Components Placement

After the Manually components placement is completed, visual checking is required again.

3.7 Remove the cardboard

Be careful when removing the FPC from the carrier board to prevent tearing.

3.8 Reflow Soldering

For flexible PCB assembly, the use of a forced thermal convection infrared reflow oven can make the temperature change on the FPC more uniform. It can also effectively reduce the occurrence of poor soldering.

Related Reading: What is the difference between wave soldering and reflow soldering?


FPC Assembly Process - Reflow Soldering
FPC Assembly Process - Reflow Soldering
Reflow Soldering Oven
Reflow Soldering finished

3.9 FPC Assembly Inspection and Testing

a. Visual inspection. The focus of the visual inspection is to check whether there is any residual glue, discoloration, gold finger immersion tin on the surface of the FPC, whether the IC pins are firmly soldered, etc.

b. AOI

C. With the help of a dedicated test fixture, flexible PCB assembly can complete ICT and FCT tests.

Related Reading:

AOI for FPC assembly
FPC Assembly Inspection and Testing

3.10 Punching

After the flexible PCB is produced, most of each piece is connected. After the assembly is completed, we need to cut it into single pieces. We recommend making a special FPC punching abrasive tool for punching and dividing the board. This can significantly improve work efficiency. At the same time, the edges of the punched flexible circuit boards are also neat and beautiful. The internal stress generated during punching is very low, effectively avoiding solder joint cracking.

Visual Checking
FPC Assembly Finished

At this point, the flexible PCB assembly has been completed. In the process of flexible electronic assembly, accurate positioning of FPC and reasonable fixed quality assurance are the keys. The flexible PCB assembly process, the pre-baking, solder paste printing, patching, reflow soldering, etc., of the flexible PCB, are pretty different from the rigid PCB assembly process. The difficulty is also more incredible. To ensure the final quality of flexible PCB assembly, each process must be strictly implemented following various regulations and the best quality control work.

The essential equipment and materials required for flexible PCB assembly include solder paste, SMT stencil, carrier board, dust-free cloth, gloves, solder paste mixer, solder paste printer, placement machine, reflow oven, cleaning machine, magnifying glass, AOI tester, ICT and FCT test fixture, etc. Different PCB assembly plants have different equipment and quantities.

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