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Flexible Printed Circuit Board 

Flexible PCBs is the process of assembling components on a flex board. The manufacture of flex PCB is quite similar to the assembly process of rigid boards. Flex PCBs are built with very thin laminates and exhibit high levels of bendability, tensile strength, and physical flexibility. These boards are also called flex boards, flexible electronics and flex circuit boards. These boards can be found in smart watches and medical equipment. These boards are very distinct due to its ability to bend and twist much than other boards. 

Flexible PCB assembly Process 

Flexible board assembly is the process of assembling components which is similar to that of rigid boards. The flow chart is describes the process flow.

Bill of Materials 

BOM or bill of materials is a required list of components to assemble a PCB. 

Flex PCB Baking 

A flex circuit board stack-up is set and sent to the baking process for reduction in the amount of moisture inside the board. The temperature and duration of the baking process depends on the overall thickness of the PCB. The thickness of flex PCB vs duration and temperature of baking is shown below.

Overall thickness of flex PCB                                Duration & temperature of baking 

Upto 1mm (39mils)                                               minimum 2 hours at 120℃ 

> 1mm upto1.8mm (70mils)                                 minimum 4 hours at 120℃ 

>1.8mm upto 4mm (157mils)                               minimum 6 hours at 120℃

Solder Paste Printing

Post baking, the board undergoes solder paste printing after that solder paste is applied on the PCB surface. The main objective is to solder pads onto the circuit board which is done by screen printing the solder paste through a stencil. Squeegee blade is used to apply the required force on the solder paste to move it across the stencil. Squeegees are generally made of metal or polyurethane.

Silkscreen Printing

Silkscreen printing is a process of creating a layer of non-conductive ink traces used to identify components, test points, parts of the circuit board, warning symbols, logos and marks, etc. This process is only performed if the customer has asked for it. Component mounting is performed after the silkscreen printing. After placing the components, the board is visually inspected and sent to the reflow soldering process.

Reflow Soldering

It is a process of pre-heating the components and melting the solder on the PCB to accomplish solder joints between the board and the components. The components are glued to the flex board by the solder paste. This solder paste melts down during the reflow soldering process and cools down to create a good solder joint. This took place in reflow ovens which have different heating zones. Each heating zone has its temperature set as per the solder profile of the assembly process. The reflow soldering consists of four stages which are shown below.

In the preheat stage, heat is accumulated on the board and the components. The temperature should gradually change because quick change in temperature can damage the components. Generally, the temperature change is no more than 2℃/second. 

During the thermal soak stage, the oxidation of pads and leads of components are reduced by activating the flux. 

In the reflow stage, the solder paste is melted and the process reaches its maximum temperature (less than the maximum allowed temperature of the components). The processed board is then cooled down and the solder alloy solidifies to create solder joints. 

In the further stages, the flex board is optically inspected and electrically tested to ensure it is 100% error free. Post testing, it is punched out of the panel and sent to final quality check (FQC). After FQC the circuit board is sent to packaging and warehousing. 

General design considerations for flex PCB assembly 

The basic flex PCB assembly technical specifications that a designer needs to know are follows.

•  Base Materials: The most common base material used in flex board is polyimide films. These materials are flexible and thin. Choose a material with good thermal resistance and electrical conductivity. 

•  Number of Layers: The number of layers in a flexible PCB depends on the type of application it is used in. For dynamic applications, a single layer board. For static, the number of layers can differ from 4 to 8. 

•  Bend radius: The bend radius of a flex circuit determines the bendability of it. Generally, the bend radius of these boards varies between 1mm and 5mm

Characteristics of flex board assembly

Application of auxiliary fixtures 

Flex boards are prone to wear and tear due to its thin and light weight feature. To successfully assemble SMT components, rigid carries are used. The basic and vital role is the positioning and consistency of the carrier in the assembly process. Lots of auxiliary fixtures are implemented in flex assembly, including board carrying tray, baking, electrical test, function test, and cutting fixtures.

Low Density 

The number of components that can be assembled on flex PCBs is relatively low when compared to rigid boards.

High quality requirements 

Generally, these boards are used in places where it requires repeated flexing. The components assembled have to meet the demands of their operating conditions. Therefore, flex circuit boards demand higher standards in terms of cleanliness and soldering reliability than rigid PCBs.

High assembly cost 

Compared to rigid PCB assembly, expenses are higher with longer manufacturing duration. The process demands more accessories and staff, and requires a well maintained manufacturing environment