Flex PCB Board Assembly

Flex PCB Board Assembly

When a PCB has no components installed, it is less ambiguously called printed wiring board ( PWB ) or etched wiring board. However, the term “printed wiring boards” has fallen into disuse. A PCB populated with electronic components is a printed circuit assembly ( PCA ), printed circuit board assembly, or PCB assembly (PCBA). Also, PCB is the traditional name for the bare board that the manufacturer uses to mount in the layout design, including the electrical components and electrical connections. In informal usage, the term “printed circuit board” most commonly means “printed circuit assembly” (with components).

Flex PCB Board Assembly is very different from rigid PCB board (Printed Circuit Board) assembly. With the development of electronic products and the continuous improvement and optimization of flex PCB assembly technology, PCB manufacturing and assembly costs gradually decrease.

If you have any requirements for Flex PCB (Printed Circuit Board) prototyping or low-volume flex PCB assembly, Venture is always your trusted partner to help you save your budget.

What Is The Difference Between Flex And Rigid PCB Assembly?

The SMT process of Flex PCB and Rigid PCB is the same, that’s because the Flex PCB (Printed Circuit Board) and the Rigid PCB all need to go through the soldering process, such as component mounting and reflow soldering. However, there are some unique aspects to Flex PCB boards (Printed Circuit Boards). If manufacturers cannot fulfill these additional requirements in the production process, it will bring great trouble. When assembling electronic components like integrated circuits on Flex PCB boards, due to the limitation of printed circuit board assembly space, the surface mounting of SMD on Flex PCB boards has become one of the development trends of SMT chip technology. However, Flex PCB boards (Printed Circuit Boards) are more challenging to assemble than Rigid PCB boards because they are not so strong to construct. Let’s learn more about the difference between creating Flex and Rigid PCB boards (Printed Circuit Boards):

1) Soldering Process (Solder Mask – Solder Paste)

Like the Rigid PCB board process, through the operation of the stencil and solder paste printer, the solder paste is covered on the Flex PCB board and the Rigid PCB (Printed Circuit Board). However, the surface of the Rigid PCB board is not flat, so we need to use some fixing devices or reinforcement for fixing. Generally, we will paste reinforcement on the component area of the Rigid PCB board.

2) SMT Component Placement

If the Flex PCB board is small, extension and wrinkles will not be a severe problem, so technicians can reduce the SMT frame or increase marking points. If you don’t want to stick the stiffener to the bottom of the component, it may need flexibility after assembly. Therefore, an SMT fixture will be a good choice.

3) Reflow Soldering Process

Manufacturers must dry the Flex PCB board before reflow soldering. That is a significant difference between the Flex and Rigid PCB component placement process. They also must stop the reflow soldering once the Flex PCB board absorbs moisture. PCB rigid boards have the same problems but with higher tolerances. Flex PCB boards need to be preheated and baked at a temperature of 225-250℃. Manufacturers must finish the process very quickly. If they don’t bake it within 1 hour, they need to store it in a dry or nitrogen storage room.

Flex PCB Assembly Production Process

Take the standard carrier board as an example to elaborate on the primary welding process of the Flex PCB layout. When using a silicone plate or a magnetic fixture, the Flex PCB is much easier to fix without using tape. The primary process such as printing, patching, and welding are the same.

The Flex board is flexible and usually not vacuum-packed when shipped out of the factory. The moisture in the air is easily entered into the Flex boards during transportation and storage. Thus it needs to be pre-baked before doing Assemblies. Otherwise, It may cause quality problems such as delamination and blistering.

1) Positioning Flex PCB Layout Before Assembly

Manufacturers need to correctly fix the Flex PCB on the carrier board before SMT. It is imperative to set the time between the Flex PCB on the carrier board, and printing, placement, and soldering need to be as short as possible. Two types of carrier boards are with and without positioning pins.

Technicians should use the carrier board without positioning pins in conjunction with the positioning boards with positioning pins. Firstly put the carrier board on the positioning pins of the template to ensure the positioning pins are 100% exposed through the positioning holes on the carrier board. The Flex PCB can be put on the exposed positioning pin piece by piece, fix it with tape, and separate the carrier board from the FPC positioning template for printing, patching, and welding. The spring positioning pins with a length of about 1.5 mm have already been fixed on the positioning board. Each piece of Flex PCB board can be placed on the spring positioning pins and finally settled with tape. The spring positioning pin can be pressed entirely into the carrier boards by the stencil in the printing process.

2) Flex PCB Board Solder Paste Printing

Flex PCBs have no special requirements for the composition of solder paste. The size and metal content of the solder ball particles depends on whether there is a fine-pitch IC on the Flex PCB boards. However, FPC has higher demands for the printing performance of the solder paste. The solder paste must have outstanding thixotropy. It needs to be easily printed and released and firmly adhere to the surface of the Flex PCB boards so that there will be no defects such as poor release, blocking the stencil leakage, or collapse after printing.

Due to the carrier board being loaded with Flex PCB board, there is a high-temperature resistant tape positioned on the Flex PCB board, which causes the inconsistent surface. As a result, the printed surface of the Flex board cannot be as flat as the Rigid PCB. Therefore it is not suitable to use a metal scraper but to use a polyurethane scraper with 80-90 degree hardness. Manufacturers usually equip the solder paste printer with an optical positioning system. The printing quality will be significantly affected without this system. Although manufacturers position the Flex PCB on the carrier board, there will always be some tiny gaps between the Flex circuit board and the carrier board.

That is the most significant difference between Rigid PCB boards. So setting the equipment parameters will also significantly impact the printing process.

3) Flex PCB Assembly Process

Manufacturers will use medium or high-speed assembly and PCB design machines according to the number of components and assembly efficiency. Since there is an optical MARK for positioning on each Flex PCB board, there is almost no difference between Flex and Rigid PCB SMD mounting. Manufacturers need to note that although the FPC is fixed on the carrier board, its surface cannot be as flat as a Rigid PCB board. There will be a partial gap between the Flex PCB board and the carrier board. Therefore, using a suction nozzle to lower the height and blowing pressure. They need to set it correctly, and the moving speed of the suction nozzle needs to be slow. Therefore, It needs the printed circuit board assembly machine to have the BAD MARK automatic identification function. It will significantly improve production efficiency.

4）Flex PCB Reflow Soldering

The forced hot air convection-infrared-reflow oven needs to be applied on the Flex PCB board assembly to make sure the temperature on the board can change more evenly, thus reducing bad soldering. If you use single-sided tape, you can only fix the four sides of the Flex PCB boards. Manufacturers will deform the middle part in the conditions of hot air. Also, the plate is easily inclined, and the liquid tin at high temperature will flow, which causes empty welding and Tin beads. This impact makes the process defect rate relatively higher.

5) Flex PCB Assembly Inspection & Testing

If the temperature is higher at the time when leaving out of the oven, then the carrier board will easily absorb heat in the stove, especially the aluminum carrier boards. So it is best to add a forced cooling fan at the outlet of the oven to help cool down quickly. Therefore, the workers can wear heat-insulating gloves to protect themselves from being burned by the high-temperature plate. When taking the Flex PCB assembly to start testing, the force should be applied evenly to protect the board from being torn or creased.

When manufacturers produce the Flex PCB assembly well, they will visually test it with five times or more magnifying glass, mainly checking if there are defects such as IC pin empty welding and continuous welding or other problems. Not all surfaces of the Flex PCB board are flat, so the Flex PCB assembly is generally not suitable for AOI inspection, but we can use unique test fixtures to complete the ICT and FCT tests.

Features of Flex PCB Assembly

• Low Density: The number of components to be assembled on a Flex PCB is relatively tiny compared with Rigid PCB boards, and the assembly density is low.

• A large Number of Panels: Technicians mainly apply Flexible PCB in the miniaturized electronics industry, which causes the area of a single piece to be small. They usually use panels to improve assembly efficiency.

• High Assembly Cost: Flex PCB assembly usually leads to high overall costs, such as a long manufacturing process, low rate of equipment utilization, higher demands for accessories, and higher requirements in the manufacturing and testing environment.

Applications of Flex PCB Assembly

Flex PCB Assembly is popularly applied in many fields such as electronics, manufacturing, automotive, medical and miscellaneous, etc.

• Electronics: calculators, baby toys, cameras, etc.
• Manufacturing: robotic Arms, processing machines, bar code equipment, etc.
• Automotive: air Bag Systems, antilock brakes, GPS Systems, etc.
• Medical: pacemakers, hearing Aids and heart Monitors, etc.
• Miscellaneous: resistors and satellites etc.

Contact us directly to know how we can meet your Flex PCB layout assembly demands.