Best Flexible PCB Soldering Guidelines for Assembly Process

As a soft and thin circuit, flexible PCB soldering presents unique challenges. Without the use of a specialized carrier board, it becomes difficult to achieve proper fixation and transmission. Additionally, basic SMT processes, such as printing, component placement, and reflow soldering, become more intricate. In this article, TechSparks will delve into the intricacies of soldering and explore effective solutions for flexible PCB.

Soldered flexible circuit board

Basic operation steps of flexible PCB soldering

Before you start soldering the flex circuit, it is very important to apply a layer of flux to the surface of the pads to help improve the tinning quality and prevent poor soldering due to oxidation.

Next, with utmost care, use tweezers to delicately position the PQFP chip on the circuit board, being extra cautious not to damage the delicate pins, and ensuring precise alignment with the corresponding pads. Once the soldering iron’s temperature is adjusted to above 300 degrees Celsius, add a small amount of solder to the tip of the soldering iron. Utilizing a special tool to aid in alignment, start by applying a touch of solder to the pins at two diagonally opposite corners of the IC chip and then gently pressing the chip to securely fix it in place. At this stage, the chip should be precisely positioned. After completing the diagonal soldering, verify once more that the chip is in the correct alignment. If necessary, make adjustments or carefully remove the chip and reposition it on the board.

Subsequently, proceed with soldering all the pins. Applying an adequate amount of solder to the soldering iron’s tip, ensure the flux covers all the pins to keep their surfaces moist. Gradually touch the tip of the soldering iron to each pin until you observe the solder flowing into the pins. To prevent overlapping solder, maintain the soldering iron parallel to the solder legs.

Once all pins are soldered, thoroughly soak them with flux to enable the solder to be properly absorbed where needed, effectively eliminating any potential shorts or laps. To ensure impeccable solder joints, meticulously inspect them using tweezers. Once the inspection is completed, remove any excess flux from the FPC and gently wipe it with an alcohol-soaked hard brush until the flux completely disappears.

For resistance components, it is advised to first tint one solder point, then secure one end of the part with pliers, and proceed to solder that end. After verifying the correctness of the solder joints, proceed to solder the other end.

Precautions when soldering flexible PCB

Fixing of board

Cardboard to fix the flex PCBSecure flex circuit boards before soldering

In the automated soldering process, the secure and precise positioning of the flexible PCB is essential as it progresses through each stage with the movement of the docking station. Depending on the carrier board, there are two types: those with positioning pins and those without.

For carrier boards without positioning pins, a positioning template with corresponding positioning pins is used. The process begins by placing the carrier board on the positioning pins of the template, aligning the positioning holes on the carrier board with the exposed positioning pins. The flexible PCB is then carefully placed on the carrier board piece by piece, ensuring that the positioning pins are exposed. To fix the flexible PCB in place, adhesive tape is used to secure it on the exposed positioning pins. After this step, the carrier board is separated from the flexible PCB positioning template, allowing for the subsequent welding process.

In contrast, the carrier board with positioning pins employs several spring positioning pins with a length of approximately 1.5mm. The flexible PCB can be directly placed on these spring positioning pins, one by one, and then securely fixed in place using tape. During the solder paste printing process, the spring positioning pins can be fully pressed into the carrier board by the steel mesh without compromising the soldering effect.

Solder paste printing

Solder paste printing when soldering flexible PCB

While the composition of the solder paste does not require special attention, the printing performance is critical. Excellent thixotropy is necessary to facilitate easy printing and release, ensuring a secure attachment to the board’s surface without defects such as poor mold release or mesh blockage.

Since the flexible PCB is loaded onto the carrier board with high-temperature-resistant tape for positioning, the surface is not as flat as traditional PCB, and its thickness and hardness may vary. As a result, using a metal scraper is not suitable. Instead, a polyurethane scraper with a hardness of 80-90 degrees is recommended. To achieve precise alignment during printing, the solder paste printing machine should be equipped with an optical positioning system.

It’s essential to note that despite the fixed positioning on the carrier board, small gaps may still exist between the circuit board and the carrier board. This fundamental difference from rigid PCB requires careful consideration when setting equipment parameters to achieve optimal welding effects. By employing the proper solder paste and equipment, the soldering process can be performed efficiently, resulting in successful and reliable flex PCB assembly.

Reflow soldering of flexible PCB

To ensure even temperature distribution and reduce poor soldering occurrences, it is recommended to utilize a forced hot air convection infrared reflow oven when soldering flexible PCB. Using single-sided tape for fixation may lead to deformation in the hot air state, causing pad tilting and molten tin flow, leading to defects like empty soldering, continuous soldering, and tin beads.

To address these challenges, a temperature curve test method can be employed. This involves placing two carrier boards with FPCs on the front and back of the test board, mimicking the actual production spacing. Components are mounted on the circuit board of the test carrier board, and high-temperature solder wire is used to attach temperature probes to test points. These probes are secured with high-temperature resistant tape, strategically placed near solder joints and QFP pins on each side of the carrier board to ensure accurate and reflective test results.

Next, setting the temperature curve is crucial for soldering quality. Due to the limited temperature uniformity of flexible printed circuit board, a heating/holding/reflow temperature curve is recommended during furnace temperature adjustment. This approach allows for easier control of parameters in each temperature zone, minimizing the impact of thermal shock on circuit board and components. It’s advisable to adjust the furnace temperature to the lower limit of solder paste technical requirements and utilize the lowest wind speed available in the reflow furnace. Ensuring good chain stability in the reflow furnace without shaking is essential.

Further Reading: Ultimate Guide to Flexible PCB Reflow

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