Ultimate Guide to Flexible PCB Reflow

As technology progresses, the minimum component size has shrunk from 03015 four years ago to now 008004, further highlighting the importance of flexible PCB in electronic manufacturing. These components are not simply pasted onto the surface of flexible PCB but are soldered, with one of the soldering techniques being reflow soldering. In this article, TechSparks will delve into some aspects related to flexible PCB reflow to facilitate your understanding of the challenges faced.

Concepts of Flexible PCB and Reflow

Let’s start our discussion by understanding the concepts of flexible PCB and reflow. Firstly, flexible PCB are a mainstream type of circuit, characterized by their ability to bend and twist to fit various spatial shapes of electronic devices. They are widely used in microelectronics, wearable electronics, and other fields, providing greater flexibility and innovation in electronic product design.

Reflow or Reflow soldering, on the other hand, is a soldering process that involves applying solder paste to the circuit board and then heating it to a high temperature, allowing components to be reliably connected to the board, ensuring smooth current flow through the traces, and activating the components.

Challenges of Reflow Flexible PCB

Despite the long history of using flexible PCB in electronic products, they face unique challenges during the reflow due to their special properties:

Reflow relies mainly on high temperature to achieve soldering, but the fixed heat source results in inconsistent soldering completion in different areas. Therefore, hot air is usually used to heat the solder joints and PCB surface to ensure uniform soldering. However, the flexible PCB is too light and thin easily causing positional displacement, leading to soldering defects. This is why experienced flexible PCB manufacturers use magnetic brackets or similar devices for fixation.

Additionally, the construction of flexible PCB includes substrates and copper foils, which have different coefficients of thermal expansion. This results in different degrees of thermal expansion during heating, which may lead to internal stress, causing delamination, warping, or cracking issues.

Although manufacturers strive to ensure quality, failures are difficult to avoid, meaning rework may be necessary. However, multiple reflows of flexible PCB may cause material degradation due to high temperatures, increasing the risk of delamination and compromising the reliability of solder joints.

Best Practices for Flexible PCB Reflow

As mentioned earlier, multiple reflows may cause material degradation, limiting the effectiveness of rework. This is also one of the main reasons for the high cost of flexible PCB. Therefore, improving the reflow quality of flexible PCB not only mitigates the time wasted due to failures but also impacts costs. To this end, TechSparks offers some practical suggestions:

Firstly, you should have a clear understanding of the characteristics and key parameters of the substrate material (such as coefficient of thermal expansion, thermal conductivity, specific heat, etc.) and the components used, including component density, and set oven parameters accordingly, such as ramp rate, dwell time, and peak temperature.

Furthermore, it is advisable to equip more advanced reflow ovens and tools. Rest assured, as a long-term investment, this is considered cost-effective, as it can reduce the cost of failures. For example, some ovens have dedicated heating zones and more precise temperature control, thereby improving soldering quality. Lastly, if conditions permit, keeping the product on the same production line may achieve consistency.

Quality control cannot be emphasized enough, including SPI inspection during solder paste printing and AOI inspection after reflow. Continuous monitoring and data collection can help you detect problems and optimize them in a timely manner.

Most importantly, close communication between customers and manufacturers is essential. The focus of communication should be on design, specifications, fixtures, etc., rather than trivial matters. Professional manufacturers are willing to cooperate with customers and provide timely feedback and solutions to problems.

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