Reflow Perspectives to Flex Circuit Assemblies

Estimated reading time: 3 minutes

At the recent NEPCON China 2018 event in Shanghai, I spoke with Ralf Wagenfuehr, plant manager at Rehm Thermal Systems (Dongguan) Ltd, to gain insight into flex circuit assembly, and the challenges of reflow soldering.

According to Wagenfuehr, flex circuits are increasingly being used in mobile phones over the past two years as manufacturers have been redesigning their phones with more curves and more functions and additional components inside.

"The PCB is getting smaller, and modules are being built on the flex printed circuits, not only the connector. Now, it’s integrated circuits in the flex boards, and sensors, which are becoming much smaller. As an example four years ago, the smallest chip component was 03015. Now, the 008004 chip component is also on the market. The 008004 component is 35% smaller than the 03015 component. The challenge of reflow soldering is how to ensure the oxygen content in the reflow chamber and how to ensure the stability of the reflow air volume to avoid such defects as tombstone," Wagenfuehr says. "You can now bring the components in mobile phones to the position that is good for the user. More and more, these applications—mobile phones, smart devices, and other similar applications—are definitely increasing the market demand for flex circuits. But I think, in big components like in automotive, it is very seldom used because of the reliability of flex circuits in such applications. They should last for years, not only just for a short time. They are using other components, for instance, the flex circuits may be integrated in housings, MID’s moldings—in automotive electronics. But flex circuits, definitely more and more in handheld devices."

One of the biggest challenges when it comes to the reflow soldering of flexible circuit assemblies is warping. "First of all, flexible circuits are very light. While some customers are trying to process flex circuits without holders or carriers, experienced manufacturers are using magnetic holders or similar things to keep the flex circuits in place. But I think soldering is not only the biggest issue. I believe, printing and chip mounting are definitely a challenge for manufacturers when it comes to flex circuits because they are not steady."

According to Wagenfuehr, mitigating this problem starts with the design. He says designers should consider not only the application but also the manufacturing. In fact, there is a clear definition of IPC-6013 with Amendment 1, Qualification and Performance Specification for Flexible Printed Boards. As long as the FPC has the correct design, it can be reasonably used in the manufacturing process.

"Some applications are very hard to realize; and some applications cannot be processed in the reflow. Some applications may be unsuitable for condensation soldering systems. So, the manufacturer of flex circuits should consider the manufacturing processes, the whole step," he explains. "Ordinarily, we are just supporting the applications in general. When we’re having issues and the customers cannot solve it, this very point that’s just mentioned, maybe the manufacturer of such boards should come with the equipment suppliers earlier in the stage. That will be much more helpful. Also, having them early at the design stage, to fit it in the whole process chain, will make it easier to manufacture the flex circuit assemblies."

Wagenfuehr also stresses the importance of communication with their customers and users. "There are a lot of limits that they should consider—limits in the design, in preparation of the fixtures—to ensure proper soldering. We can support our customers in applications where we have certain experiences, such as in mobile phone manufacturing, for instance. We also have to know more about our customers issues. We are happy to help in the process steps. We have a rich experience and we can also consider the early design stage with our application teams in China and Germany."

In fact, Wagenfuehr notes that they will be willing to join forums to address these issues. "It will be nice if it is focused on soldering flex printed circuits. There is not much forums happening in manufacturing in this front. It’s always driven by the designers—that’s the problem. They don’t ever consider the equipment. They just do the designs, which, sometimes are impossible or quite challenge to manufacture. It will be good if forums will be created to bring the designers and equipment manufacturers together to discuss the issues. That will be helpful in the future."

This article was initially published in the June 2018 issue of SMT007 Magazine.