As mentioned many times in the past in this blog, flexible circuits make up a small portion of the overall printed circuit market worldwide. The questions that typically arise quickly have to do with electrical and mechanical characteristics. Then if agreeable and a flexible circuit is determined to be the best answer for an application, the question of design practices comes up. After all of those deterrents have been dealt with, the circuit is introduced to the production department and a new set of concerns arise.
Traditional “rigid” printed circuit boards have been widely used in the United States since the 1940s. They showed up in consumer products the next decade and have been produced in very large scales ever since. The electronics industry is full of experts on the design, manufacture, and assembly of printed circuit boards (PCBs). So there isn’t a concern for our production groups when we introduce a new product and it’s made on a traditional printed circuit board. However, if the new product is a flexible circuit it arrives with many tedious questions and maybe even insecurity because of a lack of experience with this type of assembly. These concerns are understandable.
Printed circuit boards lend themselves well to automated assembly. They are after all rigid and stand up well to wave solder and IR reflow solder. Their flexible cousin however does not stand up as well. The inherent flexibility of the circuits requires that one think differently about how the assembly will travel through all the stages it must go through. Some of the process will be very similar to those we use with printed circuit boards and some will be very different.
Flexible circuit material absorbs moisture fairly well and must be baked out, as we do with traditional printed circuit boards, before being introduced to high temperatures. This process as well as the next (solder paste screening) will be very similar to those we use on our traditional printed circuit boards. However, after that point, we must take very careful steps to complete the assembly. Our next step would be to go to pick and place which puts some pressure on the circuit. Flexible circuits then must be protected during this process, temporarily, to provide the stability and even surface required to place the components reliably. They also must have that same stability as they travel through the soldering process. Whether that is wave or IR reflow. Then after all the components are soldered the circuits can be cleaned, inspected and re-worked. The stability required for travel through the assembly line is not needed any longer. We can remove the temporary stability fixture that was used throughout the assembly process and complete the task of packaging and shipment or placement into finished goods inventory.
Let’s look at some of the questions that are frequently asked.
- Can flexible circuits have components soldered to it?
Yes! This is usually the first question that is asked. Because of its small footprint in the industry there are still a lot of designers that are unaware of the ability to solder components to flexible circuits.
- Are there any components that cannot be assembled on flexible circuits?
No! Any component that can be soldered or adhered or wire bonded to a printed circuit board can be attached the same way to flexible circuits. There are some components that require very serious consideration by the assembly team, such as, BGAs and chip on board. However all components can be assembled to a flexible circuit.
- Can flexible circuits be panelized and automated?
Yes! The flexible circuit panels will require special fixtures or tooling in order to run through the automated lines. But have no fear, they can be mass produced and with astounding results, just as their rigid counterparts are.
When faced with the assembly flexible circuits, it would do you well to consult with an experienced manufacturer during the design stage. They can assist with panel layout and any fixture designs necessary to ensure that the assembly process goes as smoothly as any printed circuit assembly.