Rigid flex PCB board technology information  

flex material

As you can see from the title of this blog, I have recently studied the technology of hard and soft boards. Soft and hard boards have many benefits, many designers have not known before, because their design does not have to use this technology. However, more and more designers are now facing the pressure to build increasingly high-density electronic devices. What is more headaches is the constant reduction in manufacturing costs and manufacturing time. In fact, this is really not a new technical problem. Many engineers and designers have been headaches for a long time, and the pressure is increasing.



The combination of soft and hard boards is likely to become a trap for novices on the road to new technology development. Therefore, it is wise to understand how to make flexible circuits and soft and hard boards. In this way, we can easily find the hidden dangers in the design and prevent it from happening. Now let us know what basic materials are needed to make these boards.


rigid-flex pcb


Flexible circuit material

Substrate and protective film


First, let's consider ordinary rigid printed circuit boards whose base materials are usually fiberglass and epoxy. In fact, these materials are a kind of fiber, although we call it "rigid", if you take a single layer, you can feel its elasticity. Because of the cured epoxy resin, the layer can be made more rigid. Because it is not flexible enough, it cannot be applied to some products. However, it is suitable for many electronic products that are simply assembled and that do not continue to move.


In more applications, we need a flexible plastic film that is more flexible than epoxy. Our most common material is polyimide (PI), which is very soft and strong, and we can't easily tear it or stretch it. Moreover, it has incredible thermal stability, can easily withstand the temperature change of the reflow process during processing, and we can hardly find its telescopic deformation during the fluctuation of temperature.


Polyester (PET) is another commonly used flexible circuit material that has lower heat resistance and temperature deformation than PI film than polyimide (PI) film alone. This material is commonly used in low-cost electronic devices where printed lines are wrapped in a soft film. Since PET cannot withstand high temperatures, let alone soldering, the flexible circuit board is generally produced by a cold pressing process. I remember that the display part of this clock radio uses this flexible connection circuit, so this radio often does not work properly. The root cause is this poor quality connector. Therefore, we recommend that the soft and hard bonding board still choose PI film, and other materials are also used but not used frequently.


PI film, PET film, thin epoxy resin and glass fiber core are common materials for flexible circuits. In addition, the circuit also needs to use other protective film, usually PI or PET film, sometimes using mask solder resist ink. Like the protective layer protection circuit on the hard board, the protective film can insulate the conductor from the outside to protect it from corrosion and damage. The thickness of PI and PET films ranges from 1⁄3 mils to 3 mils, with 1 mil or 2 mil thickness being more common. Glass fiber and epoxy are thicker, typically from 2 mils to 4 mils.




Printed wires are used in the above-mentioned money-saving electronic products, usually carbon film or silver-based inks, but copper wires are still a popular choice. Depending on the application, we have to choose a different form of copper foil. If it is only to replace the wires and connectors, thereby reducing manufacturing time and cost, then electrolytic copper foil which is well applied to the suitable circuit board is the best choice. Electrolytic copper foils are also used to increase the current carrying capacity by increasing the weight of copper, thereby obtaining achievable copper skin widths, such as planar inductors.


It is well known that copper has been relatively poor in work hardening and stress fatigue. High-grade rolled toughened copper foil (RA) is a better choice if the flexible circuit in the final application requires repeated folding or repeated displacement. Obviously, the step of rolling toughening is bound to increase the cost, but the rolled toughened copper foil can be bent and folded more times before the fatigue fracture occurs. And it's more elastic in the Z-deflection direction, which is what we need, and in applications where it is often bent and rolled, it gives us a longer life. Because the rolling toughening process lengthens the grain structure in the planar direction.


Figure 2: An illustration of the rolling toughening process of an exaggerated version, non-proportional composition. After passing through the high-pressure roller, the copper foil can extend its grain structure in the planar direction, making the copper softer and increasing the elasticity of the z-axis.


A typical example is the link between the gantry and the milling cutter head, or the laser head in the blue actuator (as shown below).


rigid flex pcba

Figure 3: In a Blu-ray machine, the flexible circuit application and the connection between the laser and the main board. Please note that the flexible circuit on the board of the laser head needs to be bent at a right angle. Here, a glue bead is used to enhance the connection of the flexible circuit.


Usually, we need adhesives to bond copper foil and PI film (or other film), because unlike the traditional FR-4 rigid board, the surface of the rolled and toughened copper foil does not have many burrs, so high temperature and high pressure cannot be realized. Good adhesion. Manufacturers, such as DuPont, offer single, double-sided, corrodible copper clad laminates. It uses an adhesive of 1⁄2 mil or 1 mil acrylic or epoxy glue. This adhesive was specially developed for flexible circuit boards.


"Glue-free" laminates are becoming more and more popular due to the introduction of new processing techniques such as direct coating and deposition of copper on PI films. In HDI circuits that require finer pitches and smaller vias, the film can be used in a large amount.


When it is necessary to add protective beads to the soft and hard joints, we will use silicone, hot melt or epoxy. This will increase the mechanical strength of the soft and hard joints, ensuring that no stress fatigue or tearing occurs during repeated use. The best example is shown in Figure 3.

stack up of rigid flex pcb

Sum up

It is important to understand the materials used in flexible circuit boards or hard and soft bonded circuit boards. We can also let the manufacturers give them the freedom to choose materials based on the application, but this creates a hidden danger for the failure of the final product. The content is described in detail in Coombs, CF (editor, 2008), Printed Circuits Handbook, 6th Edition, 2008 McGraw Hill, pp 61.30 - 61.24.


Understanding the properties of materials can also help us design, evaluate, and test the mechanical parts of our products. If you are developing a product for automotive applications, heat dissipation, moisture, chemical corrosion, impact, etc., need to be carefully simulated to achieve the highest reliability and minimum allowable bend radius with the right materials. Ironically, it drives us to choose the flexible application of the soft and hard board, which is often exposed to harsh environments. For example, low-cost consumer personal electronic devices are often plagued by vibration, falls, sweat, and the like.