On the Technology of Rigid-flex PCBs

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There are many benefits of Rigid-flex PCBs panels that many designers didn't know before because they didn't have to use the technology. Now, however, more and more designers will face the pressure to build more and more dense electronic devices, and even more so, they will have to reduce the cost of manufacturing and reduce manufacturing time. In fact, this is not really a new technical problem. Many engineers and designers have had a headache for a long time and the pressure is rising.

 

Rigid-flex printed cricuit board is likely to be a pitfall for newcomers on the road to new technologies. Therefore, it is wise to know how to make flexible circuits and hard and soft connectors. In this way, we can easily find the hidden trouble in the design and prevent trouble. Now, let's see what basic materials are needed to make these boards.

flex PI material

Materials for flexible printed circuit board

 

Substrates and protective film

 

First, let's consider ordinary rigid printed circuit boards, which are usually made of glass fiber and epoxy resin. In fact, these materials are fibers, although we call them "rigid," and if you take out a single layer, you can feel its elasticity. Because of the curing epoxy resin, the plate layer can be stiffer. Because it is not flexible enough, it can not be applied to certain products. But for a lot of simple assembly, boards don't keep moving, electronics are appropriate.

 

 

In more applications, we need more flexible plastic film than epoxy resin. Our most commonly used material is the polyimide (PI), which is so soft and firm that we cannot easily tear it or extend it. It also has incredible thermal stability and can easily withstand the temperature change in the reflow welding process, and in the process of temperature fluctuation, we can hardly find its telescopic deformation.

 

Polyester (PET) is another commonly used flexible circuit material. Compared with polyimide only (PI) film, its heat resistance and temperature deformation are worse than that of PI film. This material is usually used in low-cost electronic devices, printed lines wrapped in soft film. Because PET can not withstand high temperature, let alone welding, so the flexible circuit board is made by cold pressing process. I remember that the display part of the clock radio uses this flexible connection circuit, so this radio often doesn't work properly, and the root cause is this poor quality connector. Therefore, we suggest that the soft and hard bonding board or the choice of PI film, other materials are also available but not often used.

 

PI film, PET film, thin epoxy resin and glass fiber core are common materials of flexible circuit. In addition, the circuit also needs to use other protective film, usually PI or PET film, sometimes mask resistance welding ink. The protective film can insulate the conductor from corrosion and damage, and the thickness of the PI and PET films is in the range of 1/3 mil to 3 mil, in which the thickness of 1 mil or 2 mil is commonly used. Glass and epoxy materials are thicker, generally from 2 to 4 mil.

rigid-flex pcb

electric conductor

 

Printed conductors, usually carbon films or silver-based inks, are used in the economical electronics mentioned above, but copper conductors are a common choice. According to different applications, we have to choose different forms of copper foil. If only to replace wires and connectors, thus reducing manufacturing time and costs, the best choice is electrolytic copper foil used in responsive circuit boards. Electrolytic copper foil can also be used to increase the load capacity of current by increasing the weight of copper, thus obtaining the possible width of copper sheet, such as planar inductor.

 

It is well known that copper has been relatively poor in work hardening and stress fatigue. If the flexible circuit needs to be folded or shifted repeatedly in the final application, the high grade rolled toughened copper foil (RA) is a better choice. It is obvious that the more rolling toughening process is bound to increase the cost, but the rolled toughened copper foil can be bent and folded more times before fatigue fracture. And it increases its elasticity in the Z deflection direction, which is what we need, and in applications that often bend and roll, it rewards us with a longer life. Because the rolling toughening process elongates the grain structure in the plane direction.

 

 

Exaggerated version of rolling toughening process illustrations, non-proportional composition. After the copper foil passes through the high pressure roller, it can extend its grain structure in the plane direction, make the copper softer and increase the elasticity of the z axis. A typical example is a link between a stand and a cutter head, or a laser head in a blue-ray drive (shown below).

 

 

In Blu-ray machines, flexible circuits are used to connect the laser to the main circuit board. Note that the flexible circuit on the circuit board on the laser head needs to be bent at right angles. A glue ball is used here to enhance the connection of the flexible circuit.

 

rigid-flex pcb

Adhesive

 

Usually, we need adhesive to bond copper foil and PI film (or other film), because, unlike traditional FR-4 rigid plate, there are not many burrs on the surface of rolled toughened copper foil, so high temperature and high pressure can not achieve good adhesion. It uses adhesives of acrylic or epoxy-based adhesive thickness of millet or 1 mil. This adhesive is specially developed for flexible circuit boards.

 

 

Due to the introduction of new processes such as direct coating and copper coating on PI films, "glue-free" laminates are becoming more and more common. In HDI circuits that require finer spacing and smaller perforations, the thin film can be of great use.

 

We use silicone, hot melt, or epoxy when it is necessary to add protective beads to hard and soft joints. This enhances the mechanical strength of the soft and hard joints and ensures that no stress fatigue or tear occurs during reuse.