There are many benefits to a rigid-hard board. Many designers did not understand it before, because their designs do not have to use this technology. However, more and more designers will now face the pressure to build increasingly high-density electronic devices. What is more headache is that they must continue to reduce manufacturing costs and reduce manufacturing time. In fact, this is really not a new technical problem. Many engineers and designers have been having headaches for a long time, and the pressure they are facing is constantly increasing.
Therefore, it is very wise to understand how to make flexible circuits and flexible and rigid boards. In this way, we can easily find hidden errors in the design and prevent them before they occur. Now let us know what basic materials are needed to make these boards.
Materials for flexible circuits
Substrate and protective film
First, let''s consider common rigid printed circuit boards, whose base materials are usually glass fiber and epoxy resin. In fact, these materials are a kind of fiber, although we call it "rigid", if you take out one layer, you can still feel its elasticity. Due to the cured epoxy resin, the board 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 simple-assembled electronic products whose boards do not move continuously.
In more applications, we need more flexible plastic films than epoxy. Our most commonly used material is polyimide (PI), which is very soft and strong, and we cannot easily tear it or stretch it. And it also has incredible thermal stability, can easily withstand the temperature change during reflow soldering during processing, and in the process of temperature fluctuations, we can hardly find its expansion and contraction deformation.
Polyester (PET) is another commonly used flexible circuit material. Compared with polyimide (PI) films only, its heat resistance and temperature deformation are worse than PI films. This material is commonly used in low-cost electronic equipment, where printed wiring is wrapped in a soft film. Because PET cannot withstand high temperatures, let alone soldering, cold-pressed processes are generally used to make this flexible circuit board. I remember that the display part of this clock radio uses this kind of flexible connection circuit, so this radio often doesn''t work properly. The root cause is this poor-quality connector. Therefore, we recommend that the flexible and rigid combination board be a PI film. Other materials are also used but not often used.
PI film, PET film, thin epoxy resin and fiberglass core are common materials for flexible circuits. In addition, the circuit also needs to use other protective films, usually PI or PET films, and sometimes mask solder resist ink. As with the protective layer of the welding layer on the hard board, the protective film can insulate the conductor from the outside and protect it from corrosion and damage. The thickness of PI and PET films is in the range of mils to 3 mils, of which 1 mil or 2 mils are more commonly used. Fiberglass and epoxy resins are thicker, typically from 2 to 4 mils.
Printed wires are used in the aforementioned money-saving electronics, usually carbon film or silver-based inks, but copper wires are still the common choice. According to different applications, we have to choose different forms of copper foil. If it is only to replace wires and connectors, thereby reducing manufacturing time and costs, then electrolytic copper foils that are well-suited for application to circuit boards are the best choice. The electrolytic copper foil will also be applied to the case where the copper carrying width can be achieved by increasing the current carrying capacity by increasing the weight of copper, such as a planar inductor.
As we all know, copper has been relatively poor in terms of work hardening and stress fatigue. If the flexible circuit needs to be repeatedly folded or repositioned in the final application, a high-grade rolled toughened copper foil (RA) is a better choice. Obviously, the additional step of rolling and toughening will inevitably increase the cost, but the rolled and toughened copper foil can be bent and folded more times before fatigue fracture occurs. And it has increased elasticity in the Z deflection direction, which is what we need. In applications that often bend and roll, it returns us a longer life. Because the rolling and toughening process stretches the grain structure in the plane direction.
Figure 2: Exaggerated version of the rolling toughening process illustration, non-proportional composition. After the copper foil passes through the high-pressure roller, its grain structure can be extended in the plane direction, making the copper softer and increasing the z-axis elasticity.
Typical examples are the link between the table and the milling cutter head, or the laser head in a Blu-ray drive (as shown in the figure below).
Figure 3: In a Blu-ray machine, the flexible circuit application and the connection between the laser and the main circuit board. Please note that the flexible circuit on the circuit board of the laser head needs to be bent at a right angle. Here a plastic bead is used to enhance the connection of the flexible circuit.
Generally, we need adhesives to bond copper foil and PI film (or other films). Unlike traditional FR-4 rigid boards, the surface of rolled and toughened copper foil does not have many burrs, so high temperature and pressure cannot be achieved Good adhesion. Manufacturers, such as DuPont, provide single- and double-sided, erodible copper-clad laminates. It uses ? mil or 1 mil acrylic or epoxy based adhesives. This adhesive was developed specifically for flexible circuit boards.
With the introduction of new processing techniques such as direct coating and deposition of copper on PI films, "adhesive-free" laminates are becoming more common. In HDI circuits that require finer pitch and smaller vias, this film can be of great use.
When it is necessary to add protective rubber beads to the soft and hard joints, we will use silicone, hot melt adhesive or epoxy resin. This will increase the mechanical strength of the soft and hard joints and ensure that no stress fatigue or tearing will occur during repeated use. Figure 3 is the best example.
Figure 4: Typical single-layer flexible circuit board stack.
to sum up
It is important to have a clear understanding of the materials used in a flexible circuit board or a rigid-flex circuit board. We can also let the manufacturers free, and they can choose the materials freely according to the application, but this lays a hidden danger for the failure of the final product.
Understanding the properties of materials can also help us design, evaluate, and test the mechanical parts of our products. If the research and development is applied to automotive products, then heat dissipation, moisture resistance, chemical corrosion, impact and other conditions need to be carefully simulated, so that the correct materials are used to achieve high reliability and the minimum allowable bending radius of the product. Ironically, the practical application that drives us to choose the flexible and hard-to-integrated flexible board is often exposed to harsh environments. For example, low-cost consumer personal electronic devices are often plagued by vibrations, drops, and sweat.
Building a flexible board
At first glance, a typical flexible or rigid-flex board looks bland. However, making them requires some extra steps. The manufacture of any type of flexible and rigid board starts with the manufacture of single-sided or double-sided flexible boards. As mentioned last week, manufacturing can start with a pre-pressed flexible board, or it can start with a PI film, and then laminate a copper board on the original bare board, or plate it with copper. The lamination process requires a thin layer of paste to be applied to the film, while the non-adhesive process requires copper seeding on the film. Vapor deposition techniques (such as sputtering) are commonly used to seed the "seeds" to plant condensed nodules for subsequent chemical precipitation processes. The subsequent drilling, plating, and etching processes of single-sided or double-sided flexible boards are generally similar to the processing of double-sided hard boards.