What materials are commonly used in the construction of rigid flex boards?
construction of rigid flex boards
Rigid flex board is a circuit board with rigid portions and flexible parts that can be bent for improved electrical performance. Its flexibility makes it a favorite in industries like automotive electronics and medical/life sciences where adaptable circuitry is required. It also offers cost and weight advantages over rigid PCBs. In addition to their flexibility, rigid flex boards are resistant to vibration, shock and extreme heat/radiation. These characteristics make them ideal for use in portable devices and flat panel displays.
The materials used in the construction of a rigid flex board vary depending on its application. The base material is either a woven fiberglass or epoxy resin impregnated with polyimide, which is preferred by manufacturers due to its extreme reliability in demanding applications. The copper conductor is usually rolled copper foil and can be of various thicknesses. It is sometimes coated with a thin layer of zinc to enhance its longevity. It is usually chemically treated to reduce bond degradation, increase adhesion, augment bond strength, and protect it from oxidation.
The flex portion of the rigid flex board uses an inner core that is made of a PI or PET film, which is laminated with a coverlay. The coverlay plays a similar role as the solder mask on a rigid PCB, shielding the outer surface of the copper conductor from corrosion and damage. Typically, the coverlay is polyimide as well. In some cases, the manufacturer may opt to use a low-flow prepreg material, rather than a traditional pre-preg that is rich in epoxy.
What materials are commonly used in the construction of rigid flex boards?
A flex core is also often adhesive-less to reduce flex thickness and accommodate bend requirements. The flex area of the board should also have hatched copper planes instead of solid ones, which will help to strengthen it and resist mechanical stress. The traces that are positioned in the flex area should be routed perpendicular to the bending lines, and it is important to avoid vias and pads in the flex area for structural stability.
Adhesives are the last essential component of a rigid flex circuit board, and there are many options to choose from. Acrylic and epoxy adhesives are the most common, but manufacturers can also employ hot-melt glues and silicones for more specific applications. Rigid flex boards must have high thermal resistance and low coefficient of expansion, so the adhesives used are chosen accordingly.
Depending on the manufacturer and the specific project, adhesives are also available with different levels of transparency for easy inspection. This is a key factor in reducing production time, which ultimately drives down costs. In addition, the adhesives are also formulated to ensure that there is no loss of adhesion between the substrate and the copper conductor.