Ensure Consistent Thickness in Flexible PCBs

Consistent Thickness in Flexible PCBs

Creating flexible circuits that are durable is a must for electronics manufacturers to achieve reliable performance. These specialized printed circuit boards are used in a wide variety of electronic devices to improve functionality and aesthetics. The design of a flex PCB must be carefully planned to ensure the conductive copper layer can bend and flex without affecting its conductivity or structural integrity. This requires the careful use of design rules that will help you avoid mistakes that can lead to failure.

The conductive material of a flexible pcb is etched in layers to create the traces that will transmit the signals through the circuit. These layers are typically covered with a protective coverlay to prevent contamination and damage. These layers can be as thin as 12 to 120 microns thick. This allows for the production of a flex PCB that is thinner than traditional rigid-boards. The weight reduction that results from using a flex circuit is a significant advantage in many applications.

The minimum bend radius that a flex circuit can make must be established early in the design process. The flex circuit should not be bent beyond this radius, or it will suffer stress cracking and other failures. The bending stresses can be minimized by a proper layout of the traces and careful selection of materials and stack-ups.

Ensure Consistent Thickness in Flexible PCBs

Often, the ends of a flex circuit terminate at a connector, or the ends may be embedded in rigid-flex sections. It is important to use a stiffener where the flex terminates at these points. This can be achieved with an epoxy, acrylic or hot-melt glue. However, these methods require manual dispensing and curing to ensure a good bond between the flex and the rigid section of the board. This increases cost and can be a source of quality concerns.

Another issue that should be addressed when designing a flex circuit is the location and placement of the plated through-holes. Ideally, they should be located away from the flexing zone of the board. This will reduce the likelihood of a solder joint failing during flexing, and it will also prevent the holes from ripping through the conductive copper layer or the laminate.

The layout of conductive traces on a flexible pcb should be carefully considered to accommodate the flexing and bending requirements of the circuit. The traces should be routed in a way that minimizes stress on the conductive paths and avoids sharp corners or acute angles. These types of angled traces can rip through the conductive copper and laminate, causing the circuit to fail.

To prevent this, the curved traces should be wider than those that are straight. Additionally, the curved edges should be properly terminated. For example, any inside corner that has an angle less than 180° should be finished with a tangent curve with a radius greater than 1.5 mm to avoid tearing the flex substrate material. Lastly, the plated through-holes should be staggered for better stress distribution in the rigid-to-flex zone of the board.

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