Flex Circuit Design Process

Flexible circuits, also known as flex circuits, are specifically designed to support and electrically connect components with the aid of pads, conductive tracks, and other features etched from copper sheets laminated into a non-conductive substrate. 

Flex PCBs may comprise a single layer of copper, two layers of copper, or multi sided with several copper layers. These layers are connected with the help of plated-through holes known as vias.

Given the widespread importance of flexible PCBs in electronic design, the manufacturing process has undergone numerous changes and innovations. This article will take a quick look at the design process of flex circuits.

Design process of flexible circuits

Material Selection

Flex PCBs include cladding layers, a reinforcing plate, and a flexible copper core. The core materials are usually 1 to 3 mils thick with copper amounts of between 0.5 ounce to 1 ounce. Anything than 2 ounces could adversely impact the reliability of the board. The coverlays are usually 1.5 to 2 mils thick.

Furthermore, engineers must be well trained to handle the materials during assembly to prevent creases. Any crease in the production panel can create reliability problems in the finished circuits.

Notes on Pad Plating design

All flex circuits require a pad plate method to restrict the plated copper to a small circular area surrounding the holes on the surface area. In the case of high density designs, the circular area on the surface can be planarized away for more effective trace imaging.

It is worth noting that plated copper can decrease the flexibility of the PCB and affect its mechanical bend reliability. This has repercussions for the circuit in the long run.

Designers should review the flex circuit to ensure that all current carrying requirements are met by the thickness of the base copper.

Polyimide Coverlay

Coverlay is a solid sheet of polyimide that serves the same purpose as solder mask on rigid boards. However, polyimide comes with an adhesive backing that requires a very specific manufacturing method.

All required openings (for exposing the PTH and SMT pads), should be machined using a range of methods such as laser cutting, CNC knife cutting, or drilling. Some fabricators may use a combination of these methods. Coverlays are positioned and laminated under high temperatures and pressure to the surfaces of the circuit.

Laser Cutting of Polyimide Materials

Polyimide materials can be cut with the help of lasers. They are used for creating accurate profiles and eliminating the need for laborious and error-prone die tooling. Routing can be used as an alternative but it is not recommended for creating more-complex profiles. 

It is worth noting that laser cutting can create scarring on the edge of the circuit but this is commonly accepted by most IPC standards.

Dimensional Properties of Flex Materials

Flex circuits, as the name suggests, are required to bend due to their applications in various industries (such as medical equipment and military tools). This is why the materials are supported to be thinner and contain no reinforcement (unlike rigid boards that may contain glass weave).

This can lead to a loss of dimensional stability in the materials. Moreover, flex materials can grow and shrink when they are subjected to heat, pressure, and moisture throughout production.

It is common for fabricators to use small production panel sizes compared to rigid boards. To minimize the impact of material dimensional stability.

Circuit Layers

It is important to determine the number of layers that are needed for routing, including layers that will be needed in the rigid parts of the circuit. If the circuit is to use a rigid area, the flex ribbon will need to be configured in the middle of the stackup. This could make it difficult to route when a high number of planes are required (in the case of high-speed designs).

Floorplanning

Floorplanning investigates where the flex ribbon will bend in the circuit and how the design will mount into the enclosure. The mounting and bending areas should be free of components. Some designs use stiffeners to create rigid parts in the flex circuit for high pin count ICs. You should decide where the flex ribbon will be installed on the circuit.

High Pin Density Components

Designers can place SMD components on polyimide flex ribbon. Note that components in BGA packets require more internal layers and an advanced structure for routing. Moreover, hatched plane layers may be needed in the case of RF designs to comply with the required impedance as well as isolation.

Stiffeners

Does your flex circuit require rigidity in certain areas of the board? You can control the rigidity of the board by adjusting copper weight or by placing stiffeners. If you require a very rigid polyimide section, it may be better to place a fully rigid section. Stiffeners can be used to provide mechanical support to areas of the board that contain PTH components. 

They are also used to meet ZIF connector specifications. 

For the most part, component placement is dictated by your application requirements.

Stress Concentration Points

The design of the circuit should be thoroughly evaluated for stress concentration points in the bend area. Most application failures are due to stress concentration points such as tight bends. Here are a few tips to minimize stress concentration points::

•  The cover coat should not contain any discontinuities.
• The wish and thickness of the conductor should have no variation in the bend areas.
• There should be no twists in the finished assembly because it can cause stress along the outer edges of the flex PCB.
• There should be no unrelieved slits in the flex PCB.
• The construction of the flexible circuit should be uniform with minimal variations throughout the bend area.
• Sharp corners and right angles should be avoided on flex PCBs to increase the reliability of the circuit.

Wrapping Up

When designing a flex circuit, make sure to consider the environment and application. Your circuit should be designed around the limitations with enough wiggle room for error. The end goal of the flex circuit design process is to have a reliable and defect-free product.

For more information, please get in touch with the experts at Hemeixin PCB here.