Epec's Blog | Electronics Manufacturing Solutions

In the world of flexible circuit boards, stiffeners are a common requirement in a lot of flex designs. By definition, a circuit board stiffener provides a mechanical support function and is not part of the electrical schematic of a design.

It is critical for any flex circuit design to be free of errors and violations in order to get the application to market as fast as possible without unnecessary delay. To help designers avoid common IPC PCB design standards violations in rigid-flex PCBs, this blog post will discuss three of the most common IPC Association Connecting Electronics Industries design violations.

A common question in PCB development is why flex and rigid-flex PCB tooling costs more than rigid-only circuit boards. The primary reason is process complexity. Flex circuit tooling involves more fabrication steps, tighter coordination between materials, and a higher level of hands-on engineering compared to standard rigid PCBs.

In addition to fabrication complexity, flex and rigid-flex PCB designs require more engineering programs to be manually created. Many of these steps cannot be fully automated, which increases setup time and engineering labor during the tooling phase.

Flexible circuits use either polyimide coverlay or flexible LPI solder mask to encapsulate external circuitry, each with distinct performance and design requirements. Polyimide coverlay is the most widely used due to its durability, flexibility, and dielectric properties, while LPI solder mask is applied similarly to rigid circuits but behaves differently in flex applications.

Rigid-flex printed circuit boards combine rigid and flex circuit technologies into a single, integrated construction. This integration introduces design constraints that do not exist in rigid-only or flex-only designs. As a result, specific layout rules must be reviewed and implemented during the Gerber layout phase to avoid reliability issues and manufacturing rework.

Flex and rigid-flex PCB constructions have many variations that allow for a wide range of applications and solutions. A significant difference to rigid PCB constructions is that uneven layer counts are allowed and frequently used. The primary reasons being reduced flex thickness, improved flexibility and reduced part cost.

Essential to any flex or rigid-flex printed circuit board (PCB) contruction is validation that your flex PCB design will meet your mechanical bend requirements. Minimum flex bend radius requirements were put in place so your design won't exceed the physical capabilities of copper circuitry, which would result in failed parts, long term reliability concerns, and several giant headaches.

This blog post is intended to reinforce the design support we offer at Epec for our flex and rigid-flex PCBs. Technical design and engineering is one of our core foundations which allow us to best help our customers meet their product requirements. Within the post, we'll cover design areas that require special consideration.

Flexible PCBs use one of two polyimide flex core constructions: adhesive-based or adhesiveless. The difference lies in how copper is bonded to the polyimide core, which directly affects thickness, flexibility, thermal behavior, reliability, and cost. Selecting the correct construction requires balancing electrical, mechanical, and environmental design requirements.

The copper weight and corresponding thickness used in a flexible circuit board need to be carefully selected in order to meet both the electrical and mechanical bend requirements of specific flex circuit design. Copper weight for a flex PCB is both the stiffest and most critical component.