Epec's Blog | Electronics Manufacturing Solutions

At the conclusion of our webinar, EMI Shielding Methods for Flex & Rigid-Flex PCB Designs, we had several questions submitted to our presenter, Paul Tome, Product Manager of Flex & Rigid-Flex Circuits at Epec. We have compiled these questions into a readable format on our blog.

Flex circuits have the capability of utilizing a wide variety of connector options. The list, with a couple of exceptions, includes all connector systems that can be used on rigid printed circuit boards (PCBs) plus a couple of additional systems that were specifically designed for interconnecting flexible circuits to rigid PCBs or wiring harnesses.

The last 18 months have been some of the most challenging that many of us have had to deal with both personally and professionally. It has often felt like we were riding waves, coming closer to things getting a little more normal, only to have them change radically. Looking forward to the next 18 months, I don’t see those changes getting any easier for us or our industry.

With advancements in rigid-flex PCB design technology, the use of laser-based systems in the manufacturing process are very common in the industry. Not having laser drilling and cutting available would place significant limitations on what could be accomplished in a design and what components could be used.

Many flexible circuit board applications require designs to be exposed to and or operate continuously at elevated temperatures well beyond that of standard room temperature. These requirements are above and beyond the requirements of component or connector assembly.

Flexible circuits can be manufactured using a wide range of material combinations, including different copper types, copper thicknesses, flex cores, coverlays, adhesives, stiffeners, pressure-sensitive adhesives (PSAs), shielding films, and flexible soldermasks. While this material flexibility supports a broad range of design requirements, selecting uncommon materials can significantly increase lead times.

Advancing technology has impacted rigid-flex PCB designs in the same manner as rigid printed circuit boards. Increasing interconnect requirements and smaller form factors require higher circuit densities and increased layer counts. The impact of these elements on rigid-flex circuit designs is potential reliability issues both electrically and mechanically.

In some areas of a flexible circuit board design, the trace widths and spacings, trace to pad spacings, and via pad sizes are the same as used in rigid circuit boards but will differ in other areas. This is due to the flexible nature of the polyimide materials used, the type of material used to encapsulate the external layers, and the unique plating process used for most flex circuit designs.

Impedance controlled circuits in a rigid-flex PCB design is a common requirement throughout the industry in a wide range of applications. Having Impedance control, however, does create an additional challenge for designs that have very demanding minimum bend requirements.

As with many of today’s high-speed rigid circuit board designs, flex and rigid-flex PCB designs also require controlled impedance signals. The impedance vales are the same, typically ranging from 50 ohm single ended up to 120 ohm differential pairs. However, there are differences in how the impedance values are achieved due to the mechanical bend requirements that a flex or rigid-flex circuit board must meet that a rigid PCB does not.