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8 things to consider when planning a Flex or Rigid-Flex board

Every day you can read about the Internet of Things (IoT), API, Industry 4.0, and automation in the news. The future is not something seen in movies (like we did back in the Eighties when Back to the Future seemed like Utopia), we know better. The future is now and all the new products and possibilities it brings, will for sure change how we design PCBs.

Last year we wrote about how Flex and Rigid-Flex PCBs are the ones with the most increasing demand. No surprise, but this trend continues in 2018 as well.

IoT opens up endless possibilities
The demand for Flex and Rigid-Flex circuits naturally increases when new and advanced technology requires it. The competitive situation also forces companies to bring these new products to the market at an ever-increasing rate.

Lots of products connected to IoT and flexible displays demand smaller, lighter, and more cost-effective products. Then the flexible circuit is a viable option. Flexible PCB-material is utilized to meet challenging form-factor requirements, eliminate connectors and improve performance.

A positive aspect with Flex and Rigid-Flex is the reduction of assembly time. For Rigid-Flex, reduced or no use of connectors will also increase reliability. Customers often request a reduction in both costs, time spent in assembly, fewer cables and sockets. This is in many cases solved with Flex and Rigid-Flex boards.

A huge variety
Using flexible circuits gives you a huge variety of seamless interconnections, lighter weight, improved reliability and compressed constructions that can be well worth to check out further.

With the many manufacturing options and material choices available to prospective flex circuit users, the possibilities and processing choices are numerous. So why do we talk so much about Rigid-Flex now? Let’s get back to the trends in the news.
IoT is not just a buzzword tech-bloggers toss around, it’s a completely new industry where Rigid-Flex boards fit like French cheese and wine.
With the rising possibilities IoT brings, we thought it could come in handy to give some advice on what to consider when planning Rigid-Flex printed circuits.

Rigid boards are here to stay, but the next generation PCBs will be Flex and Flex-Rigid boards.

8 things to consider when planning a Flex or Rigid-Flex board:

WEIGHT REDUCTION
The use of flexible circuits can reduce the weight of an electronic package significantly, due to the fact that they do not have reinforcements that are characteristically higher in density
than unfilled polymers.

DECREASED ASSEMBLY TIME
Seamless integrate several Rigids into one Rigid-Flex that can be assembled and soldered as one PCB will save time.
Connectors and cables are gone, and reliability is another benefit out of this.

PURCHASING COST
The purchasing cost might be higher when purchasing a Rigid Flex PCB where several pieces are connected together with flex, than purchasing each board separately. However, you do not need to buy any connectors or cables between them. And with the increased system reliability you get by not using connectors and cables, the benefit might outrun the cost.

DYNAMIC FLEXING or just a BEND AND STAY solution.
The very thin polyimide with a very thin copper foil is ideal for the dynamic flexing applications. For a bend and stay application, a thicker / many layer flex can be used.

HEAT DISSIPATION
Heat dissipation is normally better on the flat copper tracks you find on a flexible circuit, compared to around wire surrounded with insulation material. This makes the flexible circuit a preferred choice in areas where resistance to heat is crucial. Stackup for improved signal integrity is also available when using reference/shielding layers.

Flexible circuits are more resistant to heat than other printed circuits (FR4).

BENDING RADIUS
It is depending on the number of layers to be bend, thicknesses of copper and total thickness and if it is dynamic or just a few bends.
There are figures available for this.

Flex circuit type                                      Minimum Bend Radius

Single sided flex                                            3-6 times circuit thickness

Double-sided flex                                         7-10 times circuit thickness

Multilayer flex                                              10-15 times circuit thickness (or more)

Dynamic Flex applications

(Only single sided recommended)            20-40 times circuit thickness

MATERIALS
Most common materials are Polyimide (PI). But there are also other materials available such as PET (Polyester), PEN (Polyethylene Naphthalate), LCP (Liquid Crystal Polymer), and others. Polyimide is the one used for products where reliability is a top priority.

PI Polyimide Advantages:

  • Excellent flexibility at all temperatures
  • Good electrical properties
  • Good chemical resistance (except hot alkaline solutions)
  • Good tear resistance
  • Highest tensile strength

PI Polyimide Disadvantages:

  • Moisture absorption up to 3%
  • Expensive compared to PEN
  • High-temperature performance is worse if used with an adhesive system.

PET Polyester Advantages:

  • Low cost
  • Good flexibility
  • Good tear resistance
  • Low moisture absorption
  • Good electrical properties
  • Good chemical resistance
  • Thermoplastic can be formed.

PET Polyester Disadvantages:

  • Not suitable for soldering
  • Unsuitable for extreme cold (polymer becomes brittle)

DESIGN POSSIBILITIES
Designers experience an increase of design possibilities when using flexible circuits.

At Elmatica we delivered our first Flex-Rigid board in 1992, and our team of Senior Technical Advisors has been in the industry for decades.
If you want to know more about Flex, Rigid-Flex or need Design Advice, just reach out to us.

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