Copper or Epoxy? Print E-mail
Written by Mark Finstad   
Thursday, 01 March 2012 23:32

For outer layer planes, the decision comes down to when the "bend" will occur.

Question: I am designing a flexible printed circuit board, and I need planes on the outer layers. I also need it to be very flexible because I have to bend the circuit sharply at several locations. Am I better off using copper or conductive epoxy for my plane layers?

Answer: Depending on the exact application, you may be able to use either copper or conductive epoxy and still end up with a reliable circuit. Everything else being equal, the conductive epoxy version will typically yield a more flexible end product than the copper version. That does not mean that copper may not still be the best choice for your application. Multiple copper planes on a flex circuit will give the circuit a lot of memory, which is advantageous if you plan to pre-form the circuit prior to installation. If the installation operator is making the bends as the circuit is installed, the conductive epoxy version would probably be a better choice. There also are newer options available for making plane layers on a flex circuit. Here are the “pros” and “cons” of each material.

Copper “pros.”

  • Copper is the time-proven material used for plane layers on flex circuits. It has very good conductivity, and can be easily connected to other planes and internal conductors using plated vias.
  • Copper has a lot of memory, so pre-forms will hold their shape nicely.

Copper “cons.”

  • Using copper for plane layers will yield the thickest (and therefore stiffest) construction. Considering that plane layers usually end up on the outer layers on flex, they may also end up with an additional layer of copper plating, making the circuit even stiffer. Fabricators can etch a cross-hatch pattern (Figure 1) into the outer layers to improve flexibility, but unless you are removing 80%-plus of the copper, I have found that the added flexibility is minimal. For controlled impedance, a cross-hatch pattern may have unpredictable effects on high-speed signals.

Conductive epoxy “pros.”

  • Silver epoxy is the most common conductive epoxy used in shielding and plane applications on flex circuits. It has very good conductivity, and I have found that electrically it performs very close to copper.
  • Using conductive epoxy for plane layers will result in a considerably more flexible circuit than one built using copper for the plane layers.
  • Interconnection between layers is possible and does not require any additional plating to accomplish this.

Conductive epoxy “cons.”

  • Conductive epoxy has little to no memory and will not hold a form. Unless there is a significant amount of copper on internal layers to help hold the form, pre-forming may not be an option.
  • Interconnecting the outer plane layers has to be done by connecting one plane to an internal conductor, which in turn is connected to the other plane. To ensure good interconnection, redundant vias are usually employed. Since you do not want to bend the circuit anywhere near an interconnection via, having multiple vias can limit the areas you can bend or form the circuit.
  • Conductive epoxy is generally applied using screen-printing. For this reason, the edges will not be as well-defined, and there may be registration issues.
  • Since one of the main ingredients is silver, the raw material is extremely expensive. The vendor may well use hundreds of dollars of silver epoxy just to flood the screen prior to running the parts. This can result in a cost premium for the circuits.

Some flexible conductive films are used regularly in lieu of copper or conductive epoxies in shielding applications. These materials are “all-in-one” solutions that combine the conductive layer, bonding adhesive and protective cover material in a single laminate (Figure 2). These materials provide a very flexible finished product. Interconnections between layers are possible and are made in a similar manner as the conductive epoxies. These materials are moderately priced and can be used with the manufacturer’s standard processing equipment, so you will not “break the bank.”

As always, I recommend you contact your manufacturer (during the design stage) for advice on which shielding options would work best for your particular application. They are aware of the strengths and weaknesses of each material and can steer you to the material that will give you the best chance of success.

Mark Finstad is a senior application engineer at Flexible Circuit Technologies (flexiblecircuit.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . He and co-“Flexpert” Mark Verbrugge from PICA Manufacturing Solutions ( This e-mail address is being protected from spambots. You need JavaScript enabled to view it ) welcome your questions.

Last Updated on Friday, 02 March 2012 21:46
 

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