Which foil type is best for a particular application?

We are often asked about flexibility; of course, why else would you require a “flex” circuit? There are a lot of myths and misconceptions regarding the best way to increase the flexural endurance of a typical flex circuit. Over the past month or so, I have encountered one such misconception several times: “I can’t use ED copper. It will crack.” Is this true? What is the difference between ED (electrodeposited) and RA (rolled-annealed) copper and the impact on circuit longevity? Though well understood by the flex industry, we are not getting that important message to the board designer. Let’s take a moment to review these two types of foil. We will also review HDED (high ductility electrodeposited) and its role in good flex design and cost control.

To understand the pluses and minuses of these two materials, it is important to understand the benefits in both cost and performance of these two types of copper foil and, just as important, what is commercially available. A designer needs to consider not only what will work, but whether it can be procured at a price that won’t push the end-product out of the market.

At the heart of a copper conductor’s ability to bend and stretch without breaking is the grain structure of the copper. In ED foil, the grain structure is oriented in a direction perpendicular to the plane of the foil layer (Figure 1). As the conductor becomes stretched and relaxed, it can “work harden” and eventually crack along the grain direction. With such a short perpendicular grain, it may not take much to create this type of crack in the conductor. A cross-section of RA copper shows a much longer grain parallel to the plane of foil (Figure 2). This permits much more stretching before a break is likely. We record these stretching forces, and the ability of the copper to survive the stretch using an elongation test. Typical ED foil will permit an elongation of 10% before failure, whereas RA copper can often exceed 30% elongation before breaking. Then there’s HDED. Its grain structure is closer to neutral, thereby giving it much better elongation than a “traditional” ED copper, but perhaps not as good as RA. Elongation can be very close to that of RA copper.

This brings us back to my customer’s original concern about use of ED copper. Typical commercially available copper base material comes in copper weights from 5µm
to 2 oz. (I say “typical” as in readily available off-the-shelf. Other weights are certainly available, but on a custom or long lead-time basis.) Foil of 0.5 oz. is the transition point, as it is widely available in both ED and RA form. Once weight drops below 0.5 oz., all copper is ED. Weights above 0.5 oz. are likely RA.

Important: ED materials used in flex manufacturing are HDED. Do not confuse the ED copper as supplied from the raw base foil manufacturer with that applied in an additive process from the PCB manufacturer during PTH processing. These are very different in grain structure and flexural endurance. Every manufacturer of base foil tests each lot for elongation and flexibility. You could request these data but no need; there is real-world proof available. Remember our assertion that foil weights below 0.5 oz. are all ED? Guess where we use a lot of these materials? All sorts of personal electronic devices such as palm computers, tablets, flip phones, slide phones, laptops, etc. Think of the thousands upon thousands of flex cycles all of those circuits are exposed to in their lifetime. ED foils 0.5 oz. and below are the order of the day for these applications.

The key to using ED foil for ultimate longevity is to avoid additional “additive” copper plating. This can be done in two ways: one, by using “pattern plate” and only plate the vias, or by using “cap layers” and reserving internal non-plated layers for the dynamic flex areas. (Note: Most pattern plate processes still involve a “seed layer” of electroless plated copper of a few microns in thickness across the entire conductor surface.)

Do not leave this up to your flex manufacturer to decide. These issues are far too important to leave entirely to chance. Make an effort to discuss this early on in the design process. Inform your supplier of your concerns and of the actual use of the circuit. Always list the type of copper and other materials you want used in a stackup view on your drawing. I recommend using the IPC guidelines for calling out all materials (IPC-4204 in the case of base foils). This will ensure that you will get what you need and help evaluate competitive quotes, as all bids will be using the same materials.

Generally speaking, ED copper tends to be less expensive than RA when an equivalent weight is available, something to consider when pennies count. I would never, however, sacrifice performance for cost.

There is no reason to fear ED copper in dynamic applications. On the contrary, it is the de facto choice in thin, lightweight consumer applications requiring high cycle rates. The only concern is careful control of where we use “additive” plating for PTH process. RA foil is the only choice available for heavier copper weights (above 1 oz.) where heavier current applications and dynamic flexing are required. These are points of discussion to review with your flex supplier. Contact them early in the process and make sure they understand your needs.

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