GOOD AS GOLD?

Copper doesn't do a body good, but the price of other metals may leave you breathless!

Over the last several years, we have been asked several times if flexible circuits can be made with platinum or gold conductors instead of copper. Typically, the question comes from designers of medical devices, with implantable or at least human body invasive applications. There is an obvious dislike for copper invading the human body.

Since flexible circuits are produced using a photo-etch process, the manufacturer must use a foil that the chemical etchants will attack. The most commonly used etchants by flexible circuit manufacturers are cupric chloride, ferric chloride, and to a lesser extent some ammoniacal etchants. The problem is that gold and platinum are not affected by any of these etchants, so you cannot use the photoetch process to create a gold/platinum conductor flex circuit. This greatly limits the manufacturer’s options for forming the metal features on the flexible circuit. Since chemically ablating the unwanted precious metal surrounding the conductors is not an option, the only other alternatives (staying with a subtractive process) would be either laser ablating or mechanical milling. Neither of these options are viable due to the very thin nature of the materials being removed, and also by the very thin dielectric base material that must be left behind to keep the conductors in place after processing. All of the above assumes a sheet metal mill could be found that would be willing to make a roll of gold or platinum foil in the first place.?

If you do find a customer interested, they must be prepared to make a substantial investment. It would be very difficult to roll gold or platinum foil to less than 0.0005". At that thickness and the current market price for gold, the cost for the gold alone would be over $500 per sq. ft. The rolling mill would then tack on its margins, which I am sure would push up the cost an additional several hundred dollars per square foot. With the cost of raw materials for the circuit hovering just under $1,000 per sq. ft., imagine what the cost of the finished circuits would be.

The other method of producing a flex circuit would be an additive process as used in printed electronics (membrane circuits). In this process, conductive paths and insulating layers are added to the base substrate one layer at a time. Most conductive inks used in membrane circuits utilize silver particles suspended in a liquid carrier. The liquid carrier could be room-temperature curing ink solutions, or a two-part agent, like epoxy. Many types of metal particles could be used to make the ink conductive, including gold and platinum. Several conductive ink companies have indicated that they can produce this type of mix, but the cost would be up to $10,000 per ounce.

Yikes! Considering that the membrane manufacturer would need about an ounce of ink just to flood the screen prior to printing any circuits, this would be an outrageously priced product. There is a good chance that a printed gold or platinum circuit could cost more than just using gold or platinum foil.

The last method for producing this type of circuit would be to have the gold or platinum vapor deposited (sputtered) onto a flexible base dielectric film. The process is feasible, and those metals have been successfully sputtered onto flex substrates in the past. However, most vapor deposition equipment leaves an extremely thin coating of the deposited metal (usually measured in nanometers). Considering that most of these applications also include very narrow traces, this will make for a very fragile conductor with virtually no current carrying capacity.

There are ways that this type of circuit could be manufactured, but using the current processes available to PCB manufacturers, the cost is prohibitive, to say the least. Vapor deposition (sputtering) may hold more promise, but that process also has its limitations. Platinum or conductors instead of copper are possible but be prepared for some sticker shock!

Do you have a difficult application or an upcoming project you need help designing??Contact Flexible Circuit Technologies to get started! With over 250 years of combined engineering experience, FCT is the premier designer of the interconnect industry. Our state-of-the-art manufacturing facilities are certified for a variety of industry-leading standards to guarantee your product is produced with the utmost reliability and precision.?

By: Mark Finstad | Director of Application Engineering

Mark is Vice-Chair of the IPC Flexible Circuits Committee, member of IPC Technical Activities Executive Committee (TAEC), Chairman of IPC-2223 Flexible Circuit Design Committee, and is an active member on the IPC-6013, IPC-4202, IPC-4203, and IPC-4204 committees. He is a senior instructor on Flexible PCB design at IPC and PCB Design conferences.