Fabricators can build boards larger than their standard equipment, but added labor adds up.

Question: About a year ago, I had a flex circuit design that was about 16" long. We now need a flex circuit that is 25" long. We thought the cost would go up by about 25% due to the length increase, but instead we are getting quotes that are three to five times the cost of the previous design. Why is this?

Answer: To understand the reason why the circuit cost has risen so much, you need to understand the manufacturing processes and equipment that are most affected when circuit size increases. Every manufacturer has its unique equipment set and processing techniques, so some fabricators may not struggle as much as others with longer flex circuits. (This is probably why you are seeing the large cost spread on quotes.) You will probably find that with most Asian manufacturers, the price jump will start in the 14" to 16" length. This is because the standard panel sizes used in Asia are quite a bit smaller than panel sizes used in the US. In the US, the big price jump usually starts around 22". I will go through the different operations that affected most by large circuit size.

Drilling. Most PCB CNC drill tables are set up for a maximum travel area of about 24" x 24" per drill head. (Most machines have multiple drill heads.) To drill very long circuits, the manufacturer turns the panel sideways so that it spans multiple drill head working areas. The panel is then drilled by multiple drill heads. This can be done, but it is far from standard.

Imaging. If your manufacturer is using Mylar artworks for printing, it needs to place the resist-coated panels, along with the aligned artworks, in a vacuum frame.
The vacuum frames on most printers have a working area similar to a CNC drill. To print longer circuits, the fabricator will usually need to build a custom vacuum frame and modify its existing printing equipment to accommodate the flex board.

Copper plating. The majority of flex-circuit manufacturers use vertical copper plating lines, which require dedicated plating racks to support the panels as they travel through the line. These plating racks are sized to accommodate the manufacturer’s standard panel sizes. Also, most copper plating tanks have limited depth (just a little bit deeper than the fabricator’s largest standard panel), so even if a large plating rack was fabricated, it would just stick out of the plating bath. Some manufacturers will plate one end of a long panel, then flip the panel 180˚ and plate the other end. The overlap areas of the panel where the top surface of the plating solution is splashing up but not completely submerging the panel surface can look burned and ugly in many cases, so yields suffer.

Lamination. Lamination presses are offered in sizes starting at 12" x 18" and go to 36" x 36" and larger. But very few flex manufacturers have lamination presses larger than 24" x 24". (Are you starting to see a theme here?) The most common way for a manufacturer to deal with this limitation is to laminate the covers in sections with overlapping areas. This works, but each panel will require multiple lamination cycles.

Final surface finish. Most final finish plating baths are even smaller than copper plating baths, which typically limits final plated finish to the circuit ends. In the US, a lot of flex circuit manufacturers still use HASL (hot air solder leveling) machines to coat exposed copper with solder. If the HASL machine is a horizontal version, length is not an issue.

Final removal of part from processing panel. Most flex circuit manufacturers prefer to use some type of die to punch out parts. These dies can get really expensive really quickly when overall dimensions exceed ~18". Many manufacturers have a technician cut a very large flex circuit out of the panel by hand using an X-Acto knife and a microscope. This is time-consuming, and one little slip of the knife will send the part to the scrap barrel. Now that lasers are commonplace in most flex fabrication shops, most of the outline trimming for small quantities is done with a laser, so hand-trimming is becoming a lost art.

A very large circuit requires that the manufacturer jump through numerous hoops to get the product to you. And each of these hoops has a price tag attached to it. There are a few flex vendors that specialize in really long circuits, so they might be the best fit for your application. Otherwise, you may want to entertain using two smaller flex circuits on each end, and connect them with an inexpensive ribbon cable.

Mark Finstad is senior application engineer at Flexible Circuit Technologies (flexiblecircuit.com); This email address is being protected from spambots. You need JavaScript enabled to view it.. He and co-“Flexpert”
Mark Verbrugge, sales applications engineer at Amphenol Sincere (amphenol-afc.net), welcome your suggestions.