A customer sent us a Gerber file last year for a wearable fitness tracker. They specified a 50um coverlay on their flexible printed circuit. We asked them why. They said their previous supplier used 50um, and they just copied it. We told them to switch to 12.5um. They pushed back, saying they needed the thicker insulation for the battery connection. We sent them a sample: the same circuit, with 12.5um coverlay on the battery side and 25um on the rest. They tested it, and the circuit flexed 30% more times before cracking, and the insulation still passed the 500V hipot test.
The coverlay is the polyimide film that covers the copper traces on an FPC. It protects the copper from oxidation, provides electrical insulation, and adds mechanical strength. But it also makes the circuit stiffer. Choose the wrong thickness, and your flexible circuit will not bend as many times as you need it to. Choose it too thin, and you risk insulation breakdown.
Here is how we choose the coverlay thickness on every FPC design we build.
The three standard coverlay thicknesses
There are three standard coverlay thicknesses in the industry:
12.5um (0.5 mil) - this is the thinnest coverlay available. It is used for circuits that need to flex millions of times, like the hinge in a clamshell phone or a medical catheter. The tradeoff is that the insulation is thin, so you need to design the trace spacing wider to avoid breakdown. It is also the most expensive, because the thin film is harder to handle during lamination.
25um (1 mil) - this is the most common coverlay thickness. It balances flexibility and insulation, and it works for 90% of FPC applications. It can flex hundreds of thousands of times, and it provides enough insulation for most low-voltage applications (up to 50V).
50um (2 mil) - this is the thickest standard coverlay. It is used for circuits that do not flex much, but need high insulation, like a power supply board or a battery management system. It is also used for circuits that need to be stiff enough to stand on their own, like a display panel that snaps into a plastic housing.
FPC with 25um coverlay, showing the exposed contact pads at the end.
What determines the right thickness
We ask four questions before we recommend a coverlay thickness:
1. How many times will the circuit flex? If the circuit flexes more than 100,000 times, we recommend 12.5um. If it flexes 10,000 to 100,000 times, 25um is fine. If it flexes fewer than 1,000 times (like a circuit that bends once during assembly and never moves again), 50um is acceptable.
2. What is the voltage? If the circuit carries more than 50V, we recommend 25um or 50um, because the 12.5um coverlay has a breakdown voltage of about 200V, which is enough for most low-voltage applications but not for high-voltage ones. For 220V AC, you need 50um minimum.
3. What is the trace spacing? If the traces are spaced less than 0.2mm apart, you need a thicker coverlay, because the thin coverlay can flow into the gap during lamination, causing a short. We recommend 25um minimum for trace spacing below 0.3mm.
4. How stiff does the circuit need to be? If the circuit needs to be stiff enough to insert into a connector without curling, you need 50um. If it needs to be flexible enough to wrap around a 5mm radius, you need 12.5um.
Four-legged stainless steel domes mounted on an FPC with 25um coverlay.
Can you mix thicknesses?
Yes. You can use different coverlay thicknesses on different parts of the same circuit. This is called "selective coverlay." For example, on a wearable device, you can use 12.5um coverlay on the flexing area (the part that goes around the wrist), and 25um coverlay on the rigid area (the part that holds the battery and the processor).
The way we do this is by laminating two layers of coverlay: a 12.5um layer that covers the whole circuit, and a 25um layer that is die-cut to only cover the non-flexing areas. The result is a circuit that is flexible where it needs to be, and stiff where it needs to be.
This is more expensive than using a single thickness, but it is often cheaper than redesigning the whole circuit to fit a stiffer coverlay. We have seen customers save money by using selective coverlay instead of switching to a more expensive flexible material.
Common mistakes we see
The most common mistake is using 50um coverlay on a circuit that needs to flex. The customer copies the coverlay thickness from a previous design, without thinking about whether the new circuit flexes more. The result is a circuit that cracks after a few thousand flex cycles.
The second most common mistake is using 12.5um coverlay on a high-voltage circuit. The customer wants the thinnest possible circuit, and they forget that the thin coverlay has a lower breakdown voltage. The result is an insulation failure after a few months in the field.
If you are designing a new FPC, send us your Gerber file and your flex requirements. We will review the coverlay thickness and recommend the right material for your application. We can also build a sample with different coverlay thicknesses, so you can test which one works best before you commit to production.
We also produce metal domes, dome sheet arrays, and silicone buttons that mount on the FPC. If you need a complete keypad assembly, talk to our engineering team. We can design the whole thing - the FPC, the domes, and the overlay - as one integrated assembly.
- John Smith, Senior Application Engineer, Shanyo
