February 21, 2017
The most common question PFC receives during the DFM process (Design for Manufacturing) with our customers, has to do with the bend radius of the circuit we are designing. As we have mentioned many times IPC calls out the bend radius to be ten times the thickness of the circuit. The industry response is seven times the thickness and depending on the materials used in the design, we can go to five times the thickness.
Flexibility in the copper comes from multiple factors. Of course the thinner the copper, the more flexible. In addition to the thickness (or thinness), copper grain also affects flexibility. There are two common types of copper that are used in the PCB and flex circuit markets. Electro Deposited (ED) and Rolled Annealed (RA).
With ED copper, there is a huge diversity of foils regarding surface roughness, treatments, grain structure, etc. As a general statement, ED copper has a vertical grain structure. The standard ED copper typically has a relatively high profile or rough surface as compared to rolled annealed copper. ED copper tends to lack flexibility and does not promote good signal integrity.
RA copper has been used extensively in the flex industry for decades. The grain structure and smooth surface is ideal for dynamic, flexible circuitry applications. Another area of interest with rolled copper types exists in the high-frequency signals and applications. It has been proven that copper surface roughness can impact high-frequency insertion loss and a smoother copper surface is advantageous.