The main conductor material used in PCBs is copper foil, which is used to transmit signals and currents. At the same time, copper foil on PCBs can also be used as a reference plane to control the impedance of the transmission line, or as a shield to suppress electromagnetic interference (EMI). At the same time, in the PCB manufacturing process, the peel strength, etching performance and other characteristics of copper foil will also affect the quality and reliability of PCB manufacturing. PCB Layout engineers need to understand these characteristics to ensure that the PCB manufacturing process can be successfully carried out.
Copper foil for printed circuit boards have electrolytic copper foil (electrodeposited ED copper foil) and calendered annealed copper foil (rolled annealed RA copper foil) two kinds, the former through the electroplating method of manufacturing, the latter through the rolling method of manufacturing. In rigid PCBs, electrolytic copper foils are mainly used, while rolled annealed copper foils are mainly used for flexible circuit boards.
For applications in printed circuit boards, there is a significant difference between electrolytic and calendered copper foils. Electrolytic copper foils have different characteristics on their two surfaces, i.e., the roughness of the two surfaces of the foil is not the same. As circuit frequencies and rates increase, specific characteristics of copper foils may affect the performance of millimeter wave (mm Wave) frequency and high speed digital (HSD) circuits. Copper foil surface roughness can affect PCB insertion loss, phase uniformity, and propagation delay. Copper foil surface roughness can cause variations in performance from one PCB to another as well as variations in electrical performance from one PCB to another. Understanding the role of copper foils in high-performance, high-speed circuits can help optimize and more accurately simulate the design process from model to actual circuit.
Surface roughness of copper foil is important for PCB manufacturing
A relatively rough surface profile helps to strengthen the adhesion of the copper foil to the resin system. However, a rougher surface profile may require longer etching times, which can affect board productivity and line pattern accuracy. Increased etching time means increased lateral etching of the conductor and more severe side etching of the conductor. This makes fine line fabrication and impedance control more difficult. In addition, the effect of copper foil roughness on signal attenuation becomes apparent as the circuit operating frequency increases. At higher frequencies, more electrical signals are transmitted through the surface of the conductor, and a rougher surface causes the signal to travel a longer distance, resulting in greater attenuation or loss. Therefore, high-performance substrates require low roughness copper foils with sufficient adhesion to match high-performance resin systems.
Although most applications on PCBs today have copper thicknesses of 1/2oz (approx. 18μm), 1oz (approx. 35μm) and 2oz (approx. 70μm), mobile devices are one of the driving factors for PCB copper thicknesses to be as thin as 1μm, while on the other hand copper thicknesses of 100μm or more will become important again due to new applications (e.g. automotive electronics, LED lighting, etc.). .
And with the development of 5G millimeter waves as well as high-speed serial links, the demand for copper foils with lower roughness profiles is clearly increasing.
Post time: Apr-10-2024