The main difference between high frequency PCB and conventional PCB is that they use laminated material, and the former material shows very special characteristics. Compared with the conventional FR-4 material, the high frequency plate uses more advanced composite materials, which have very specific requirements for dielectric constant, loss tangent and thermal expansion coefficient.
Manufacturing high frequency PCB also requires special skills that conventional manufacturers using standard PCB technology may not have. High frequency boards not only need special design techniques, but also manufacturers must use special laminates to manufacture them. At the same time, they should pay attention to specific material selection, DFM inspection and high quality control to ensure that the products can play a role in the whole life cycle.
Several manufacturers have made laminates for high frequencies. Some famous names are Taconic, polyclad, Isola, Hitachi, asaki, metclad and Rogers. However, different applications have different requirements and budgets, and designers must negotiate with the contract manufacturer to use the most appropriate material for their application and budget.
Manufacturing high frequency board
All laminates have individual manufacturing problems. In order to obtain the required quality and reliability, it is necessary to follow the corresponding manufacturing instructions for each material for processing, storage, surface treatment, inner layer treatment, drilling, bonding, deburring and electroplating.
Special process is needed to make PCB with HF material. For example, preparation of plated through holes is a critical process, especially for PTFE substrates. PTFE needs special back etching process and plasma etching settings to prepare the hole surface for electroless copper plating. Therefore, with the correct selection of materials, the proper manufacturing method is also essential for obtaining high-quality plates and working at high frequencies.
Problems in manufacturing high frequency boards
Weak adhesion: if resin blocked buried through holes are densely placed in PCB, the adhesion between resin and prepreg is very low, and delamination may occur in these areas after high-temperature welding, thus reducing the high-frequency performance. The manufacturer must use plugging resin compatible with base material and have equivalent CTE and TG. This allows both materials to expand and contract in the same way, thereby reducing the possibility of delamination caused by temperature fluctuations.
Drilling: because conventional methods cannot eliminate the heat generated during drilling, it is difficult for manufacturers to drill holes in high-frequency, high-speed Multilayer PCB. This leads to the residue of melted cuttings on the hole wall, which greatly damages the quality of the hole wall. High frequency and high-speed PCB materials need to use resin cover instead of conventional aluminum cover, because it can better absorb the heat generated during drilling, and increase the vacuum and suction pressure. In this way, the temperature of the drill bit can be reduced, the lubrication time of the drill bit can be kept longer, the drilling pollutants can be reduced, and the drilling quality can be improved at the same time.
Reverse drilling: through holes on high-frequency boards can cause signal integrity problems, because the extra roots of the through holes usually act as reflection paths, increasing insertion loss and damaging signal transmission. In order to solve this problem, the manufacturer must drill a hole larger than the through-hole in the back to eliminate the excess residual part. However, burr generated during reverse drilling may cause problems. Rapid printed circuit board can avoid drilling before etching, thus avoiding burr.
When selecting materials for RF and HF PCBs, several issues must be considered when selecting materials for RF and HF PCB. While some are related to manufacturing to achieve the most robust PCB, others are designed to achieve the best electrical performance for a given application. Due to various trade-offs, the materials with the highest electrical properties for high frequency applications may be different from those used to make the most robust PCBs.