Multi-layer PCBs encompass a few or more conductive layers divided by insulating material, making it possible for for intricate circuit designs in the compact form component. In short, the 4-layer board has an inner layer. Regarding the method, some lines might be etched in the inner layer shaped by lamination. https://10-layer3-levelhdigoldfin50593.blogchaat.com/31271565/the-definitive-guide-to-16-layer-5-order-hdi-high-frequency-storage-server-pcb