The steps of this method are reasonably uncomplicated, and The crucial element should be to plug the pores and cleanse the plugged solder mask. In short, the four-layer board has an inner layer. Regarding the method, some strains is going to be etched through the inner layer formed by lamination. https://stephenntvvb.weblogco.com/31467506/6l-8l-10l-12l-layer-second-order-pcb-circuit-board-options