The last of those layers, substrate, is made of fiberglass and is also known as FR4, with the FR letters standing for "fire retardant. "
ສຸດທ້າຍຂອງຊັ້ນທີ່, substrate, ແມ່ນຂອງ fiberglass ແລະຍັງເປັນທີ່ຮູ້ຈັກເປັນ FR4, ມີຕົວອັກສອນ FR ຢືນສໍາລັບການ "retardant ໄຟ.