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.