In the lead-free electronic assembly process, due to the increase in temperature, the degree of bending of the printed circuit board due to heat is increased. Therefore, in the SMT, a board with a small degree of bending is required as far as possible, such as the FR-4 type substrate. Because of the influence of the expansion and contraction stress on the device after the substrate is heated, it will cause the electrode to peel off and reduce the reliability. Therefore, the material expansion coefficient should be paid attention to when selecting the material, especially when the device is larger than 3.2×1.6 mm.
PCBs used in surface mount technology require high thermal conductivity, excellent heat resistance (150°C, 60min) and solderability (260°C, 10s), high copper foil bond strength (above 1.5x104Pa) and bending strength (25 ×104Pa), high electrical conductivity and small dielectric constant, good punching (accuracy ±0.02mm) and compatibility with cleaning agents, in addition to the appearance of smooth and flat, can not appear warping, cracks, scars and rust spots.