In order to ensure the quality of the appearance of the circuit board and, the surface of the circuit board PCB assembly have high request for smoothness, high flatness, fine lines, high precision of the circuit board baseplate surface defect is strict, especially for substrate flatness requirement is more strict, SMB warp degree requirements of control within 0.5%, and the general demands of SMB PCB warp degrees is 1% ~ 1.5%. At the same time, SMB also has higher flatness requirements for the metal coating on the pad.
When the solder pads of PCB are plated with tin-lead alloy, the surface tension of tin-lead alloy after melting in the hot melting process generally presents a circular arc surface, which is not conducive to accurate positioning and mounting of SMD. Vertical hot air leveling coating solder PCB, due to the effect of gravity, generally lower than the upper convex welding plate, is not flat, is unfavorable to SMT SMD, vertical hot air leveling of the printed circuit board and uneven heating, plate bottom heating time is longer than the upper, prone to warping, therefore the SMB unfavorable use tin-lead alloy plating and hot melt vertical solder coating layer of hot air leveling, calls for horizontal hot air leveling technology, gold-plated process or preheat flux coating process.
Pcba patch processing
In addition, welding resistance graphics on SMB also require high accuracy. It is difficult to meet the high precision requirement of screen printing resistance graph, so the resistance graph on SMB mostly USES liquid photosensitive flux. Since the SMD can be assembled on both sides of the SMB, the SMB also requires both sides of the board to be printed with solder resistance graphics and symbols. In addition, with the reduction of the volume of electronic products and the improvement of the assembly density, it is difficult to meet the requirements of single or double-sided printed circuit board, so multilayer wiring is required. Generally, SMBS of today are mostly 4-6 layers, up to about 100 layers at most. In conclusion, compared with PCB, SMB requires far more than PCB in both the selection of base material and the manufacturing process of SMB itself.