Flatness requirements for PCB assembly?

In order to ensure the appearance and quality of circuit board, PCB assembly of circuit board has very high requirements for flatness. High flatness, fine wire and high precision have strict requirements on the surface defects of circuit board substrate, especially for the flatness of substrate. The warpage of SMB is controlled within 0.5%, while that of general non SMB printed circuit board is 1% ~ 1.5%. At the same time, SMB also has higher requirements for the flatness of the metal coating on the pad.

When tin lead alloy is plated on the pad of PCB circuit board, due to the effect of surface tension after melting of tin lead alloy in hot melting process, the surface is generally circular arc, which is not conducive to the accurate positioning and mounting of SMD; for PCB coated with solder by vertical hot air leveling, due to the effect of gravity, the lower part of pad is more convex than the upper part, which is not smooth enough, and is not conducive to SMD mounting, Moreover, the PCB with vertical hot-air leveling is not evenly heated, and the heating time of the lower part of the PCB is longer than that of the upper part, so it is easy to warp. Therefore, it is not suitable for SMB to use hot-melt Sn Pb alloy coating and vertical hot-air leveling solder coating. Horizontal hot-air leveling technology, gold plating process or preheating flux coating process are required.

PCBA chip processing

In addition, the solder mask on SMB also requires high precision. It is difficult to meet the requirement of high precision for screen printing solder mask, so liquid sensitive solder resist is widely used in solder mask. Since SMD can be assembled on both sides of the SMB, the solder mask graphics and marking symbols should be printed on both sides of the board. Moreover, with the reduction of electronic product volume and the increase of assembly density, it is difficult to meet the requirements of single-sided or double-sided printed circuit boards. Therefore, multi-layer wiring is required. Generally, most of the current SMBs are 4-6-layer boards, up to 100 layers. To sum up, compared with the plug-in PCB, the requirements of SMB far exceed the plug-in PCB in terms of the selection of substrate and the manufacturing process of SMB itself.