PCB assembly printed circuit board requires flatness
In order to ensure the appearance and quality of the board, the surface pcb component of the board has extremely high requirements for flatness. High flatness, fine lines and high precision strictly affect the surface defects of the board substrate, especially the flatness requirements of the board. More strictly, SMB warpage is required to be controlled within 0.5%, while general non-SMB printed circuit board warpage requirements are 1% to 1.5%. At the same time, SMB also has a higher flatness requirement for the metal coating on the mat.
When the tin-lead alloy is plated on the pad of the PCB circuit board, the surface tension of the tin-lead during the hot-melting process is generally a circular arc surface, which is not suitable for the precise positioning of the SMD; the vertical hot air leveling coating the solder printed circuit board due to gravity The effect is that the lower part of the pad is generally convexer than the upper part, not flat enough, which is not conducive to the installation of the SMD, and the vertical hot air leveling printed circuit board is unevenly heated, and the lower part of the board is heated. Time is more prone to warping than the minister. Therefore, SMB should not use a hot-melt tin-lead alloy coating and a vertical hot-air leveling solder coating. Horizontal hot air leveling techniques, gold plating processes or preheating fluxes are required. Painting process.
In addition, the solder mask pattern on the SMB also requires high precision. The commonly used screen printing solder mask pattern method is difficult to meet the high precision requirements, so most of the SMB solder mask patterns use liquid photosensitive solder resist. Since SMD can be assembled on both sides of the SMB, the SMB also needs to print solder mask patterns and mark symbols on both sides of the board. Moreover, as the volume of electronic products decreases and the assembly density increases, single-sided or double-sided printed circuit boards are difficult to meet requirements, and thus multilayer wiring is required. Generally speaking, today's SMEs are mostly 4-6 floors and up to 100 floors. In summary, SMB is more demanding than plug-in PCBs than PCBs, whether it is the choice of substrate or the manufacturing process of SMB itself.