What Are The Requirements Of Pcb For The Flatness Of SMB Pads

In order to ensure the appearance and quality of the circuit board, the PCB assembly on the surface of the circuit board has extremely high requirements for flatness. High flatness, thin lines, and high precision require strict surface defects on the circuit board substrate, especially the requirements for the flatness of the substrate are more strict. The warpage of SMB is required to be controlled within 0.5%, while the warpage of non-SMB printed circuit boards is generally required to be 1% to 1.5%. At the same time, SMB also has higher flatness requirements on the metal plating on the pad.

When tin-lead alloy is electroplated on the pad of PCB circuit board, due to the effect of surface tension after the tin-lead alloy is financialized in the process of hot melting, it is generally an arc-shaped surface, which is not conducive to SMD accurate positioning placement. vertical hot air leveling coating for the printed circuit board of solder, due to the effect of gravity, the lower part of the general pad is more convex than the upper part, which is not flat enough, and it is not conducive to mounting SMD. Moreover, the printed circuit board flattened by vertical hot air is unevenly heated, and the lower part of the board is heated longer than the upper part, and it is easy to warp. Therefore, SMB should not use hot-melt tin-lead alloy coating and vertical hot air leveling solder coating, which requires horizontal hot air leveling technology, gold plating process or preheating flux Coating process.

In addition, the solder mask pattern on the SMB also requires high precision. The commonly used screen printing solder mask pattern method has been difficult to meet the high precision requirements, so most of the solder mask patterns on SMB use liquid photosensitive solder resist.

Since SMD can be assembled on both sides of the SMB, SMB also requires solder mask graphics and marking symbols to be printed on both sides of the board. Moreover, as the volume of electronic products decreases and the assembly density increases, it is difficult for single-sided or double-sided printed circuit boards to meet the requirements. Therefore, multilayer wiring is required. Generally, today's SMBs are mostly 4-6 layers, and can be up to about 100 layers.

In summary, compared with plug-in PCBs, SMB requires much more than plug-in PCBs, whether it is the choice of substrate or the manufacturing process of SMB itself.