/Pcb assembly printed circuit board flatness requirements ?
In order to ensure the appearance and quality of the circuit board, the surface pcb assembly of the circuit board has extremely high requirements on the flatness. High flatness, thin line and high precision, be strict on the surface defects of the circuit board substrate, especially the flatness requirement of the substrate are more stringent, SMB warpage requirements are controlled within 0.5%, while general non-SMB printed circuit board warpage is required to be 1% to 1.5%. At the same time, SMB also has a higher flatness requirement for the metal coating on the welding pad.
When the tin-lead alloy is plated on the pcb circuit board welding pad, tin-lead in the hot-melt process generally present a circular arc surface under surface tension, which is not benefit for accurate positioning of the SMD; vertical hot air leveling coating Solder printed circuit board, due to the effect of gravity, the bottom part of the general pad is more convex than the upper part, not flat enough, and is not conducive to mounting SMD, and the vertical hot-air leveling printed circuit board is unevenly heated, and the heated time of bottom part is longer than the upper part and is prone to warpage. Therefore, SMB should not use hot-melt tin-lead alloy coating and vertical hot-air leveling solder coating. Horizontal hot air leveling technology, gold plating craft or preheating flux coating craft are required.
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 the SMB solder mask pattern mostly uses liquid photosensitive solder resist. Since the SMD can be assembled on both sides of the SMB, the SMB also requires solder mask graphics and marking symbols printed on both sides of the board. Moreover, as the volume of electronic products miniature and the assembly density increase, single-sided or double-sided printed circuit boards are difficult to meet the requirements, so multi-layer wiring is required. Generally, today’s SMBs are mostly 4-6 layers, most up to 100 layers. In summary, compared with the PCB, the SMB is much more demanding than the plug-in PCB, whether for the choice of substrate or the manufacturing craft of SMB itself.
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