Printed circuit boards used as PCB assembly and interconnection must adapt to the rapid development of current PCB assembly technology. PCB assembly printed circuit boards have become the mainstream products of current PCB manufacturers. Almost 100% of PCBs are SMBs, and their functions are The through-hole plug-in PCB is the same, but because the SMC/SMD is directly mounted on the SMB in the process, its requirements are much higher than the plug-in PCB, and the design and manufacturing are much more complicated. Compared with plug-in PCB, SMB has the following main features:

1. High density. Since the number of SMD pins is as high as hundreds or even thousands, and the center distance of the pins can reach 0.3mm, the SMB on the circuit board requires fine lines and fine spacing, and the line width is reduced from 0.2~0.3mm to 0.1mm or even 0.05mm. , There have been 4, 5 or even 6 wires with double wires between 2.54mm grids. The fine lines and fine pitch greatly increase the assembly density of SMB.

2. Small aperture. Most of the metallized holes in SMB are not used to insert component pins, and no soldering is performed in the metallized holes. The metallized holes are only used as electrical interconnections between layers, so they should be reduced as much as possible. The small aperture provides more space for SMB. The hole diameter has changed from 0.5mm to 0.2mm, 0.1mm or even 0.05mm.

3. Low thermal expansion coefficient. Any material will expand when heated, and polymer materials are usually higher than inorganic materials. When the expansion stress exceeds the material's bearing limit, the material will be damaged. Due to the many and short SMD pins, the CTE between the device body and the PCB is inconsistent, and the damage of the device caused by thermal stress often occurs. Therefore, the CTE of the SMB substrate should be as low as possible to adapt to the matching with the device. Sex.

4. Good high temperature resistance. Most of today's SMBs require double-sided mounting components. Therefore, SMBs are required to withstand two reflow soldering temperatures. And nowadays, lead-free soldering is used, and the soldering temperature requirements are higher, and SMBs are required to be small and non-deformed after soldering. Foaming, the pad still has excellent solderability, and the surface of the SMB still has a high degree of smoothness.