SMT (surface assembly technology) does have strict requirements for the surface of PCB boards, which are mainly based on ensuring the smooth progress of patch processing and the high quality of the final product. The following is a detailed analysis of the requirements for the surface of PCB boards:

1. Surface flatness and smoothness

Flatness: The surface of the PCB board must be flat, without warping or unevenness. Warped or uneven PCB boards may cause cracks, affect the life of steel mesh and scrapers during solder paste printing and patching. Therefore, the PCB board should meet certain bending and twisting standards, such as the upward bending degree is less than 1.2mm, the downward bending degree is less than 0.5mm, and the twisting degree (maximum deformation height divided by diagonal length) is less than 0.25.

Smoothness: The surface of the PCB board should be smooth, without defects such as scratches, rust spots, and oxide layers. These defects may affect the quality of positioning, mounting and welding.

2. Cleanliness and no dirt

Cleanliness: The surface of the PCB board should be able to withstand cleaning with detergents and should not produce adverse reactions during the cleaning process, such as bubbles, discoloration, etc. This usually means that the surface of the PCB board should be free of pollutants such as grease, dust, fingerprints, etc.

No dirt: The surface of the PCB board must be clean, free of dirt, rust spots, etc. These dirt may affect the welding quality and the placement accuracy of components.

3. Surface color and identification

Color difference: There should be a clear difference between the surface of the PCB board and the surrounding color, which helps visual inspection and positioning during the SMT process.

Clear identification: The identification on the PCB board (such as Mark points, text, symbols, etc.) should be clearly visible and not deformed. As the key positioning reference for mounting components, the Mark point has strict requirements on its shape, size, position and surrounding environment. For example, the Mark point usually adopts a standard circle, square or triangle with a size between 0.8 and 1.5 mm, and there should be no obstacles with a significantly different color from the mark within 1 mm around it.

4. Other special requirements

Thermal conductivity and heat resistance: The PCB board should have good thermal conductivity and heat resistance to adapt to the high temperature welding environment during the SMT process. PCB boards with good thermal conductivity can reduce the defective products produced during the welding process. At the same time, with the widespread application of lead-free technology, the heat resistance requirements of PCB boards are also higher, usually requiring them to withstand high temperatures of 260 degrees Celsius for more than 10 seconds.

Conductivity: As a carrier of electronic components, PCB boards must have good conductivity. PCB boards with poor conductivity may cause poor connections between components and affect the performance of the entire product.

In summary, SMT's requirements for the surface of PCB boards include flatness and smoothness, cleanliness and dirt-free, surface color and clarity of logos, and other special requirements such as thermal conductivity, heat resistance and conductivity. These requirements are intended to ensure the smooth progress of SMT patch processing and the high quality of the final product.