The performance parameters of PCBA are closely related to the manufacturing process of the PCB and the design requirements of the PCB designer. Among the numerous PCB parameters, the main factors that can affect the SMT process are its geometric dimension parameters, which mainly include the flatness of the entire PCB and PCB manufacturing tolerances and PCB surface treatment and other aspects of the impact. Now let's introduce that for you as below.
According to the characteristics of PCBA and the manufacturing process, we know that it is formed by one-layer adhesion and crimping. During the manufacturing process, stress is generated, and the material and thickness of the pad layer and the middle electrical isolation layer are different. The coefficient of thermal expansion is also different, so the PCB may undergo plastic deformation (Bow) or twist (Twist) in the process of packaging and transportation. The degree of plastic deformation is related to the thickness and area of the PCB and the symmetry of its design. .
The allowable value for PCB bending and twisting is 0.75% of the length of the PCB, which is (L×0.75%) mm, but the maximum cannot exceed 3.175 mm.
If the flatness exceeds the allowable range, the shape of the reference point in the camera will change. For example, the shape of the standard circle reference point in the camera will become an ellipse, which will cause the center-to-center distance reading between the reference points to change, which will affect The position of the component pads on the PCB relative to the reference point may directly cause the component to deviate. In addition, due to warping of the PCB in the chip processing, the component may slide on the solder paste on the pad during the depressing process, or the solder paste on the bottom of the component may be squeezed to form a solder ball or lead to bridging of adjacent components. .
2, the impact of manufacturing tolerances
PCB manufacturing tolerances will also affect the placement quality, mainly reflected in the Fiducial contour and position deviations, as other errors on the PCB, such as manufacturing tolerances of vias will directly affect the after welding The electrical performance, but this does not belong to the placement machine.
3, surface treatment
The effect of surface treatment on placement is mainly reflected in the coverage of fiducials. It can mainly affect the accuracy of the visual system and quickly identify the fiducials. The important principle is that the solder mask must not cover the reference point and the auxiliary resolution areas around it.
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