On PCB performance parameters, it is closely related to the PCB manufacturing process and PCB designers to design requirements , in many PCB parameters for placement process can affect primarily its geometry parameters , including the entire PCB Fu Dao flatness and PCB manufacturing tolerances and surface treatment effects of PCB 's and other aspects will be discussed separately below .
1 . Flatness
According to the characteristics of the PCB manufacturing process , as we know it by the adhesive layer is a layer formed by crimping , there is stress generated during the manufacturing process , and the material and thickness of the pad layer and the intermediate electrically insulating layer are different , and therefore the thermal expansion coefficient is different , and therefore packaging and transportation process , PCB may be associated with plastic deformation of the bending (Bow) or twisted (Twist), whether the plastic deformation degree of the PCB thickness and area , and its lamellar design symmetry .
PCB bending , twisting allows PCB length L is 0.75% , that is, (L × 0.75%) mm, but the maximum should not exceed 3.175 mm.
If the flatness exceeds the allowable range , the reference point of the camera in the shape change , e.g. , a standard reference point a round shape in the camera becomes the ellipse , which causes the change occurred between the center reference point from the readings , which may affect components on the PCB pad relative to the reference point , which may be a direct result of component placement side. In addition, due to the PCB solder components can cause warping process is relatively lower pressure on the pads from sliding , or to the bottom of the squeeze paste elements , thus resulting in the formation of tin beads or adjacent components bridging short circuit.
2 . The impact of manufacturing tolerances
PCB manufacturing tolerances also affect the placement quality , mainly at the reference point (Fiducial) contour and position deviation , the other error on the PCB , manufacturing tolerances , such as vias will directly affect the welding electrical performance, but this does not affect the placement machine belong .
3 . Surface treatment
For mounting surface treatment effects , mainly in the areas covered by the reference point (Fiducial) , it can affect primarily visual system can accurately and quickly identify the reference point . Important principle is that the solder layer not covered by the reference point and the area around it to distinguish the secondary .
