PCBA Tech - About SMT patch computer control system

The computer control system of the SMT placement machine usually adopts secondary computer control: the sub-level is composed of a dedicated industrial control computer system, which completes the control of the movement of the mechanical mechanism; the main control computer uses a PC to realize programming and man-machine dialogue. Among the relevant technical parameters of the placement machine, placement accuracy, speed and adaptability are the three important characteristics of the placement machine. Accuracy determines the types of components that can be mounted by the placement machine and its applicable fields. High-precision machines can mount fine-pitch devices, such as FQFP, CSP, and FC, but the price of this type of equipment is much more expensive. When purchasing a placement machine, a balance point should be determined between accuracy and price. Speed determines the production efficiency and ability of the placement machine. It is clear in the selection of the placement machine, but the gap between the theoretical speed and the actual speed should be considered, as well as the type of placement component or the adaptation of the placement machine placement component. Sex, which size components can be attached. At present, there is no placement machine that can mount all different types of components flawlessly. More precisely, there is no placement machine that can balance placement accuracy, speed and component size at the same time. .

1. The placement accuracy of the placement machine

The placement accuracy is an important indicator of the placement machine. The placement accuracy refers to the mechanical accuracy of the y-rail movement of the placement machine and the z-axis rotation accuracy. It can be characterized by positioning accuracy, repeatability and resolution. Accuracy is a statistical concept, so from a statistical perspective, placement accuracy is the characteristic distribution of placement machine placement quality, which can be characterized by the average value (boat) and standard deviation (b); it can also be used for placement The operating capability of the machine in a controlled and normal working state is represented by the process capability index Cp/Cpk value.

1) Positioning accuracy

The positioning accuracy is the offset of the target mounting position relative to the printed board standard after the component is mounted. The positioning accuracy of the placement machine mainly depends on the movement accuracy of the placement head on the *, y guide rails and guide rails, and the rotation accuracy of the placement head z-axis, as well as the resolution of the CCD, PCB design, component size accuracy errors, programming and other factors related. Since the components are stored randomly in the package, the components have three degrees of freedom *, y, and o after the pick-and-place head, and there are A*, Ay, and A in the process of aligning with the pad position on the PCB. For the three errors, the center and Ay are caused by the displacement of the placement machine's mechanical positioning system, which is also called displacement error; A0 is caused by the z-axis rotation correction system in the placement head, which is also called rotation error.

2) Repeatability

Repeatability refers to the ability of the placement machine to repeatedly return to the set placement position. To be precise, the guide rail,> guide rail and e of each motion system have their own repeatability. Their combined results reflect the placement accuracy of the placement machine. Therefore, the sample accuracy given by the placement machine is usually It is characterized by the repeatability of the placement machine.

3) Resolution

Resolution refers to the smallest increment of mechanical displacement of the placement machine. It depends on the resolution of the rotation or linear encoder on the servo motor and shaft drive mechanism, and it is the means adopted by the placement machine to achieve high-precision placement. At present, the resolution of a good placement machine has achieved 0.0024. /Pulse, that is, when the placement head receives a pulse command, it will only rotate 0.0004. . Under normal circumstances, the resolution of a mounter using a light ruler/magnetic ruler is higher than that of a mounter using a code. Resolution is rarely used when describing machine performance in a comprehensive manner, so it is generally not included in the technical specifications of the placement machine. The performance index of resolution is used only when comparing the performance of the placement machine.

The relationship between the above three is interrelated, usually resolution is the basis, and the use of high-resolution means determines the good positioning accuracy of the placement machine. But sometimes a well-positioned machine, due to improper assembly and poor adjustment, will also have the problem of regular deviations in one direction after placement, but it is regular. At this time, re-adjust the machine to correct it.

The placement accuracy in actual production refers to the deviation of the alignment between the device pins and the corresponding pads. However, because the placement process is a dynamic process, the PCB it is placed on is constantly replaced, and each PCB is manufactured by corrosion methods, including reference point marks, pads, device size errors, patch glue and solder paste All have an impact on it, so in addition to the accuracy of the placement machine itself, the placement accuracy of the placement machine should also include PCB pad positioning error, pad size error, PCB light drawing error, and chip component manufacturing error.

4) Process capability index

The process capability index reflects the ability to ensure product quality when the process is in a normal state, and it is expressed quantitatively by a numerical value. The meaning of the process capability index q value and Cpk value of the placement machine refers to the placement capability that the placement machine meets the quality requirements under normal working conditions, and it is quantified.