Repeatability Accuracy of the Chip Mounter

Repeatability Accuracy of the Chip Mounter

Repeatability accuracy deSCRibes the ability of the mounting head of the chip mounter to repeatedly return to a certain set position. It can also be defined as a standard deviation when mounting different components at the same position on different circuit boards, and is sometimes also called repeatability. It reflects the degree of convergence and divergence among the deviations when the mounting head reaches a mounting position multiple times, which is equivalent to the concept of precision in metrology. However, as mentioned in the previous section on mounting accuracy, the characteristics and working mechanism of the chip mounter are more similar to those of a CNC machine tool. In the GB101131 - 811 standard for evaluating the repeatability accuracy of CNC machine tools, the concept of repeatability accuracy in the American NMTBA standard is referenced. It is stipulated that when approaching unidirectionally, under the same conditions, approaching a certain given point multiple times results in a degree of dispersion centered around the average position μ; when approaching bidirectionally, under the same conditions, approaching a certain given point multiple times in the positive and negative directions results in a degree of dispersion centered around the average position μ, which is expressed as ±3σ, as shown in the figure below. Regarding the meanings of μ and σ, follow the official account for subsequent explanations.

Fundamentally, the motion system of the chip mounter, including the X - guide, Y - guide, Z - movement, and Z - axis rotation, all have their respective repeatability accuracies. The combined result of these repeatability accuracies and the mounting accuracy determines the mounting precision, which ultimately affects the welding process quality in the subsequent processes. Therefore, in the IPC - 9850 standard, the comprehensive consideration of various factors is used as the standard for evaluating the accuracy of the chip mounter.
Currently, high - precision chip mounters can provide a repeat positioning accuracy as high as the micron level (0.001mm).

The repeatability accuracy of mounting can also be understood in terms of the degree of dispersion between the actual mounting position and the target position. The relationship between accuracy and repeatability accuracy is shown in the figure below.

Analysis of SMT Mounter Mounting Precision Parameters and How to Measure Them