PCB Blog - AOI Exploration of 6 Sigma PCBA Assembly

How to check assembly defects in modern inspection systems, root cause analysis of PCBA assembly defects, and 6Sigma exploration of measurement systems.

1 Introduction

How do assembly defects arise? Why do some PCBA assembly produced by the assembly line meet the requirements, while others are endlessly reworked and reworked due to multiple assembly errors? Why does the assembly quality vary from batch to batch? More importantly, from these variations What experience should we gain, and what should be done to exclude variation in PCBA assembly. The above problem is the traceability of 6 Sigma production, Sigma is a Greek letter that describes the distribution or dispersion of the mean value of any process parameter, that is, the standard deviation. 6 Sigma is the use of statistical techniques to determine the state of the process by measuring the process capability, and then through comparative analysis to find out the main variables that affect the process capability, use process optimization methods to find out the law of change, and then eliminate it. Or control, through a continuous measurement-analysis-improvement-control cycle, the process capability is continuously improved and finally reaches or exceeds the 6 Sigma level.

2. 6Sigma and PCBA assembly

Variability refers to any variation that has a potential negative impact on product quality. PCB board design should comprehensively consider its electrical and mechanical performance reliability, such as component pad design tolerance, pad pattern design, etc. Secondly, the dimensions and quality of components and materials used to assemble PCBA will also affect the assembly quality. , the variation of the assembly process itself will also affect the quality of PCBA assembly. In PCBA assembly, variation is the "enemy". After eliminating the obvious sources of variation in design and materials, the rest is the variation of the PCBA process itself using PCB boards, components, solder paste, etc. Attribute data represents unsuspected, fixed defects due to process variation, and attribute data is usually yes/no, good/bad, I/O type data. The variable data records the degree of process variation, which is not directly indicated as a defect but is data of digital type, measurement type, etc. that must be recorded and related to attribute data, unsuspected defects, or the probability of occurrence of defects. Attribute data inspection is a way to observe the presence or absence of unacceptable variation. The characteristics and frequency of attribute data are related to the source of variation. Defects are usually found during in-circuit testing, functional testing, automated optical image analysis or manual visual inspection, or other means of inspecting PCBAs. Some variations in the PCBA manufacturing process are unavoidable, and measures need to be taken in advance to prevent them from occurring, which are called "acceptable process variations". APV is usually the tolerance of the assembly process or the acceptable mechanical variance in components, raw materials, etc. APV produces variable data, but it does not become the source of defects in the final product. If there are unquestionable defects or fixed defects due to APV, design or manufacturing problems must be improved in advance. Unacceptable process variations are those variations that are undetected and necessarily lead to defects or have a high probability of defects. Jinli's process should accept APV, detect and reject UPV. 6 Sigma is used to define the method and necessary error to distinguish APV from UPV. In order to identify variation and provide a continuous measure of variation and its resulting defects, we must understand the various data and attribute data when the PCBA is actually produced. In order to implement the measurement, the measurement mechanism of the measurement variable data and attribute data in PCBA production needs to be understood. Attribute data testing is the key to inspection and testing in current PCBA production. Modern PCBA assembly factories are usually equipped with modern inspection systems such as automatic optical image analysis, online testers, and functional testers to scan and detect defects and send the inspection results to operators.

3. The main source of defects in electronic product assembly

Because all variation can lead to defects, variation is the "enemy" of production. In combination with the PCBA production process, we mainly discuss the origin of defects in SMT production. Combined with the main processes of solder paste printing, patch and reflow soldering, etc., the discussion is as follows:

Solder paste printing: mistakes (problems): solder paste missing printing, solder paste short circuit, solder paste contamination. Difference (variation): solder paste coverage area, solder paste coverage height, solder paste coverage volume, solder paste coverage pattern. Inspection: solder paste coverage/missing, adjacent pad inspection, solder paste coverage area inspection. Measurement: solder paste coverage area, solder paste coverage height, solder paste coverage volume, solder paste coverage pattern. SMD: Error (problem): Missing components, wrong direction of components, damaged components, wrong components. Differential bite): x-Y-z axis, component/pad registration, assembly registration. Inspection: pasted/missing pasted components, component orientation signs/signs, component packaging shapes. Measurements: x-y-z axis, component/pad alignment, assembly alignment. Reflow failure (problem): component placement characteristics, component erection, tombstone phenomenon, solder beads, solder short circuit, etc. Inspect all wing lead solders, all J lead solders, solder shorts check, check for discrete components (floating), random contamination of components, etc.

4. Automatic optical image detection

During the assembly process, the amount of solder paste and the shape of the solder joints of the semi-finished product, the thickness of the wires of the bare circuit board, and the defects of the wires should be continuously inspected, which are generally undetectable by online tests or functional tests. Visual inspection has many errors and low efficiency, and automatic optical image inspection is a recognized and effective method. At present, automatic optical image inspection adopts two methods: design rule inspection and graphic recognition. The design rule inspection method is to check the circuit pattern according to two given rules, such as all connections should be terminated with solder joints, the width of all leads should not be less than 0.127mm, and the interval between all leads should not be less than 0.102mm. This method can guarantee the correctness of the tested circuit from the algorithm. Pattern recognition is the comparison of stored digitized images with actual work. The inspection is carried out in accordance with the inspection documents established by inspecting a good printed circuit board or glass model, or in accordance with the inspection procedures compiled in the computer-aided design. Accuracy depends on the resolution and the inspection procedure used. Modern automatic optical image inspection systems can ensure that extremely small x, Y, θ (rotational) position deviation variations are measured and tracked when detecting component placement characteristics. The inspection process is very sensitive, and some variations that should be eliminated, such as position, Dimensions, and images are measured, and some acceptable process variations such as component supplier changes, nominal dimensions, logos or colors default (allowed), and the positional characteristics of the component placement process are recorded.

5. R&R research on automatic optical image detection system

The repeatability of measurement results refers to the consistency between the results obtained from consecutive and multiple measurements of the same measurand under the same measurement conditions. Reproducibility of measurement results refers to the consistency between measurement results of the same measurand under changing measurement conditions. For modern AOI systems, the repeatability of the measurement results is very important. Because it is possible to identify key variations with an AOI system, but to draw accurate conclusions about the trend of the variation requires good measurement repeatability of the AOI system to distinguish the process variation from the variation of the measurement system itself. According to the requirements for the detection capability index, the selection of the standard device usually follows the one-third principle, that is, the ratio of the accuracy of the standard device to the measured measuring instrument should be kept at a ratio of 1/3. In the inspection of parts in the machinery industry, the ratio of the measurement limit error to the tolerance is called the coefficient of accuracy, which should usually be kept within the range of 1/3 to 1/10. The detailed calculation of the measurement uncertainty (RaR) of the AOI system will not be listed here. Modern AOI systems have measurement uncertainty better than ±0.4mils at a confidence factor of 3, which means that 99.73% of the measurements fall within the upper and lower specification limits. In fact

6. In the production of Sigma PCBA, what is the measurement uncertainty required by the AOI system? It is generally considered that the current SMD component is 0201 size. If it is required to detect a 50% deviation from the pad, the required measurement value is 0.127mm. Using the above The 1/10 principle of the AOI measurement system requires that the measurement uncertainty of the AOI measurement system is less than 0.0127mm when the confidence factor is 3. For the current QFP package IC, its size is 0.4064mm×0.2032mm, and the detection requirement is also 50% off the pad, that is, when the confidence factor is 3, the measurement uncertainty of the AoI measurement system is required to be less than 0.01016mm. The 6Sigma PCBA test mentioned above means that the variation from the center of the specification value ±3 Sigma is considered as an "iE normal or acceptable" variation.

7. Measure patch (pick-and-place) capability

In the SMD process inspection, in order to ensure the repeatability of 6 sigmas, how do select the inspection standard? The following is the QFP0402 component with repeatability of ±0.0508mm (confidence factor of 3) and spacing of 0.0508mm based on the SMD process capability. Take 50% of the pads as an example of the SMD process inspection required for inspection: First, formulate the average value to determine the measurement result distribution of the process statistic with the SMD placement repeatability of ±0.0508mm at the 3 Sigma confidence factor. And the distribution drift of the mean value over time, temperature and maintenance cycle, etc. This specification is part of the inherent characteristics of the device, and its origin is very important. If this is a device feature, the user needs to reconsider them. Does this feature represent the delivered, pick-and-place comprehensive characteristics of the SMD process, including variations in SMD component size, PCB board supplier, PCB board deformation, etc. It needs to be decomposed into actually delivered equipment characteristics or at different times and temperature conditions, test a series of products, and calculate the distribution drift of different batches of test samples. Second, we must recognize that the 50% off-pad inspection requirement is the limit for pick-and-place inspection applications in the SMD process, and many products actually specify 30% or less as a tolerance. Third, the deviation of the component from the pad should be calculated by 50%. For 0402QFP components, the deviation of 50% from the pad represents the deviation of 0.127mm. Therefore, when an AOI inspection is performed, the measurement uncertainty of the AOI measurement system should be less than 0. 0127mm. It can be calculated that when the confidence factor is 3, for the process distribution of ±2mils, 50% deviation from the pad is used as the detection requirement, which represents the placement detection limit of 7 Sigma (assuming that the mean distribution remains stable).

8. Conclusion

6 Sigma PCBA production will be our goal. Combining 6 Sigma with modern automatic optical image inspection equipment, a significant reduction in total PCBA assembly errors has been demonstrated. And in the component placement process, it can provide precise and repeatable position measurements to confirm its 6 Sigam performance. To ensure 6 Sigam performance, automatic optical image inspection is critical. The third generation of the modern automatic optical image inspection system, its repeatability, performance, and speed can meet the modern PCBA assembly requirements. At the same time, it provides key measurements of the assembly process to the manufacturer and combines the statistical results of the inspection with the patch process to provide comprehensive closed-loop control to ensure the quality of PCBA assembly production.