1.Automated Optical Inspection Machine Finding PCB Defects Fast at Low Cost

The biggest problem engineers face when manufacturing PCBs with surface mount components is how to further increase production yield while still ensuring the shortest time and lowest cost. As the density of PCB boards increases and the size of components decreases, the performance of component placement processes gradually reaches its limits, and improving placement performance is the key to achieving high yields in large-scale production. The reason for the low yield is complicated, and one of them is that the mounting can not be done very well, so it is often necessary to adjust the X-Y data of the placement machine for the defect-free assembly. Defects can occur for a variety of reasons, and if not corrected quickly, these defects quickly add a lot of test-adjustment-retest work to the manufacturer, creating a “bottleneck” that limits plant throughput.

 

2.Automated Optical Inspection Machine Reduce the Generation of Defects

Finding defects is an important function of AOI, but reducing the generation of defects is even more important. Digital analysis shows that as component densities increase, the yield is greatly reduced regardless of whether the defect rate increases or remains constant. Therefore, the analysis of the placement process is the most basic step to reduce defects. Statistical analysis can be used to determine some of the tendencies that lead to defects in the production process, but the key is to be corrected in the shortest possible time to avoid the impact of production capacity. The “bottleneck” phenomenon.

Traditional： most surface mount manufacturers seldom make changes to the initial machine parameters of the placement equipment based on production conditions after the equipment is installed. Even if the placement operator implements process control, it is a very slow offline method that uses the most basic placement analysis method, not for different actual production processes. In fact, the main reason for using these methods to eliminate bottlenecks is very slow, and such bottlenecks are closely related to the time of test-adjustment-repair.

The current practice: the placement equipment suppliers have done a lot of fruitful work, and the industry organizations have also standardized the characteristics of the placement system. These methods are all related to the accuracy, repeatability and reliability of component placement. The key parameters establish a standard performance evaluation system. However, these methods currently focus on “first-level” process analysis or just the ideal performance conditions for using standard glass and substrate mount systems. Although these tasks are absolutely necessary to establish machine performance measurement standards, there is a certain gap between theoretical performance and actual production conditions.

Factors to consider also:

Regardless of the method used to perform closed-loop control of the placement process, it is necessary to take into account the defects associated with the placement process as well as the placement components (dimensions, shapes and different shapes), PCB boards (circuit patterns, solder mask pairs). The position of the bit, the reference point, and the degree of warpage of the board, as well as the mounting equipment itself (positioning error of XY-θ) are all related.

At the same time, the error generated by the online AOI test system inspection must be compared with the difference caused by other parts of the process. Some statistical techniques such as ANOVA (variable analysis) can be used to evaluate the interaction between process variables. If you want a real-time SPC system to work, the errors produced by the Automated Optical Inspection Machine should be as small as possible compared to changes due to placement systems, components, and PCBs.

Combining the theoretical machine performance obtained from the ideal standard with the actual machine performance also has some difficulties, because the actual performance is related to the materials and conditions used in modern surface mount manufacturing itself. An AOI device can analyze and evaluate the placement/PCB/component assembly process as a major part of the analysis system if it can acquire accurate and reproducible data at a very high speed from the actual production board. And the ultimate control.

Problems involved in the Automated Optical Inspection Machine

The performance of AOI equipment used for data acquisition in closed-loop systems is very important. If an Automated Optical Inspection Machine cannot provide high-quality data quickly, its corrective effect may be counterproductive. The main factors to consider when evaluating an AOI for a closed-loop system are: data acquisition reliability, accuracy, repeatability, speed, and ease of use.

System reliability will determine the effectiveness of AOI probing components and the correct identification of component data, such as missing parts, incorrect polarity, or component errors. The criteria for evaluating reliability are faulty false positive rate (FAR) and qualified false positive rate (FFR). The former indicates that the real defects (such as missing parts) are missed by the Automated Optical Inspection Machine, while the latter reflects the Automated Optical Inspection Machine’s passing points. It is considered a defect. The FAR of the Automated Optical Inspection Machine in actual production should be relatively low to avoid real defects entering the assembly process. Recent new AOI technologies such as color images (pictured) and adaptive techniques such as multi-level classification (MVC) can reduce FAR and FFR on products with high density 0402, 0201 and 0.4 mm pitch components.

Accuracy is defined as the difference between the measured mean and the true value, which is important to ensure that the Automated Optical Inspection Machine can accurately correct the production process. Accuracy is influenced by many factors and can be evaluated using specially prepared boards that mimic various shapes of components.

Another important indicator for determining the performance of an Automated Optical Inspection Machine is repeatability (GR&R), which measures the consistency of the Automated Optical Inspection Machine under different conditions. Ideally, AOI equipment measurements should not exceed 10% of the tolerances of the component under test. For example, on a 0402 component with a placement requirement of ±100μm in the XY direction, the repeatability of the Automated Optical Inspection Machine should not exceed 10% of the required or ±10 μm. This indicator can also be expressed in terms of accuracy/tolerance (P/T) ratio. Theoretically, P/T of 10% or less is ideal, but can be up to 30% under certain qualifications.

In addition, speed is also important because the PCB is 100% fully inspected to ensure that no defects are missing. Another important consideration is the ease of programming because the Automated Optical Inspection Machine is too long or bad to use, which inevitably affects production.

 

3.Automated Optical Inspection Machine Provide Closed-loop control

Although manufacturers use a large number of internal sensors to monitor the use of the placement machine, there is no way to monitor the results after the end of the placement process to determine if the defect is caused by a placement error or poor control. The complexity of the problem is that the surface mount production line usually consists of multiple placement machines and is different brands. Therefore, the current enterprises mostly use the open-loop method for component placement, so the defects are usually after reflow or later. The test phase will only be discovered.

Obstacles

The two biggest obstacles to true closed-loop control of the placement process are: 1) how to correlate the component data collected by the Automated Optical Inspection Machine with the operation of the placement machine (ie, which suction is used for each component placement) Mouth, placement head or machine); 2) How to modify the operation of the placement machine to correct the errors found during the measurement. The placement closed-loop control can be divided into three steps: 1) the placement system provides the feedforward phase of the critical data of the board to be assembled; 2) the feedback phase after the Automated Optical Inspection Machine measures the board; 3) the correction phase of the closed loop process.

The feedforward phase closed-loop control process collects data from each patch device at this stage, including component type, position data, feeder number, nozzle, placement head, machine, camera, visual alignment data, and general information (such as board Model and serial number) and so on. Since all of the data required for the feedforward phase may be dedicated or, in some systems, cannot be determined due to the dynamic configuration of the nozzles, the closed loop process uses a common data interface and then the vendor provides specialized data conversion. Obviously, closed-loop placement systems can help to obtain useful information, and the placement equipment used in closed-loop systems should provide a full set of relevant data, including nozzles, placement heads, machines, and feeders.

The feedback phase Automated Optical Inspection Machine will feed back all the characteristic data (such as missing parts, polarity errors and wrong parts) and variable data (XY-θ position deviation) to the closed-loop system. The changed data can be used to update the system database and the AOI variables. Linked to the components that are placed on each device.

Revision stage

At this stage, the closed-loop system analyzes the measured data, calculates the mean, standard deviation and Cp/Cpk, and calculates corrective measures related to the measurement results. For a fully closed-loop control system, the deviation correction data is automatically sent to the placement device, and the user can also specify the actions performed by the machine at this stage. This type of correction data is usually based on statistical trends rather than on a single placement. This statistical trend can be continually corrected if there are major problems such as missing parts, wrong polarity, or wrong parts. Can stop production. If the placement equipment is capable of remote control, the closed loop system will be used as a servo controller in the placement process to make the effect more visible.

4. Conclusion

To increase surface mount yield and reduce the time spent on defects, the Automated Optical Inspection Machine can be used to collect real-time data to drive a closed-loop patch system. Closed-loop placement is the first step in automating defect correction, which identifies process changes and automatically corrects itself to accommodate new placement systems. In addition, the system can also achieve high-volume production at high yields by greatly eliminating the “bottleneck” caused by defects (test-adjustment-retest).