过程品质控制：通往零缺陷制造的途径.doc

过程品质控制：通往零缺陷制造的途径Process quality control: the approach to zero defect manufacturingProcess quality control: the approach to zero defect manufacturingThe ultimate goal of adopting AOI will determine where to place the AOI on the production line and what process control information will be produced.As printed circuit assembly becomes smaller and more dense, automatic AOI (automated optical inspection) devices are increasingly used to monitor and guarantee the quality of board (PCB, printed, circuit). In addition, the efficient use of AOI with specialized purposes can produce process control information of different types and levels of detail.There are four types of targets for implementing AOI inspections:1. final quality (End, quality). Manufacturers who focus their attention on the final quality are interested in the status of the product when they leave the production line. This goal is preferred when production issues are clear, product mix is high, and quantity and speed are critical factors. AOI will usually be attached to the end of the line. In this position, the device can produce a wide range of process control information.2. process tracking (Process tracking) uses inspection equipment to monitor the production process. Typically, detailed defect classification and element placement offset information are included. This is a priority for manufacturers when product reliability is important, high-volume manufacturing is low, and supply of components is stable. This often requires moving the inspection equipment to several locations and monitoring specific performance online. Process control information is usually less than the final quality goal, but it can directly find specific process problems. Support the steps of special process process control information may be more quantifiable than the other target.3., ICT (ICT, test, in-circuit), pass through rate (FPY, first-pass, yields), reaction process capabilities and any difficulties in the steps. The AOI device improves the pass rate of the ICT by finding the defect plate and sending the defect plate to repair before the first ICT. When production volumes are large and demand times are short, manufacturers are given priority in order to make testing and delivery of products a critical issue. Because AOI is placed near the final end of the line, process control information is typically qualitative.4. characterization (Characterization). In the production of hazardous assemblies, i.e. medical or military applications, the inspection equipment must be adjusted to find all possible abnormalities. As a result, there is almost no omission rate, but it produces false positives and requires longer inspection times. AOI is typically used at the end of the production line to check solder points, but it can be removed to monitor the special process. The key production factors for this goal are the type of production, the risk of site failure, and the allowable monitoring time. Qualitative and quantitative process control information can be generated, depending on the location and setting of the equipment.In many applications, the AOI machine settings will be no different for any of these goals. The difference is to check how the machine is used. For example, check the machine typically only signs of serious defects, however, if the feature as the inspection target, the user may browse to the (or to check the equipment abnormal report) just enough serious, not enough to fully reflect the characteristics of the product.The inspection facility must support all of these inspection targets and move from one to the other. Similarly, process control information may vary depending on the use of the device. For example, although component mount information can be collected and checked by reflow soldering, the use of this information will be checked before reflow soldering. At this site, defects associated with mounting machine errors may be less relevant.Implementation strategy (Implementation, Strategies)The location placed by the machine can realize or hinder the inspection target, and different positions can produce different process control information. The placement of AOI is determined by the following factors:1. special production problems. If the production line has specific problems, the inspection equipment can be added or moved to this location to monitor defects and detect duplicate problems as early as possible.2. implementation objectives. For inspection equipment, there is no best place to handle all production line defects (Table 1). If the goal of implementing AOI is to improve the overall final quality, placing the machine in front of the process may not be worth much later. One argument that the machine puts in front is to avoid adding value to the products that are already defective. Also, the cost of maintenance in the early stages of the process is substantially less than the cost of maintenance before and after delivery. However, many defects occur late in the production, meaning that no matter how many defects have been detected before, a comprehensive visual inspection is required before shipment.Place of placement (Placement, Location)Although AOI can be placed behind the part of the assembly process, there are three check locations that are primary:After solder paste printing (after, solder, paste, deposit) (Figure 1). If the solder paste printing process meets the requirements, the number of defects found by ICT can be dramatically reduced by 1. Poor printing may be related to the following problems:1. solder on soldering pad is insufficient.2. solder on soldering pad too much.3. solder butt welding disc bad reclosing.Solder bridge between 4. pads.On ICT, the probability of defects relative to these conditions is directly proportional to the severity of the situation. A slight lack of tin rarely causes defects, while severe conditions, such as the fundamental Wuxi, almost always cause defects in the ICT. Insufficient soldering may be a cause of component loss or open solder joints. Still, deciding where to place AOI requires recognizing that component loss may have occurred for other reasons, and that these reasons must be included in the review plan. This location check most directly supports process tracking and characterization.Quantitative process control data at this stage include information on the offset and solder volume, and qualitative information about the solder print will also be generated.The inspection before reflow (Pre-reflow) (Fig. two) is done before the component is placed on the solder paste inside the board and the PCB is fed into the reflow furnace. This is typically a placement check machine location, as most defects from solder paste printing and machine bonding can be found here.The quantitative process control information generated at this location provides information on the calibration of the tablet machine and the dense pitch element mount device.This information can be used to modify component placement or to indicate that the mounter needs calibration. This location check supports all AOI implementation goals.After reflow (Post-reflow) (Figure three) inspection is the final step in the SMT process, before the completion of the shipment ICT, functional testing and system testing. This is the most popular choice for AOI, because this location can detect full assembly errors. The reflow test provides a high level of safety because it identifies errors caused by solder paste printing, component mount and reflow processes. This location check supports all AOI implementation goals.Although quantitative information can be generated at this point, it may be difficult to correlate errors with deterministic causes. Nevertheless, the resulting qualitative information can provide the most accurate indicator of the whole process state, since at this point the SMT process has been completed.If each location is ideal for identifying a particular defect, the challenge of implementing AOI is to put the inspection device in a position that can identify and correct the most defects as soon as possible.Process control information (Process, Control, Information)The AOI device produces two types of process control information. Quantitative information, such as component offset measurements, can be used to monitor the state of the patch device. The subsequent logic is that the mount accuracy is a good indicator of the quality of the whole production process. Qualitative information can determine the quality of the entire assembly process by directly reporting defect information. This information can be used to determine system defects in the manufacturing process. The steps for analyzing information are as follows.Select a control chart as the master view. This diagram is often displayed on inspection machines or rework stations. The operator looks at the chart to determine if the process is out of tolerance. A check line for reflow monitoring of a monitoring element. The chart is typically a discrete diagram of component offsets. When the drawing points exceed the preset limit, the operator can correct the defects and adjust the mounter if possible. For reflow applications, the master supervisor view will either be a Pareto diagram, or a FPY diagram. Figure four shows a typical FPY quality control chart. This diagram compares the FPY data generated by the operators from the AOI device and the PCA rework station. Because FPY is a good indicator of overall process line performance, this diagram is generally used as a critical process monitoring tool. At any time, the operator can select one point for further investigation and produce a more detailed defect classification map.Fig. five is a summary of the defects reported in the Pareto diagram classification. A process control diagram such as this one is typically presented when an FPY graph has some kind of exception. The diagram tells the process engineer what types of defects are emerging. In this case, the most important defect is the tin bridge, which accounts for 42% of the defects. The linear graph shows the cumulative percentage of defects associated with the Pareto bar graph. It shows that up to three defects account for 75% of the errors that occur on the product. If these defects are eliminated, significant process improvements can be achieved. To further investigate this data, it is possible to determine the location of the solder short circuit.Figure six shows where the solder short fault occurs.A process engineer can better determine the root cause of a defect by examining the occurrence of a particular defect one by one. In this case, the most defective position causes 15% of the total number of tin bridges. As this is critical, the root of the defect will require further investigation.Qualitative quality control charts are given to operators and process engineers to improve tools for finding root defects and highlighting system defects.The future of AOI (Future, of, AOI)The AOI industry is trying to reach the ultimate goal of zero defect manufacturing, but not yet. The present situation is that a series of statistical quantities tell the process engineer what has happened, but it cannot tell the operator what will happen. Similarly, it is assumed that the root of the defect is the control chart type produced by the AOI system, but the system rarely determines its root. The component mount / component offset check cycle is only just beginning to approach a point. The point before reflow is that the tester can send an offset correction signal directly to the mounter and modify its mount program.In the process of quality control, AOIs future will involve intelligent software system, the system of statistical data, and the quality of the relevant information and data from other production equipment variables to translate, predict the defects occurred before, to find out the possible root defects in a moving process. Further increase in system capability will make automatic corrective action possible.conclusionThe four major inspection objectives are suitable for all types of manufacturers. Where AOI is placed on the production line depends on what the ultimate goal is. Location also affects the quality of the process control system produced by the inspection equipment. This must be taken into account to really meet the goals of AOI implementation.Inspection objectives change as the process changes and matures. AOI devices must be flexible to accommodate changes. It is important that the equipment is easily moved to different locations on the production line to accurately identify and classify defects. In the same way, different process statistics will be necessary for different periods. When the descriptive statistics produced by AOI are suitable for looking for process changes, the understanding of this is the next stop on the road to zero defect manufacturing.