Basic knowledge of PCB circuit board inspection method

- May 24, 2016-

A few years ago, we started using the term "process monitoring", rather than the inspector, because we wanted to change the idea of the place of production from a passive reaction to an active prevention. An inspector usually sits at the end of the assembly line to inspect the product. In an ideal situation, the process of monitoring activities is a balance between product inspection and process monitoring - for example, to confirm the correct process parameters are used, measuring machine performance, and establish and analysis of control chart. Process monitoring is a leading role for these activities; they help machine operators complete the tasks. Training is a key factor. Process monitoring and machine operators are required to understand the concepts of process standards (e.g., IPC-A-610), process monitoring, and related tools (e.g., control charts, Pareto charts, etc.). Process monitors also improve product quality and process monitoring. As one of the key members of the manufacturing team, the monitor encourages a defect prevention approach rather than a method of finding and repairing.

Over checking is a common problem. In many cases, too much inspection is caused by the dislocation of the IPC-A-610 process standard. For example, in the installation of the components, many of the inspectors also hope that the two sides of the board of the perfect welding round foot, through the hole is full. However, this is not required by the IPC-A-610. The quality of the examination fluctuates with the degree of concentration and concentration of the inspector. For example, fear (stress) may increase the concentration of the production site, the quality of a period of time may improve. However, if a large number of inspection is the main examination method, then the defective products may also be produced, and may go out of the factory.

Another term that we should avoid is the repair welding (touch-up). In the industry, many employees believe that repair welding is a normal and acceptable part of the assembly process. This is very unfortunate, because any form of rework and repair should be seen as a no hope. Rework is often seen as a no hope, but it is a necessary information to instill in the entire manufacturing organization. It is important to set up a manufacturing environment that is to be avoided and the least wanted.

Manual inspection is the first line of defense for most companies. The inspector uses a variety of magnifying tools, closer look at the components and welding points. IPC-A-610 based on the pad width of the inspection components to establish a number of basic amplification guidelines. The main reason for these guidelines is to avoid excessive checks caused due to excessive amplification. For example, if the pad width is mm 0.25~0.50, then the desired magnification is 10X, if necessary, can also be used as a reference 20X.

Solder paste inspection. The printing is a complex process, it is very easy to deviate from the desired result. A process monitoring strategy is required for a clear definition and proper execution to maintain the process control. At least manual inspection covers the area and thickness, but it is best to use automated coverage, thickness and volume measurements. Use range control chart (R chart X-bar) to record the results.

Solder paste inspection equipment has simple amplification 3X mirror to automatic online machine expensive. The first level tool uses optical or laser measuring thickness, while the two level tool uses laser to measure the coverage area, the thickness and the volume. The two tools are offline. Three level tools also measure coverage area, thickness and volume, but online installation. The speed, precision and repeatability of these systems are different, depending on the price. The more expensive the tool provides better performance.

For most of the assembly lines, especially high hybrid production, the preferred medium level performance, it is offline, the installation of the table tool, measuring the coverage area, thickness and volume. These tools are flexible and cost less than $50000 dollars, generally provide the desired number of feedback information. Obviously, the cost of automation tools are much more expensive ($200000 - $75000 USD). However, they are faster and more convenient, because it is online. The most suitable for large volume, low mixed assembly line.

Initial product (first-article) confirmation. Companies are usually on the assembly line down from the first board to carry out a detailed inspection, in order to confirm the machine settings. This method is slow, passive and not accurate enough. Common to a complex board containing at least 1000 components, many of which are not marked (value, part number, etc.). This makes it difficult to check. Verification of machine settings (components, machine parameters, etc.) is a positive approach. AOI can be effectively used for the inspection of the first board. Some hardware and software vendors offer feeder (feeder) setting software.

Coordinate machine setup verification is an ideal role for process monitoring, and he leads the machine operator through the production line confirmation process through an inspection table. Besides the verification device for setting, process monitor should use existing tools to check carefully the first two plates. After reflow soldering, process monitoring should be carried out on key components (close spaced elements, BGA, polar capacitors, etc.) for quick and detailed inspection. At the same time, the production line continues to assemble the plate. In order to reduce the down time, the process monitoring member to check the initial two pieces of the back flow of the plate, the production line should be filled with the board before the return. This may be a little dangerous, but by verifying the machine settings you can get the confidence to do so.

Data collection is one thing, but using these data to improve performance and reduce defects is the ultimate goal. Unfortunately, many companies collect a lot of data and make no use of it. Review and analysis of data may be laborious, and often see that the work is carried out only by engineering designers, not including production activities. Without accurate feedback, the production is carried out blindly. Weekly quality meeting is likely to be an effective way to communicate key information to the engineering design and production department and to promote the necessary improvements. These meetings require a leader that must be well organized, in particular for a short time (30 minutes or less). The data presented at these meetings must be user friendly and meaningful (e.g., Pareto charts). When a problem is identified, it must be assigned immediately to a researcher. In order to ensure a successful conclusion, the meeting leader must make an accurate record. End means root cause and corrective action.

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