PCB assembly inspection methods
Check to constantly remind you that your assembly process have too many variables. Even in your manufacturing process to achieve zero defect production continued after some form of inspection or monitoring to guarantee the desired quality level is necessary. Surface mount assembly is a very complex series of events and a large number of individual actions. Our trick is to create a balance check (inspection) and monitoring (monitering) strategy, without the need for 100% inspection. Discussed in this article is to examine the methods, techniques and manual inspection tools and review the use of automated checking tools and test results (number and type of defects) to improve the process and product quality.
Inspection is a product-centric activities, the monitoring is based on technology-centered activities. Both are needed for a quality program, but the long-term goal should be less product inspection, process monitoring and more. Passive product inspection (defect has occurred), and monitoring of the process is active (defect can prevent) - It is clear that prevention is better than the defects that have been reactive to be useful to make more.
Check is actually a screening process, as it attempts to find the product unacceptable to repair. Very clear the fact that a large number of checks does not necessarily improve or ensure product quality. Deming (Deming) two p.m. The third said, "Do not expect a large check." Deming stressed that the process should be a strong focus on creating a stable, repeatable, and statistical process monitoring objectives, rather than the large quantities of checks. Inspection is a subjective activity, even if a considerable degree of training, it is also a difficult task. In many cases, you can call a group of inspectors to evaluate the weld point, but get several different views.
Operator fatigue is why the 100% inspection of a manufacturing defect is often unable to find the reasons for each of the other, this is a high cost, worthless value-added operations. It rarely achieve higher product quality and customer satisfaction desired goal.
A few years ago, we began to use "process monitoring" the term, not the censor, because we want to produce ideas workplace change from a reactive to proactive prevention. An inspector usually sit at the end of the assembly line, inspection products. In an ideal case, the process monitoring activities, product inspection and monitoring of the process is a balance between - for example, verify that the correct process parameters are used to measure the performance of the machine, and the establishment and analysis of control chart. Monitoring of the process assumes a leadership role in these activities; they help the machine operator to complete these tasks. Training is a key factor. Process monitors and machine operators must understand the process standards (for example, IPC-A-610), monitoring of the process concept and related tools (eg, control charts, Pareto diagrams, etc.). Process monitors can improve product quality and process monitoring. For manufacturing a key team member monitors a defect prevention approach to encourage, rather than a way to find and repair.
Excessive checking is also a widespread problem. In many cases, over-inspection is due to the IPC-A-610 process standards caused by the dislocation of understanding. For example, insert the installation of components, many on both sides of the censor board would also like a perfect circle of welding foot hole completely filled. However, this is not the IPC-A-610 required. Check the quality inspector's attention as the degree of tension and concentration fluctuations. For example, the fear of (the management of pressure) may increase the degree of attention on production sites, the quality may improve over time. However, if a large number of checks is the main way to check, then the defective product may have, and may be out of the factory.
Another term that we should avoid is the welding (touch-up). In being an industry, many employees think that welding is a normal and acceptable part of the assembly process. It is very unfortunate, because any form of rework and repair should be seen as undesirable. Rework is often seen as undesirable, but it is instilled in the necessary information throughout the manufacturing organization. Important to establish a defect and rework as to avoid and most do not want the manufacturing environment.
For most companies, manual inspection is the first line of defense. Inspectors use a variety of zoom tools, more recently, to view components and welding points. IPC-A-610 check the component pad width based on the establishment of the amplification some basic guidelines. The main reason for these guidelines is to avoid over-amplification caused by the excessive checks. For example, if the pad width is 0.25 ~ 0.50 mm, then the desired magnification is 10X, 20X, if necessary can also be used for reference.
Each inspector has a favorite checker; there is a mechanic with three folding pocket magnifying glass lens is better. It can carry a maximum magnification of 12X, which is just suitable for fine pitch solder joints. Perhaps, the most common diagnostic tool is the microscope, magnification range of 10-40X. But the fatigue caused by continuous use of microscopes, often lead to excessive checking, because the magnification is usually more than IPC-A-610 guidelines. Of course, need to carefully check for possible defects or useful.
For a general examination of choice can be equipped with a zoom lens (4-30X) and high-resolution color video monitor system. These systems are easy to use, more importantly, less prone to fatigue than the microscope. High-quality video systems cost less than $ 2000, a good microscope, also in this price range. The additional benefit of video systems more than one person can see the object, which inspectors need training or a second opinion is helpful. Edmund Scientific Company (edmundscientific.com) amplification of a large number of tools, from hand-held magnifying glass to a microscope to the video system.
To sum up, the establishment of a balance between 0-100% inspection monitoring strategy is a challenge. From this point, the key check point, we will discuss the inspection equipment. ...
Automation is wonderful; in many cases, more accurate than the inspectors, fast and efficient. But can be quite expensive, determined by their degree of complexity. Automated inspection equipment may dilute the people's consciousness, give people a false sense of security.
Solder paste inspection. Solder paste printing is a complex process, it is easy to deviate from the desired results. Need a clearly defined and appropriate strategy for the implementation of process monitoring to keep the process under control. Manual inspections at least cover the regional and measuring the thickness, but the best use of automated coverage, thickness and volume measurements. Using the range control chart (X-bar R chart) to record the results.
Solder paste inspection equipment has simple 3X magnifying glass to expensive automated online machine. A tool for measuring the thickness using optical or laser, while the two tools use laser measurement coverage, thickness and volume. Both tools are offline use. Three instruments also measured coverage area, thickness and volume, but the line is installed. These systems are speed, accuracy and repeatability are different, depending on the price. The more expensive tools to provide better performance.
For most of the assembly line, especially in high-mix production, the preferred medium level of performance, it is offline, install the tool table, measuring coverage, thickness and volume. These tools have the flexibility, cost less than $ 50,000 U.S. dollars, generally provide the desired amount of feedback. Obviously, costs are much more expensive automation tools ($ 75,000 - $ 200,000 U.S. dollars). However, they check the board faster, more convenient, because it is online installation. Most suitable for high-volume, low mix of assembly line.
Rubber check. The distribution of plastic is another easy to deviate from the desired result of complex processes. And the solder paste printing, the need for a clear definition and proper implementation of the process monitoring strategies to keep the process under control. Manual inspection recommended diameter of the gel-point. Using the range control chart (X-bar R chart) to record the results.
Cycle in a epoxy around the board at least two isolated drops to represent the gel-point diameter of each point is a good idea. This allows the operator to compare Timor during the dispense cycle gel-point quality. These points can also be used to measure the diameter of the gel-point. Gel-point inspection tool is relatively inexpensive, portable or desktop is basically a measuring microscope. Do not know whether there are specifically designed for gel-point inspection of the automated equipment. Some automated optical inspection (AOI, automated optical inspection) machine can be adjusted to accomplish this task, but may be overkill.
The initial product (first-article) for confirmation. The company down the assembly line usually the first piece of board to conduct a detailed examination to confirm the machine's settings. This method is slow, passive, and not accurate. Common to a complex containing at least 1,000 plate elements, and many are not marked (value, part number, etc.). This makes inspection difficult. Verify machine settings (components, machine parameters, etc.) is a positive approach. AOI can be effectively used for the first piece of board examination. Some hardware and software vendors also provide feeder (feeder) to set recognized software.
Coordination of the machine is a technology validation set ideal role of monitors, he passed a checklist to help lead the machine operator the confirmation process through the production line. In addition to verifying the settings feeders, the process monitors the existing tools should be used carefully check the first two panels. In the reflow soldering, the process monitors should be of critical components (fine pitch components, BGA, polar capacitors, etc.) fast but detailed examination. At the same time, the production line to assemble boards. To reduce downtime, the process monitors check back after the first two panels at the same time, the production line should be back before the full board. This may be a bit dangerous, but the machine settings can be validated the confidence to do so.
X-ray examination. Based on experience, X-ray forBGA assembly need not be mandatory. However, it certainly should have a good hand tool, so if you can afford. Assembly of the CSP should be recommended to use it. X-ray welding short-circuit the inspection is very good, but the results look almost welded open. Low-cost X-ray machines can only look down on the inspection of the welding circuit is adequate. You can check the objects in the tilt of the X-ray machine to check the open is better.
Automated optical inspection (AOI). Ten years ago, optical inspection is used as the quality of each person to solve the problem of tool. Later, the technology is not stopped, because it can not keep up with the pace of assembly technology. In the past five years, it as a desirable technology again. A good process monitoring strategy should include a number of overlapping tools, such as online testing (ICT), optical inspection, functional testing and visual inspection. These processes overlap and complement each other, can not alone provide sufficient coverage.
Two-dimensional (2-D) AOI machine can check the device is lost, for the error, incorrect part number and polarity reverse. In addition, the three-dimensional (3-D) machine to evaluate weld quality. Some vendors provide open desktop ,2-D AOI machines, the price is less than $ 50,000 dollars. These machines for the initial inspection and the product of small quantities of sample plan is ideal. In the high performance category ,2-D stand-alone or on-line machine prices in the $ 75,000-125,000 U.S. dollars, while the 3-D machine at $ 150,000-250,000 dollars. AOI technology has considerable future, but the processing speed and programming time is still a limiting factor.
Data collection is one thing, but use these data to improve performance and reduce the defects is the ultimate goal. Unfortunately, many companies do not collect a lot of data and effective use of it. Review and analysis of data may be arduous, often see this work is only carried out by the engineering staff, not including production activities. No accurate feedback, production blindly. The quality of the weekly meeting of the engineering design and production departments to communicate critical information and promoting the necessary improvements may be an effective method. The meeting called for a leader, must be well organized, especially in shorter time (30 minutes or less). Raised at these meetings must be user friendly and meaningful data (eg, Pareto chart). When a problem is recognized, you must immediately assign a research staff. In order to ensure a successful conclusion, the meeting leader must keep accurate records. Means the end of its causes and the corrective action.