Basic knowledge of PCB circuit board inspection methods
In recent years, the corresponding inspection technology and SMT manufacturing technology have undergone rapid development. The main inspection methods in the initial stage were through manual visual inspection and contact inspection methods. Visual inspection methods mainly include magnifying glass, binocular microscope, projector, etc.; contact inspection methods include functional test and online test (TCT).
Checking can often remind you whether there are too many variables in your assembly process. Even after your manufacturing process can achieve continuous zero-defect production, some form of inspection or monitoring is necessary to ensure the desired level of quality. Surface mount assembly is a series of very complex events with a large number of individual actions. Our trick is to establish a balanced inspection (inspecTIon) and monitoring (monitering) strategy without the need for 100% inspection. This article discusses inspection methods, techniques and manual inspection tools, as well as a review of automatic inspection tools and the use of inspection results (number and types of defects) to improve the quality of the process and products.
Inspection is a product-centric activity, while monitoring is a process-centric activity. Both are required for a quality plan, but the long-term goal should be less product inspection and more process monitoring. Product inspection is passive (defects have occurred), while process monitoring is active (defects can be prevented)-obviously, prevention is more valuable than reactive reactions to existing defects.
Inspection is actually a screening process because it attempts to find unacceptable products to repair. The facts are very clear, a large number of inspections do not necessarily improve or guarantee product quality. The third of Deming’s fourteen points said, “Don’t expect mass inspections.” Deming emphasized that a strong process should focus on establishing stable, repeatable, and statistically monitored process goals, rather than mass inspections. Inspection is a subjective activity, even with a considerable degree of training, it is a difficult task. In many cases, you can call a group of inspectors to evaluate a solder joint, but get several different opinions.
The common testing methods for PCB boards are as follows:
1. Manual visual inspection: Use a magnifying glass or a calibrated microscope to use the operator’s visual inspection to determine whether the circuit board is qualified or not, and to determine when a correction operation is required. This is the most traditional inspection method. Its main advantages are low upfront cost and no test fixture, while its main disadvantages are human subjective error, high long-term cost, discontinuous defect detection, and difficulty in data collection. At present, due to the increase in PCB production and the shrinking of the wire spacing and component volume on the PCB, this method has become increasingly infeasible.
2. On-line testing. Find out manufacturing defects and test analog, digital and mixed-signal components through electrical performance testing to ensure that they meet specifications. There are several testing methods such as bed of needle testers and flying probe testers. The main advantages are the low test cost of each board, strong digital and functional testing capabilities, fast and thorough short-circuit and open-circuit testing, programming firmware, high defect coverage, and easy programming. The main disadvantages are the need to test fixtures, programming and debugging time, high cost of making fixtures, and difficult use.
3. Functional testing. Functional system testing uses special testing equipment in the middle and end of the production line to conduct a comprehensive test on the functional modules of the circuit board to confirm the quality of the circuit board. Functional testing can be said to be the earliest automatic testing principle. It is based on a specific board or a specific unit and can be completed with various equipment. There are final product tests, the latest physical models and stacked tests. Functional testing usually does not provide in-depth data such as foot-level and component-level diagnostics for process improvement, and requires specialized equipment and specially designed test procedures. It is complicated to write functional test programs, so it is not suitable for most circuit board production lines.
4. Automatic optical inspection, also known as automatic visual inspection, is based on optical principles and comprehensively adopts multiple technologies such as image analysis, computer and automatic control to detect and process defects encountered in production. It is a relatively new confirmation manufacturing Defective method. AOI is usually used before and after reflow and before electrical testing to improve the pass rate of electrical processing or functional testing. At this time, the cost of correcting defects is much lower than the cost after final testing, often reaching more than ten times.
5. Automatic X-ray inspection uses the difference in X-ray absorption rates of different substances to see through the parts that need to be inspected and find defects. It is mainly used to detect defects in ultra-fine pitch and ultra-high-density circuit boards, as well as bridging, chip loss, poor alignment and other defects generated during the assembly process. It can also use its tomographic imaging technology to detect internal defects in IC chips. It is the only way to test the soldering quality of the ball grid array and the blocked solder balls. The main advantage is the ability to detect BGA welding quality and embedded components, without fixture cost; the main disadvantages are slow speed, high failure rate, difficulty in detecting reworked solder joints, high cost, and long program development time. This is a relatively new test. The method remains to be further studied.
6. Laser inspection system, which is the latest development of PCB testing technology. It scans the printed board with a laser beam, collects all measurement data, and compares the actual measurement value with the preset qualified limit value. This technology has been proven on the bare board and is being considered for assembly board testing. The speed is sufficient for mass production lines. Fast output, no fixtures and visual non-covered access are its main advantages; high initial cost, maintenance and use problems are its main disadvantages.
7. Dimension detection, using a two-dimensional image measuring instrument to measure the hole position, length, width, position and other dimensions. Since PCB is a small, thin and soft type of product, contact measurement can easily deform and cause inaccurate measurement. The second-dimensional image measuring instrument has become the best high-precision size measuring instrument. The image measuring instrument of Sirui Measurement can realize fully automatic measurement after programming, which not only has high measurement accuracy, but also greatly shortens the measurement time and improves measurement efficiency.
Operator fatigue is the reason why 100% inspection usually fails to find every manufacturing defect. In addition, this is a costly and value-added operation. It rarely achieves the desired goal of higher product quality and customer satisfaction.
A few years ago, we started to use the term “process monitoring” instead of inspectors because we wanted to change the mindset of production sites from reactive to proactive prevention. An inspector usually sits at the end of the assembly line and inspects the product. In an ideal situation, process monitoring activities are a balance between product inspection and process monitoring-for example, confirming that the correct process parameters are being used, measuring machine performance, and establishing and analyzing control charts. Process monitoring assumes a leadership role in these activities; they help machine operators complete these tasks. Training is a key factor. Process monitors and machine operators must understand the process standards (for example, IPC-A-610), the concept of process monitoring and related tools (for example, control charts, Pareto charts, etc.). Process monitors also improve product quality and process monitoring. As a key member of the manufacturing team, the monitor encourages a method of defect prevention rather than a method of finding and repairing.
Excessive inspection is also a common problem. In many cases, over-checking is only caused by a misunderstanding of the IPC-A-610 process standard. For example, for components that are inserted and installed, many inspectors also hope that the two sides of the board are perfectly welded with round feet, and the through holes are completely filled. However, this is not required by IPC-A-610. The quality of inspection fluctuates with the intensity of the inspector’s attention and concentration. For example, fear (management pressure) may increase the concentration of the production site, and the quality may improve over a period of time. However, if mass inspection is the main inspection method, then defective products may still occur and may leave the factory.
Another term we should avoid is touch-up. In this 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 considered undesirable. Rework is usually seen as undesirable, but it is necessary to instill necessary information throughout the manufacturing organization. It is important to establish a manufacturing environment where defects and rework are considered avoidable and least desirable.
For most companies, manual inspection is the first line of defense. Inspectors use various magnifying tools to get a closer look at components and solder joints. IPC-A-610 establishes some basic magnification guidelines based on inspecting component pad widths. The main reason for these guidelines is to avoid over-examination due to over-magnification. For example, if the pad width is 0.25~0.50 mm, the desired magnification is 10X. If necessary, you can also use 20X as a reference.
Every inspector has a favorite inspection tool; a three-lens folding pocket magnifier used by a mechanic is better. It can be carried with you, and the maximum magnification is 12X, which is just suitable for close-spaced welding points. Perhaps the most common inspection tool is a microscope with a magnification range of 10-40X. However, continuous use of the microscope causes fatigue, which usually leads to over-examination, because the magnification usually exceeds the IPC-A-610 guidelines. Of course, it is useful when you need to carefully examine possible defects.
For general inspections, a video system equipped with a zoom lens (4-30X) and a high-definition color monitor is preferred. These systems are easy to use and, more importantly, are less prone to fatigue than microscopes. The price of a high-quality video system is less than $2000, and the price of a good microscope is also in this range. The additional benefit of the video system is that more than one person can see the object, which is helpful when training or when the inspector needs a second opinion. Edmund ScienTIfic (edmundscienTIfic.com) has a large number of magnification tools, from handheld magnifiers to microscopes to video systems.
In summary, it is a challenge to establish a balanced monitoring strategy between 0-100% inspection. From this point, the key checkpoint, we will discuss checking equipment…
Automation is fantastic; in many cases, it is more accurate, faster and more efficient than inspectors. But it can be quite expensive, depending on its complexity. Automated inspection equipment may dilute people’s consciousness and give people an illusion of safety.
Solder paste inspection. Solder paste printing is a complicated process, and it can easily deviate from the desired result. A clearly defined and properly implemented process monitoring strategy is needed to keep the process under control. At least manually inspect the coverage area and measure the thickness, but it is best to use automated coverage, thickness and volume measurements. Use the X-bar R chart to record the results.
Solder paste inspection equipment ranges from simple 3X magnifying glasses to expensive automatic online machines. Primary tools use optics or lasers to measure thickness, while secondary tools use lasers to measure coverage area, thickness, and volume. Both tools are used offline. The three-level tool also measures coverage area, thickness and volume, but is installed online. The speed, accuracy, and repeatability of these systems are different, depending on the price. The more expensive tools provide better performance.
For most assembly lines, especially high-mix production, the middle-level performance is preferred. It is an offline, countertop tool that measures coverage area, thickness, and volume. These tools are flexible, cost less than $50,000, and generally provide the desired amount of feedback. Obviously, automation tools are much more expensive ($75,000-$200,000 USD). However, they are faster and more convenient for checking boards because they are installed online. Most suitable for high-volume, low-mix assembly lines.
Glue inspection. Glue dispensing is another complicated process that easily deviates from the desired result. As with solder paste printing, a clearly defined and appropriately implemented process monitoring strategy is required to keep the process under control. It is recommended to manually check the glue dot diameter. Use the X-bar R chart to record the results.
Before and after a dispensing cycle, it is a good idea to drop at least two separate dots on the board to represent the diameter of each dot. This allows the operator to compare the quality of the glue dots during the Tei glue cycle. These points can also be used to measure the glue dot diameter. Glue spot inspection tools are relatively inexpensive, basically there are portable or desktop measuring microscopes. It is not known whether there is an automatic device specially designed for glue spot inspection. Some automated optical inspection (AOI, automated opTIcal inspection) machines can be adjusted to accomplish this task, but they may be overkill.
Confirmation of the first-article. The company usually conducts a detailed inspection of the first board off the assembly line to verify the machine’s settings. This method is slow, passive and not accurate enough. It is common for a complex board to contain at least 1000 components, many of which are unmarked (values, part numbers, etc.). This makes inspection difficult. Verifying machine settings (components, machine parameters, etc.) is a positive method. AOI can be effectively used for the inspection of the first board. Some hardware and software vendors also provide feeder setting confirmation software.
Coordinating the verification of machine settings is an ideal role for a process monitor, who leads the machine operator through the production line to confirm the process with the help of a checklist. In addition to verifying the feeder settings, the process monitor should carefully inspect the first two boards using existing tools. After reflow soldering, process monitors should conduct a quick but detailed inspection of key components (fine-pitch components, BGAs, polar capacitors, etc.). At the same time, the production line continues to assemble the panels. To reduce downtime, while process monitors check the first two boards after reflow, the production line should be filled with boards before reflow. This may be a bit dangerous, but you can gain confidence in doing so by verifying the machine settings.
X-ray inspection. Based on experience, X-rays are not necessarily mandatory for BGA assembly. However, it is of course a good tool you should have on hand, if you can afford it. It should be recommended for CSP assembly. X-ray is very good for checking welding short circuit, but it is not very effective for finding welding open circuit. Low-cost X-ray machines can only look down and are sufficient to check for welding short circuits. An X-ray machine that can tilt the object under inspection is better for the inspection.
Automatic optical inspection (AOI). Ten years ago, optical inspection was used as a tool that could solve everyone’s quality problems. Later, the technology was discontinued because it could not keep up with the pace of assembly technology. In the past five years, it has reappeared as a desirable technology. A good process monitoring strategy should include some overlapping tools, such as online testing (ICT), optical inspection, functional testing and visual inspection. These processes overlap and complement each other, and none of them alone can provide sufficient coverage.
The two-dimensional (2-D) AOI machine can check for missing components, misalignment, incorrect part numbers and polarity reversals. In addition, three-dimensional (3-D) machines can evaluate the quality of solder joints. Some suppliers offer desktop, 2-D AOI machines for less than $50,000. These machines are ideal for initial product inspection and small batch sample planning. Among the higher-performance categories, 2-D standalone or online machines cost $75,000-125,000, while 3-D machines cost $150,000-250,000. AOI technology has considerable promise, but processing speed and programming time are still a limiting factor.
Data collection is one thing, but using this data to improve performance and reduce defects is the ultimate goal. Unfortunately, many companies collect a lot of data without using it effectively. It may be laborious to review and analyze data. It is often seen that this work is only carried out by engineering designers and does not include production activities. Without accurate feedback, production proceeds blindly. Weekly quality meetings may be an effective way for the engineering design and production departments to communicate key information and promote necessary improvements. These meetings require a leader to be well organized, especially in a short time (30 minutes or less). The data presented at these meetings must be user-friendly and meaningful (for example, Pareto charts). When a problem is confirmed, an investigator must be assigned immediately. In order to ensure a successful conclusion, the meeting leader must make accurate records. The end means the root cause and corrective action.