source:Other information release time:2024-01-03 Hits: Popular:PCB Assembly company
The PCB manufacturing industry has a dream, hoping that one day it can start from "near-perfect" raw materials and designs, process them through "near-perfect" equipment, and finally produce "near-perfect" products, without any of the time-consuming and Spend money on testing and inspection overhead. Unfortunately, the increasing complexity of PCB products, rapidly changing market dynamics and internal management challenges from enterprises, coupled with many uncertain and random factors in the entire manufacturing process, have offset the gains brought solely by advances in raw materials, design, equipment and processes. Come the benefits.
Today, the one-time yield of general PCBs still hovers between 60% and 90%. If it is not tested or repaired, this will undoubtedly cause huge waste; and once more than 5% of defective products flow into the market, the consequences may is catastrophic. It seems that the dream of producing "nearly perfect" products will eventually come to nothing, and manufacturers will still have to continue to spend heavily on testing equipment. They will also have to purchase new equipment to cope with the new testing challenges brought by new technologies. It all seems endless, and the quality medal hanging around the manufacturer's neck is getting heavier and heavier.
The question now is whether expensive testing and inspection equipment can be reasonably invested, configured and used to maximize their effectiveness. This way, manufacturers can feel a little more at ease when they take a lot of money out of their pockets. Fortunately, optimized inspection solutions do exist that allow manufacturers both money and peace of mind to control both production costs and product quality. These plans have been tested in practice and are effective, but at their essence, they are only based on a few simple facts and principles:
1. Control the spread of defects - the earlier product defects are discovered, the smaller the economic losses caused by defective products.
2. Reduce redundant detection--the fewer repetitive defect detections, the lower the detection cost.
3. Control the main enemy of quality---sorting PCB defect ratios and suppressing the number one quality killer is an effort to get twice the result with half the effort.
These simple facts and principles constitute the three elements for optimizing PCB inspection programs. Their comprehensive application can help construct different inspection programs. Manufacturers should use actual production data to conduct quantitative analysis of the program to evaluate the gains and losses of the program. Let us now examine the above three aspects in more detail.
Control the spread of defects
Just like a disturbance on the water, its scope will expand as the disturbance propagates. If defects in PCB manufacturing are not discovered in time, the losses caused will gradually expand as the PCB moves from the upstream to the downstream of the manufacturing line. Data from manufacturers indicate that for moderately complex PCBs (for example, around 3,000 solder joints and around 500 component counts), if defects escape current inspection and are not discovered until the next step of inspection, the average cost of board rework will be The cost will increase approximately 6 to 7 times. At the same time, losses caused by circuit board scrapping have also skyrocketed.
The simple model shown in Figure 2 reveals to us the general idea of rationally utilizing testing and inspection equipment to reduce overall production costs. At the same time, the model also lists some major factors that need to be considered in testing benefit analysis. First of all, testing equipment should be reasonably configured according to the types of defects introduced in each production process, so that defects can be discovered in a timely manner and provide as accurate defect location information as possible for circuit board rework. Secondly, the detection rate of relevant defects by each testing equipment should not be lower than a certain lower limit, otherwise the configuration of testing equipment will not produce benefits. This lower limit can be estimated based on the factory's profit target. Finally, it should be kept in mind that any testing equipment may cause damage to the circuit board itself during the testing process (such as static electricity during manual visual inspection, backside drive current during ICT testing, etc.), and the testing process may also cause damage to the entire production line. The pace slows down. These negative factors can be considered in a more refined analysis of detection benefits.
Reduce redundant detection
In the "spectrum" of PCB fault defects, not every detection equipment only covers a "narrow band". In short, one inspection equipment may detect defects in multiple PCBs. Figure 1 is a simplified fault coverage allocation table, which shows that each type of defect may be repeatedly inspected multiple times. For example, when the production line is equipped with manual visual inspection (MVI), automatic X-ray inspection (AXI), and in-circuit testing (ICT), PCB solder joint open defects will be repeatedly inspected three times. For military or medical PCBs, redundant testing may be useful, and sometimes even necessary, but for most civilian PCBs, such redundant testing may not be worth the gain. Both inspection equipment suppliers and PCB manufacturers have realized that they can do something here to reduce inspection costs while maintaining product quality. The so-called "distributed" testing and inspection solutions are aimed at reducing redundant detection.
The distributed detection solution, in addition to trying to reduce redundant tests, also strives to improve the overall test coverage of the PCB. The key to achieving this is to have a complete understanding of PCB defect distribution rules, and at the same time master the pros and cons of various testing technologies and PCB CAD data, so that appropriate testing methods can be used to detect appropriate defects at the appropriate production stage. Fault type. Currently, there are commercial software on the market for automatically generating distributed detection solutions, such as GenRad's GR FORCE/Strategist. The software takes PCB CAD/BOM files as input and coordinates automatic optical inspection (AOI), automatic X-ray inspection (AXI), flying probe test system (FPT), production line defect analyzer (MDA), and online testing. Test resources including system (ICT) and functional test system (FCT) to output optimized distributed detection solutions.
Control the main enemy of quality
Just like fighting a battle, in the battle against PCB manufacturing defects, you must understand the main opponent. There is a so-called "80%-20% rule" in the electronics manufacturing industry, which means that 80% of quality problems are often caused by 20% of manufacturing defects, and this 20% is the main killer of product quality. A PCB testing program that pays equal attention to efficiency and quality should be based on a complete understanding of the distribution of defects and seize the main problems to prescribe the right medicine.
The PCB defect distribution shown in Figure 4 is general, but not every PCB and manufacturing process will produce the same defect distribution. Manufacturers must make more precise quantifications of defect distribution based on specific PCBs and their processes.
The current overall defect distribution trend in PCB manufacturing is: structural defects introduced by manufacturing (such as solder joint problems, mounting deviations, etc.) account for about 70%, while electrical performance defects of components (such as electrical parameter deviations, IC functional failures, etc.) Accounting for about 30%. It is this defect distribution trend that has changed the inspection scheme that was mainly based on electrical performance testing in the past, and has made structural defect testing equipment represented by optical (including X-ray) inspection widely used in production.
In depth, due to the advancement of packaging technology (μBGA, CSP, etc.) and the trend of miniaturization, electrical performance testing has become powerless against nearly 70% of structural defects introduced in PCB assembly. Therefore, the focus of the testing program must be Mainly control structural defects.
Understanding the priorities of defect control is also crucial to constructing a reasonable sampling inspection plan. The frequency of sampling is basically proportional to the frequency of failures caused by defects, and the resulting sampling inspection overhead tends to be the lowest.
Summarize
We have briefly described here the three elements for optimizing PCB inspection solutions. How to mix these elements and prepare a satisfactory solution requires collecting a large amount of data from the production line and calming down to make careful calculations. They may help you answer questions such as this qualitatively or quantitatively: Can you purchase a sealed AOI equipment to increase the pass rate of ICT testing by 5%? Can you shorten the inspection time by 10% without reducing the defective rate? An increase of 1%? But mastering these elements does not necessarily accurately answer all questions about PCB testing solutions. Because even for a "nearly perfect" manufacturing process, there will be inevitable and irredeemable random factors that blur our estimates, not to mention that the actual manufacturing process may be full of human errors and additional factors that affect product quality. However, it is gratifying that the "80%-20% rule" mentioned above also holds true here, so that we can use fewer main factors to help construct a PCB inspection program to answer most of the relevant control questions we care about. Quality and cost issues.
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