As automated test equipment becomes part of the overall electronics assembly process, DFT must include not only traditional hardware usage issues, but also knowledge of test equipment diagnostic capabilities.
The design for test (DFT, design for test) is not a single person's work, but a group of representatives from the design engineering department, the test engineering department, the manufacturing department, and the purchasing department.The design for test (DFT, design for test) is not a single person's work, but a group of representatives from the design engineering department, the test engineering department, the manufacturing department, and the purchasing department.Design engineering must specify functional products and their error requirements.Test engineering must provide a strategy that achieves the highest possible first pass-through yield (FPY) with the lowest cost and minimal rework.The manufacturing and quality departments must provide input on production costs, what has been done in similar products in the past, what has not been done, and the help of DFV (design for volume) to increase production.The Purchasing Department must provide information on the availability of components, especially reliability.The test and procurement departments must work together to purchase components on the on-board test hardware to ensure that they are available and easy to implement. The test system is often used as a sensor to collect historical data to achieve process improvement, which should be the goal of the quality team. So these functions should be done before placing/removing any node selection. The test and procurement departments must work together to purchase components on the on-board test hardware to ensure that they are available and easy to implement. The test system is often used as a sensor to collect historical data to achieve process improvement, which should be the goal of the quality team. So these functions should be done before placing/removing any node selection.
[Printed Circuit Boards Specifications]
Preparation and understanding are key before developing a policy for the test environment. The parameters that affect the test strategy include:Accessibility. Full access and large test pads are always the goal of manufacturing a printed circuit board. There are four reasons why you cannot usually provide full access:
1. The size of the board.
The design is smaller; the problem is the "extra" footprint of the test pad. Unfortunately, most design engineers believe that the accessibility of test soldering is less important on printed circuit boards (PCBs). When the product has to be debugged by the design engineer because of the inability to use the simple diagnostics of the in-circuit tester (ICT), the situation is quite another. Test options are limited if full access is not available.
2. Function.
The performance lost in high-speed designs affects the board, but can gradually reduce the impact on product testability.
3. Board size / number of nodes.
This is when the physical board size cannot be tested on any existing device. Fortunately, this problem can be solved by adding a budget on new test equipment or using an external test facility. When the number of nodes is larger than the existing ICT, the problem is more difficult to solve. The DFT team must understand the test methods that will allow the manufacturing department to output good products with minimal time and money. Embedded self-test, boundary scan (BS) and function block tests can do this. The diagnosis must support the unit under test (UUT); this can only be achieved by an in-depth understanding of the test methods used, the existing test equipment and capabilities, and the fault spectrum of the manufacturing environment.
4. DFT rules are not used,
observed or understood. Historically, DFT rules have been enforced by an engineer or group of engineers who understand manufacturing environments, process and functional test requirements, and component technology. In the real world, the process is lengthy and requires communication between design, computer-aided design (CAD) and testing. This ubiquitous repetitive work is prone to human error and often rushes through time-to-market pressure. Nowadays, the industry has begun to use automatic "productivity analyzers" to evaluate CAD files using DFT rules. When a contract manufacturer (CM, contract manufacturer) is used, multiple sets of rules can be classified. Rule continuity and error-free product evaluation are the advantages of this approach.
[Test equipment availability Methods ]
The DFT team should be aware of the existing testing strategy. As OEMs turn to relying on more and more CMs, the equipment used varies from plant to plant. Without a clear understanding of the manufacturer's process, too many or too few tests may be used. Existing test methods include:Manual or automated visual testing, using vision and comparison to confirm component placement on the PCB. There are several implementation methods for this technique:
1. Manual vision is the most widely used online test
Manual vision is the most widely used online test,but this approach becomes infeasible due to increased manufacturing throughput and shrinking of boards and components. Its main advantages are low pre-cost and no test fixtures, and its main drawbacks are high long-term cost, discontinuous defect detection, data collection difficulties, no electrical testing and visual limitations.
2. Automated optical inspection (AOI)
Automated optical inspection (AOI),usually used before and after reflow, is a relatively new method of confirming manufacturing defects. It is a non-electric, fixtureless, online technology that uses "learn and compare" programming to minimize the ramp-up time. Automated vision is better for polarity, component presence and non-existence, as long as the latter components are similar to the originally "learned" components. Its main advantages are easy to follow diagnostics, fast and easy program development, and no fixtures. The main disadvantages are poor short circuit identification, high failure rate and not electrical testing.
Automated X-ray inspection (AXI) is the only method currently used to test the quality of ball grid arrays (BGA) and the occluded solder balls. It is a non-electrical, non-contact technology that finds flaws in the early process, reducing work-in-process (WIP). Advances in this area include pass/fail data and component level diagnostics. There are now two main AXI methods: two-dimensional (2-D), looking at the complete board, three-dimensional (3-D), taking multiple images at different angles. Its main advantages are the unique BGA welding quality and embedded component inspection tools, no fixture costs. The main disadvantages are slow speed, high failure rate, difficulty in detecting rework solder joints, high cost per board, and long program development time.
The manufacturing defect analyzer (MDA) is a good tool for high-volume/low-mix environments where the test is only used to diagnose manufacturing defects. Repeatability between testers is a problem when no residual reduction techniques are used. Also, the MDA does not have a digital driver and therefore cannot functionally test components or firmware on the programming board. The test time is less than the visual test, so the MDA can catch up with the beat speed of the production line. This method uses a needle bed so the output can be diagnosed.
Its main advantages are lower upfront cost, lower WIP, low programming and program maintenance costs, high output, easy follow-up diagnostics, and fast full short and open circuit testing. The main disadvantage is that it cannot confirm whether the bill of material (BOM) conforms to the unit under test (UUT), has no digital confirmation, has no functional test capability, cannot call firmware, and usually has no test coverage indication. , board and board line-to-line repeatability, fixture cost, and usage issues.
ICT will identify manufacturing defects and test analog, digital and mixed-signal components to ensure they meet specifications. Many devices have the ability to program on-board memory, including serial numbers, pass/fail, and genealogy data. Some devices make the program easier. It is easy to implement multi-version testing and firmware transformation by embedding the tool into an easy-to-use graphical user interface (GUI) and storing the code into a special file. of. Some devices have complex instrumentation that will confirm the functional aspects of the UUT and interface with commercially available instruments. Today's test equipment has an embedded computer-aided design (CAD) interface and a non-multiple environment to reduce development time. Finally, some testers provide in-depth UUT coverage analysis that clarifies the components being tested or not tested.
The main advantages of ICT are low test cost per board, digital and functional test capability, high output, good diagnostics, fast and thorough short and open circuit testing, programming firmware, defect coverage, and ease of programming. The main disadvantages are fixtures, programming and commissioning times, fixture costs, expected expenses and usage issues.
The flying-probe tester has been popular in the past few years due to advances in mechanical precision, speed and reliability. In addition, the market requirements for fast-switching, fixture-free test systems required for prototype manufacturing and low-volume manufacturing have made flying probe testing a desirable test option. The best probe solution provides learning capability and BOM testing, which automatically increases monitoring during the testing process. The probe software should provide an easy way to load CAD data, as X-Y and BOM data must be used during programming. Because node accessibility may be incomplete on one side of the board, the test generation software should automatically generate a non-repeating split program.
The probe uses a vectorless technique to test the connection of digital, analog, and mixed-signal components; this should be done by a capacitive plate that the user can use on both sides of the UUT.
The main advantage of the flying probe tester is that it is the fastest time-to-market tool, automatic test generation, no fixture cost, good diagnostics and easy programming. The main disadvantages are low production, limited digital coverage, fixed asset expenses and usage issues.
Functional test, arguably the earliest automatic test principle, has seen rejuvenation in importance. It is a specific board or a specific unit and can be done with a variety of devices. A few examples:
The final product test is the most common functional test method. Testing the final unit after assembly is costly and reduces operational errors. However, the diagnosis is non-existent or difficult, which increases the cost. Only test the final product and have the opportunity to damage the product if there is no software or hardware protection provided by the automated test. The final product test is also slow and usually takes up a lot of space. This method is usually not used when the standard must be met, as it usually does not support parameter measurements.
The main advantages of final product testing are the minimum initial cost, one assembly, and product and quality assurance. The main drawbacks include low diagnostic resolution, lack of speed, high long-term cost, FPY, damage to the board or machine due to undetected short circuits, high cost of repair, and no parameter testing capability.
The latest hot mock-ups are usually placed in different assembly stages, not just in the final test. In terms of diagnosis, it is better than the final product test, but the cost is higher because of the need to set up a special test unit. The solid model may be faster than the final product test if the program debug only tests a specific board. Unfortunately, due to lack of protection, the test bed may be damaged if the short circuit is not diagnosed during the previous process.
Its main advantage is the low initial cost. The main disadvantages are low space efficiency, cost of maintaining test equipment, damage to the UUT due to short circuits, and no parameter testing capability.
A software-controlled, commercially available instrument is often referred to as a "rack and stack" test because the instruments are purchased separately and then connected. The software for the sync device is usually fully customizable. Commercially available instrument comparison integration solutions are not expensive and, if done correctly, allow independent UUTs to be effective. But this "homemade" system is usually slower, and engineering changes and production site support are difficult because these applications are under-documented.
Its main advantages are protection of UUT damage, faster output, small footprint requirements, and independent/industrial acceptable calibration. The main drawbacks are time-consuming, difficult to support, and updated and used at remote facilities.
Commercial, custom integrated systems couple software and hardware on a test platform, such as IEEE, VXI, Compact PCI, or PXI. File archiving, software support, and standard manufacturing concepts make these systems easy to use and support. The upfront cost is higher than the internal build plan, but this cost is adjustable because of higher performance, output, and repeatability. It is also easy to support on-site and during new product development.
The main advantages are fast output, less floor space required, the easiest to support and reset, the best repeatability, and the ability to provide independent industrial acceptance. The main drawback is the high initial cost.
Non-contact testing methods such as laser systems are the latest developments in PCB testing technology. This technology has been proven in the bare-board area and is being considered for testing on populated boards. This technique uses only line-of-sight, non-masked access to detect defects. Each test is at least 10 milliseconds and is fast enough for a mass production line.
Fast output, no fixture required, and line-of-sight/non-covered access are its main advantages; unproductive trials, high initial cost, high maintenance and usage issues are major drawbacks.