Circuit card assembly testing: Techniques and Best Practices
When the printed circuit board is assembled, testing is an essential step to determine whether the final PCB can be put into use. Therefore, mastering circuit board assembly is essential for every PCB manufacturer.
What is circuit card assembly test?
The testing of circuit card components (CCA) (also known as printed circuit board components (PCBA)) is an integral part of manufacturing quality control. With the increase of the complexity, component density and number of circuit cards, the effectiveness of electronic testing must also be improved. Defects may occur during the assembly process and need to be screened out through testing to prevent delivery to customers. Circuit card component testing is an effective quality verification step in circuit card assembly, which we will further elaborate on in the following sections.
Benefits of circuit card assembly testing
Even with advanced assembly machines and intelligent automatic optical inspection (AOI), the circuit card assembly test cannot be skipped, because the circuit card assembly test is a very important process to verify the quality of the circuit board. The following is a list of comprehensive advantages of circuit card assembly testing：
Due to sophisticated tools, built-in software and automation, circuit card assembly and test equipment require huge capital investment. Nevertheless, due to the cost itself, circuit card assembly testing is still a prerequisite before shipment. Through the circuit card assembly test, you can ensure that the circuit board works as expected, thereby avoiding expensive costs, repairs and customer dissatisfaction. Early feedback on assembly problems can immediately prevent the problem from affecting larger batches, thereby avoiding serious waste of resources.
Process changes cannot be 100% avoided. Circuit card assembly testing is a method of detecting pre-assembly problems caused by process changes. As mentioned earlier, visual inspection cannot guarantee that the printed circuit board is “good”. Circuit card assembly testing ensures that the PCB to be shipped can be operated electrically as designed. Circuit card assembly testing is also important to ensure that the test program can capture defects. Due to loopholes in the test program and program development, it is not common for substandard products to escape the test. Before being deployed to mass production, rigorous testing and verification are required. The lack of testing and verification due to the rush to mass production may lead to design defects and serious failures.
Through the circuit card assembly test, you can better understand the performance of the equipment. Detected electrical faults can be analyzed to identify potential causes and improved by implementing preventive measures. The faulty PCB can be immediately analyzed for faults to find defects, the affected components can be identified, and the solution can be reported to the assembly team or design engineer.
Types of circuit card assembly tests
There are two types of circuit card component testing, each with advantages and disadvantages: flying needle testing and online testing (ICT). Both types use a test needle to apply an electrical signal and determine the corresponding output signal.
Flying needle test is a kind of circuit card assembly test, using a probe that can be moved quickly to test different contact points of the circuit card. In this method, no fixture is required, but due to the limited number of movable probes, the test time is long. Flying needle testing is usually used for small-scale and small-batch production, but there are also efforts to optimize and accelerate flying needle testing by increasing the number of probes, reducing the number of tests required, and minimizing the movement of probes.
Online testing is another kind of circuit card assembly test, which consists of a fixture with a probe for contacting circuit nodes and tracking electrical performance. The number of probes is equal to the number of test points in the PCB. Online testing has fast testing and diagnosis time. ICT can quickly detect manufacturing failures, such as wrong components, wrong chip orientation, and open and short circuits. The programming sequence can be easily derived from the Gerber file of the circuit card tool. Some disadvantages of using ICT are the high cost of the tools and automation mechanisms required. Due to the need for automation, ICT design modification is also more difficult, and the entire tool must be replaced, which will generate costs.
Functional testing is done by detecting faults during the simulation of the operating environment. The functional tester inputs a signal and measures the response of the printed circuit board. The functional tester has the ability to pass or fail the test quickly and the slow diagnosis time. Functional tests usually provide general rejection messages, which need to be explained by the test project.
Circuit card tester settings
The following are the relevant inspection items that the circuit card tester should carry out at least before production and use：
Functional inspection means knowing whether the tester can correctly detect defective and good products. It involves testing the main sample (good and defective parts) to see if the tester can provide correct readings.
Spring needles are consumables and have a determined service life. Due to the frequent contact points with the circuit board, the life of the spring needle calculated on the basis of the contact point must be tracked every time it is used to determine whether it has exceeded the specified limit. It is also necessary to check for foreign objects or contaminants in the spring needles, as they may interfere with the testing of components.
The test operator or technician must check whether the tester is calibrated and maintained at a given frequency. The tester should be affixed with a PM (preventive maintenance) and calibration label, which contains the date of the last calibration or maintenance. The technician or operator will use this information as a reference to determine whether the tester meets the conditions for use.
Potential causes of circuit card assembly test failure
Failures in circuit card assembly testing may come from various sources. The following are the circuit card test faults, which can be divided into three categories：
PCB test failures may also be caused by material-related problems, such as component failures and material defects. These material problems are why it is essential to carry out incoming quality control before the circuit board is assembled and on top of the supplier's material factory inspection.
The second type of potential cause of test failure is circuit board design problems. Common design-related problems include excessive noise, signal distortion, and component incompatible. This is why efforts must be made in the prototyping process to ensure the normal operation of the design.
Certain defects may occur during assembly and cause electrical failures. An extremely low first-time pass rate of less than 85% may indicate a problem with assembly. Test failures such as short circuits may be caused by process defects, such as solder bridging, conductive material contamination, and component misalignment, while circuit breaks may be caused by insufficient solder, broken lines, or component misalignment. The potential causes of these types of defects vary depending on the machine settings.
The electrical results of some circuit boards may be at the boundary of the test specification, which may be caused by physical or chemical changes in the components under thermal and mechanical stress. Dangerous situations that may occur are potential failures that cannot be detected by automatic optical inspection (AOI), X-ray inspection, or circuit board assembly testing. These failures are usually caused by cracks or inconspicuous stratification. These cracks or stratification may have passed electrical tests, but they will be aggravated in field applications.