PCBA Tech - Reliability Test Design of SMT Electronic PCB Assembly Process

The primary problem of PCBA reliability test design is to define the purpose of reliability test and select appropriate reliability test scheme. This is because the purpose of various reliability tests is different, and the test requirements are not the same. The distribution rule of the working time before the failure of the product, the design sampling scheme used, the treatment and evaluation method of the test results are also different. According to different test purposes of products, different test schemes can be selected to correctly evaluate or infer the reliability level of products.

For products requiring reliability test, the purpose of reliability test is generally determined and the test scheme is selected according to the stage of the product. For example, when the product is in the development stage, the purpose of reliability test is to expose the weak links of the product, so as to take improvement measures to improve the inherent reliability of the product. Generally, the reliability growth test scheme is selected. When the product is in the identification stage after design finalization, production finalization or major technical improvement, the purpose of the test is to verify whether the reliability of the product is consistent with the specified reliability requirements, and the life test with regular truncation should be selected to infer the reliability of the product, etc. After selecting the test scheme, in order to ensure that the PCB assembly reliability test can be carried out economically, reasonably and scientifically, the sequence of reliability test should be defined. To determine the reliability test sequence, the cost and time of the test should be considered first. The time-consuming and expensive tests, such as salt spray test and constant high humidity test, should be arranged at the end of the test as much as possible. This is because if the previous test fails, the subsequent test can be omitted, so as to save the test cost and time. Secondly, we should take into account the purpose of the test, the environmental factors in actual use and the degree of influence of each factor on the reliability of the product. Specifically, the principle for determining the test sequence is to study the performance of the sample. According to the test purpose, we should obtain as much test data and information as possible. Generally, we should carry out small destructive environmental test items for the product first, and then carry out large destructive environmental test items, This principle is also applicable to complex, small and expensive sample type tests.

In order to verify the effectiveness of the design, we hope to obtain the relevant test data at the fastest speed, so the test sequence should be from the most destructive test to the less destructive test, that is, to carry out the severe environmental test first, and after the failure of the previous test item, it is not necessary to carry out the next item. Determine the test sequence according to the actual environmental factors encountered by the product. This kind of test sequence has strong authenticity and is applicable to the type test of products. The prerequisite is to know the environmental conditions of the actual use of the product.

The test sequence is determined according to the impact of the test on the product, that is, the previous test item can strengthen and induce the product failure caused by the next test, and maximize the exposure of product defects. For example, if the damp heat test is followed by the low temperature test, the moisture will be absorbed in the previous test, and condensation will occur in the low temperature test, which will aggravate the destructive effect of low temperature. This sequence is especially applicable to product type test in unknown use environment.

The procedure of SMT reliability test generally starts from the sampling of test samples, through sample pretreatment, initial detection, entry test, recovery after test, and final detection until the samples are returned. For products produced in batches, the sampling of reliability test samples shall follow the principle of statistical sampling, and random sampling shall be conducted from batches of products that pass the verification. Pretreatment refers to the treatment of samples before the formal test. Its purpose is to eliminate the impact of samples before the test, including the surface cleaning, positioning and static treatment of samples, which are usually carried out under the specified standard test conditions.

Initial test refers to the appearance inspection, safety performance inspection, electrical and mechanical performance inspection of samples under the specified test conditions. It is required to be as comprehensive as possible, so as to compare with the test data under various test conditions during the test and confirm whether it is invalid. Place the sample in a specific space and apply the test stress according to the test requirements, that is, enter the test state. According to the test requirements, in the test state, the product is subject to appearance inspection, safety performance inspection, electrical performance and mechanical performance inspection, which is called intermediate inspection. Recovery refers to the treatment of samples after the test. Generally, the sample is placed under the specified conditions for 1-2h, with the purpose of restoring the sample after the test to a stable state before the final test.

At the end of the last test of the reliability test, the samples in the final test room will be subject to appearance inspection, safety performance inspection, electrical and mechanical performance inspection under the specified test conditions, as well as the initial test. After the test, the sample shall be restored to the factory state, and necessary repair and replacement can be carried out during this process. After commissioning, the product shall be re-inspected by the quality inspection department and returned to the original sample storage department after it is judged to be qualified.

In case of failure of the tested product, the failure time and failure phenomenon shall be recorded in detail. The test conditions in case of failure include environmental conditions and working conditions, etc., and the tested product in case of failure shall be restored to normal temperature and withdrawn from the test. Conduct fault analysis on the tested product, and conduct verification test after maintenance to confirm the repair results. The test can be resumed only after confirming that the functions and indicators are normal. If the tested product repaired by the replaced device has the original fault symptom again in the continuous test, and the fault analysis has misjudged, the original failed part shall be reinstalled and the cause of the fault shall be investigated.

Investigate and verify the test conditions of the failed device or the whole piece, and preliminarily determine the main environmental factors or electrical stress factors that cause the failure. At the same time, analyze whether the failure of the product is caused by problems in the operation of the test equipment. For example, after the low temperature test of the product is completed, due to the rapid heating rate, water accumulated inside the machine, which leads to the failure of the high temperature test, which is easy to cause PCBA misjudgment, At least we will ignore the problems related to product failure and humidity.