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PCB Tech

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PCB Tech

PCB Tech

Reliability screening method of electronic assembly process

Reliability screening method of electronic assembly process
At present, the country has higher and higher requirements for environmental protection and greater efforts in link governance. This is a challenge but also an opportunity for PCB factories. If PCB factories are determined to solve the problem of environmental pollution, then FPC flexible circuit board products can be at the forefront of the market, and PCB factories can get opportunities for further development.
1. Visual inspection and microscopy screening
Visual inspection or microscopic inspection (microscopic inspection) is an important screening method in the manufacture of electronic products. It can be used to find out work and stickiness, defects, damages and poor connections, etc., and then remove them. The microscopic inspection standards should be reasonably formulated based on the main failure modes and mechanisms, combined with the specific failure process conditions. Years of experience has recognized that this method is one of the easiest and most efficient methods. It is very effective for inspecting various defects on the chip surface (such as metallization layer defects, chip cracks, oxide layer quality, mask quality and diffusion defects, etc.), as well as observing internal lead stitching, chip bonding and packaging defects. There are already automatic microscopy systems using scanning electron microscopes and computers in foreign countries.

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2. X-ray screening

X-ray is a non-destructive screening, which is used to inspect the package for excess objects, potential defects in bonding and packaging processes, and cracks on the chip after the device is sealed.

3. Infrared filter

Use infrared detection or photography technology to reveal the heat distribution characteristics (hot spots and hot spots). When the design is unreasonable, there are defects in the process, and there are certain failure mechanisms in the production process, a hot spot or hot zone will be generated in a certain part of the product. In this way, unreliable components can be screened out in advance. The advantage of infrared screening is that it does not damage the components during the inspection process, which is especially suitable for the inspection of large-scale integrated circuits.

4. Power aging

Power aging is a very effective screening method, and it is one of the screening methods that high integrated circuits must carry out. Power aging imposes excessive electrical stress on the product, prompting the potential defects of the early failure device to be exposed as soon as possible and be rejected. It can effectively eliminate process defects, thin metallized films, scratches, and surface stains during the device production process. Power aging is usually to place integrated circuit products under high temperature conditions and apply the maximum voltage to obtain a large enough screening stress to eliminate early failure products. Put the products under high temperature conditions and apply the maximum voltage to obtain sufficient pulse power. Ageing. The former is mostly used in small-scale digital circuits, while the latter is used in medium and large-scale integrated circuits, so that the components in the circuit can withstand the maximum power consumption and stress under working conditions during aging. Although super-power aging shortens the aging time, it may also cause the instantaneous load of the device to exceed the maximum rating, damage the qualified device, and even cause instant degradation or breakdown. Some products may still work temporarily, but their life span is shortened. Therefore, for the super power aging, it is not that the more the super power is, the more effective it is, but the best super load should be selected. The more consistent method now is to apply the maximum rated power to the device and extend the aging time appropriately, which is a more reasonable method of screening for electrical power aging.
5. Temperature cycling and thermal shock screening
Temperature cycling can accelerate failures caused by thermal mismatch effects between materials. Potential defects such as chip assembly, bonding, packaging, and metallized film on the oxide layer can all be screened by temperature cycling. The typical conditions for temperature cycle screening are -55~+155℃ or -65~+200℃ for 3 or 5 cycles. Each cycle is maintained at the highest or lowest temperature for 30 minutes, and the transfer time is 15 minutes. After the test, the AC and DC parameters are tested. Thermal shock screening is an effective method to determine the strength of integrated circuits with rapid temperature changes. For example, there are two water tanks at 100°C and 0°C. After immersing in a high temperature tank for 15s, take it out, and move it into a low temperature tank for at least 5s and then 3s within 3s. Move inside into the high temperature tank. Do this reciprocating operation 5 times. For some products, if the thermal expansion and contraction properties of the internal parts of the materials do not match, or the parts have cracks, or defects caused by poor SMT process, the early failure parts can be advanced under the temperature impact of the high and low temperature alternating environment Invalidate. This method has a better screening effect.
6. High temperature storage screening
High temperature can accelerate the chemical reaction inside the product. If the integrated circuit package contains water vapor or various harmful gases, or the chip surface is not clean, or there are various metal components in the bonding area, chemical reactions will occur, and high-temperature storage can accelerate these reactions. Because this screening method is easy to operate, can be carried out in batches, the screening effect is good, and the investment is small, so it is widely used.
7. High temperature work screening
High-temperature work screening generally includes high-temperature DC static, high-temperature AC dynamic, and high-temperature reverse bias 3 screening methods, which are very effective for eliminating the failures caused by potential defects in the surface, body, and metalization system of the device. High-temperature reverse bias is an experiment in which reverse bias working voltage is applied at high temperature. It is carried out under the combined action of hot spots, and is very close to the actual working state, so it is better than pure high-temperature storage and screening.
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