summary
Jiang Wei, a researcher at Mianyang Weibao Electronics Co., Ltd., wrote an article in the 10th issue of "Electrical Technology" magazine in 2018, introducing the characteristics of commonly used potting materials for current sensors , analyzing in detail the key processes of the potting process, and proposing specific solutions based on problems encountered in actual production. This potting process has been verified through multiple batches of production and fully meets the product design requirements.
With the development of national science and technology, power sensors have been widely used in various fields such as military industry, power grid [1], railway, and new energy vehicles. Their usage conditions are becoming more and more diverse and the usage environment is becoming more and more severe, thus putting forward higher and higher reliability requirements for power sensors.
Depending on the different performance characteristics of the product, different materials are selected to pot the electrical components, allowing them to penetrate into all gaps in the electrical circuit system components and parts, and giving the product mechanical support, encapsulation protection, waterproofing, moisture resistance, corrosion resistance, vibration resistance, and improved insulation strength.
This article introduces the key process points and problems encountered in the potting process, identifies solutions to these problems, and improves potting quality.
1 Commonly Used Potting Materials
Potting materials are the foundation of the potting process, and selecting the right material is crucial to its success. Different materials with varying properties should be chosen based on the product's specific performance and operating environment. Currently, there are many types of potting materials, but epoxy resin, silicone rubber, and polyurethane are the most widely used. Based on the characteristics of our existing products, we primarily use epoxy resin and silicone rubber as potting materials.
1.1 Epoxy Resin
Epoxy resin potting compound refers to an epoxy resin liquid encapsulation material made by adding a certain amount of toughening agent, curing agent, filler and other functional additives in proportion, and then adding a certain proportion of pigment according to requirements.
Epoxy resin has good adhesion, good insulation properties, strong corrosion resistance, excellent mechanical strength, and low price, so it has developed rapidly in recent years. At present, most foreign semiconductor devices are encapsulated with epoxy resin potting materials, which are not only diverse in type, but also highly specialized and have excellent performance [2-3].
Currently, the performance of epoxy potting materials used in the domestic electronics industry is still insufficient compared with that of advanced foreign countries in some aspects, such as the presence of internal stress during the curing process and the tendency to crack.
1.2 Silicone rubber
Silicone rubber is a semi-inorganic and semi-organic polymer elastomer whose main chain is composed of alternating silicon atoms and oxygen atoms, and whose silicon atoms have organic groups[4].
Silicone rubber has low linear shrinkage, excellent high and low temperature resistance, good weather resistance, and good electrical insulation [5].
Furthermore, silicone rubber has a simple potting process, cures quickly at room temperature, and offers good repairability. Based on these excellent properties, silicone rubber has been widely used as an encapsulation material for electronic components in recent years. However, silicone rubber also has certain inherent drawbacks, such as poor mechanical properties, low wear resistance, and insufficient adhesion.
2. Encapsulation process for electrical sensors
2.1 Potting process flow
The conventional process flow for potting a current sensor is shown in Figure 1.
Figure 1. Conventional process flow for potting a power sensor
2.2 Key Process Points
1) Apply conformal coating. All components in the product that require potting should be coated with conformal coating. The thickness of the conformal coating should be 15-20 μm. The coating should be uniform, glossy, and free from defects such as pinholes, build-up, and flow marks.
2) Dehumidification. Any material surface will absorb moisture under natural conditions. The presence of moisture not only reduces the adhesive strength of the potting compound but also drastically reduces the insulation of the potting material. Therefore, the potted components need to be dried and dehumidified. After drying, potting should be carried out promptly once the temperature drops to room temperature, generally within 2 hours; otherwise, moisture will be absorbed again. Considering the component's lifespan and the amount of residual water molecules, after multiple tests, the dehumidification temperature for the power sensor is controlled at 50±5℃, and the dehumidification time is 2 hours.
3) Mixing and Stirring. Based on the product characteristics and the requirements of relevant process and technical documents, mix the potting compound according to the specified proportions. The mixed potting compound should be stirred immediately. Stirring is generally divided into manual stirring and mechanical stirring. Manual stirring is prone to uneven mixing and can introduce air, making it difficult to guarantee the stability of potting quality during mass production; mechanical stirring can ensure the greatest possible mixing uniformity. Considering the high reliability requirements of the current sensor, a stirrer is generally used for mixing.
4) Vacuum degassing. The presence of bubbles not only affects the appearance quality of the product, but more importantly, it poses serious quality risks to the electrical and mechanical properties of the product. In silicone rubber, only its impact on electrical properties is generally considered. For epoxy potting materials, bubbles affect electrical properties, but more importantly, they prevent the stress in the colloid from being transmitted continuously and uniformly, causing stress concentration at the bubble location, which leads to damage to electrical components, cracking of the colloid, etc., resulting in potting failure [6].
5) Potting. Currently, potting methods include static potting, pressure potting, vacuum potting, and vacuum pressure potting [7]. Based on product characteristics and production efficiency, our company selects pressure potting process for potting. The pressure potting process involves drawing the defoamed potting compound into a syringe, and then injecting the potting compound into the dehumidified product cavity under certain pressure. To ensure potting quality, the product should be placed in a suitable position during potting, so that the potting surface is horizontal, and the potting compound should be slowly injected into the potted area from one point.
3 Common Problems and Solutions in Power Sensor Encapsulation
3.1 Common Problems in Encapsulation of Power Sensors
In the early stages, some of our products encapsulated with silicone rubber and epoxy resin experienced quality abnormalities during high and low temperature tests, which seriously affected product quality. The main issues were: separation of the silicone rubber compound from the casing and cracking of the epoxy resin compound and damage to electrical components, as shown in Figure 2.
Analysis revealed the following main reasons: ① Poor adhesion of silicone rubber colloid, with inconsistent expansion coefficients between the shell and the colloid; ② Excessively high curing temperature and excessively rapid cooling rate after curing of the part; ③ Inappropriate potting process design.
Figure 2. Schematic diagram of abnormal filling quality.
In most cases, cracking and damage to electrical components are primarily caused by internal stress. Internal stress includes shrinkage stress and thermal stress. Shrinkage stress is the stress exerted on the device due to the shrinkage of the epoxy resin during the curing process; while thermal stress is the stress generated when there is a significant temperature change due to the large difference in the coefficients of thermal expansion between the resin and the potting compound in the potting product.
Under internal stress, potting materials cause defects and minor cracks of varying degrees to expand, leading to colloid cracking and damage to electrical components. Therefore, the existence of internal stress is the root cause of cracking and component damage.
3.2 Solution
1) Improvement of potting process
By adopting a composite process of silicone rubber and epoxy resin, a 1mm thick layer of silicone rubber is first coated onto the surface of the component, followed by epoxy resin encapsulation. Utilizing the excellent insulation, high and low temperature resistance, and low linear shrinkage of silicone rubber, a protective film is formed between the component and the epoxy resin, isolating the component from the direct impact of internal stress from the epoxy resin. Then, the excellent adhesion, corrosion resistance, and good mechanical strength of epoxy resin are used to seal the product, effectively reducing the impact of internal stress on the component and increasing the overall environmental adaptability of the product.
2) Improved curing temperature
Using segmented curing reduces the peak exothermic temperature. While segmented curing doesn't change the total heat release with the same potting material, it does alter the peak exothermic temperature. Segmented curing breaks down a high exothermic peak into multiple smaller peaks, promoting more uniform heat release and reducing the peak exothermic temperature. Our company's use of segmented curing not only enhances the crack resistance of our products but also effectively solves the problem of air bubbles in the potting compound.
in conclusion
Many factors affect the potting quality of power sensors. Engineers can only obtain satisfactory products by rationally formulating potting process plans based on the characteristics of each product and strictly implementing them.
After multiple production trials, our sensor products have not experienced any further colloid cracking or device damage after high and low temperature tests. This proves that the improvements to the above-mentioned process scheme and the control of the process can further improve the potting quality of the products and ensure that their protective and insulating properties fully meet the design requirements.
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