Electronic potting adhesives are mainly used for bonding, sealing, potting and coating protection of electronic components. The potting compound is liquid before uncured and has fluidity. The viscosity of the glue differs depending on the material, performance and production process of the product. It can be used after it is fully cured. After curing, it can be waterproof, moisture-proof, dust-proof, insulated, heat-conductive, confidential, anti-corrosive, temperature-resistant and shock-proof. There are many kinds of electronic potting glues. Here, we mainly introduce epoxy resin potting glue, polyurethane potting glue, silicone potting glue and so on.
Epoxy resin potting
Through the EU ROHS specified standards, the cured product has high hardness, smooth surface and good gloss. It has the characteristics of fixed, insulated, waterproof, oil-proof, dust-proof, anti-theft, corrosion-resistant, aging-resistant, and resistant to thermal shock. It is used for packaging of electronic transformers, AC capacitors, negative ion generators, aquarium pumps, ignition coils, electronic modules, LED modules, etc. Suitable for potting of small and medium-sized electronic components, such as automobile, motorcycle igniter, LED drive power supply, sensor, toroidal transformer, capacitor, trigger, LED waterproof light, circuit board for confidentiality, insulation, moisture (water) Potting.
Advantages: Epoxy potting is mostly hard, and there are very few modified epoxy resins. The material has the great advantage of good adhesion to the material and good insulation, and the cured product has good acid and alkali resistance. Epoxy resin is generally resistant to temperature of 100 ° C. The material can be used as a transparent material and has good light transmittance. The price is relatively cheap.
Disadvantages: weak resistance to cold and heat, cracks easily occur after thermal shock, resulting in water vapor infiltrating into the electronic components from the cracks, poor moisture resistance; high hardness and brittleness after solidification, high mechanical stress Easy to pull electronic components; epoxy resin can not be opened due to high hardness after potting and curing, so the product is a "lifetime" product, the replacement of components can not be achieved; transparent epoxy resin materials generally have poor weatherability, It is prone to yellowing under light or high temperature conditions.
Applications: Generally used for potting of non-precision electronic devices such as LEDs, transformers, regulators, industrial electronics, relays, controllers, power modules, etc.


2. Polyurethane potting compound
The polyurethane potting compound is further formed into a PU potting compound, usually consisting of a polyol of polyacetate, a polyether and a polydiene, and a diisocyanate, a glycol or a diamine as a chain extender, and is gradually polymerized. to make. Potting compounds can usually be prepared by a prepolymer process and a one-step process.
Polyurethane potting materials are characterized by low hardness, moderate strength, good elasticity, water resistance, mold resistance, shock resistance, transparency, excellent electrical insulation and flame retardancy, no corrosion to electrical components, steel, aluminum, copper, tin Other metals, as well as rubber, plastic, wood and other materials have good adhesion. The potting material protects the installed and commissioned electronic components and circuits from vibration, corrosion, moisture and dust.
Advantages: Polyurethane potting compound has excellent low temperature resistance, the material is slightly soft, and has good adhesion to general potting materials. The bonding force is between epoxy resin and silicone. It has good waterproof, moistureproof and insulation properties.
Disadvantages: Poor heat resistance and easy foaming, vacuum defoaming must be used; after curing, the surface of the colloid is not smooth and the toughness is poor, anti-aging ability, anti-shock and ultraviolet rays are weak, and the colloid is easily discolored.
Application range: Generally used for potting of electronic components with low heat generation. Transformers, chokes, converters, capacitors, coils, inductors, varistors, linear engines, fixed rotors, circuit boards, LEDs, pumps, etc.


3. Silicone potting compound
There are many types of silicone potting glues. Different types of silicone potting glues have great differences in temperature resistance, water resistance, insulation properties, optical properties, adhesion and adhesion properties to different materials, and hardness. Silicone potting compounds can be added with some functional fillers to impart properties such as conductive, thermal, and magnetic properties. The mechanical strength of silicone potting glue is generally poor, which is to borrow this performance, so that it can be "opened" for maintenance, that is, if a component fails, it only needs to open the potting glue and replace it with new ones. After the original, you can continue to use it.
The color of the silicone potting compound can generally be adjusted as needed. Or transparent or opaque or colored. Silicone potting adhesives perform very well in shock resistance, electrical properties, water resistance, high and low temperature resistance, and anti-aging properties.
Two groups of silicone potting compounds are the most common, and such glues include both condensed and additive types. Generally, the condensation type has poor adhesion to the components and the potting cavity, and volatile non-molecular substances are generated during the curing process, and the shrinkage rate is obvious after curing. Additive (also known as silicone gel) has very low shrinkage and no low molecular weight during curing. Can be heated quickly to cure.
Advantages: The silicone potting compound has a soft material after solidification. It has two forms of solid rubber and silicone gel, which can eliminate most mechanical stress and provide shock absorption protection. It has stable physicochemical properties, good resistance to high and low temperature, and can work in the range of -50~200 °C for a long time. Excellent weather resistance, it can still play a good protection role for more than 20 years outdoors, and it is not easy to yellow. It has excellent electrical performance and insulation ability, effectively improves the insulation between internal components and lines after potting, and improves the stability of the use of electronic components. With the ability to repair, the sealed components can be quickly and easily removed for repair and replacement.
Disadvantages: The bonding performance is slightly worse.
Applications: Suitable for potting all kinds of work in harsh environments as well as high-end precision/sensitive electronic devices. Such as LED, display, photovoltaic materials, diodes, semiconductor devices, relays, sensors, car stabilizers, automotive ECUs, etc., mainly for insulation, moisture, dust, shock absorption.


Second, the main factors affecting settlement


The potting glue is mainly composed of base resin, filler, curing agent, cross-linking agent and other additives. The filler and auxiliary agent are the main factors affecting the settlement of potting glue. Therefore, when designing the potting compound formula, it should be Considered in general, it is mainly from the perspective of packing. (based on the commonly used powder silicon powder in potting glue)
1. Particle size.
The same kind of thermally conductive filler, the finer the particle size, the better the sedimentation.
This is because the fine powder has a large specific surface area, a large surface hydroxyl group content, and strong hydrogen bonds between the particles, resulting in a large viscosity, thereby slowing the sedimentation of the filler, but the excellent anti-settling property caused by the gelation can cause potting The viscosity is high, so it makes no sense. Common potting compounds use different sizes of fillers for thickness matching. This compounding not only forms dense packing in the system, but also the addition of coarse powder can improve the thermal conductivity. More importantly, the coarse powder increases the viscosity of the system. The smaller, coarse and fine powders are matched with each other, and the viscosity of the system can be flexibly adjusted to adjust the sedimentation.
2. The amount of filler added.
Common fillers are inorganic powders. In the field of electronic potting, the common powder is silicon micropowder (see the figure below). Compared with silicone oil, the density is large and the surface active groups are small; the compatibility with silicone oil is poor, and as the standing time is prolonged, the inorganic powder gradually settles, causing oil powder to separate.
However, when the amount of powder added reaches a certain amount, the viscosity of the colloid increases sharply, and the sedimentation speed of the thermally conductive filler is slowed down, and the separation of the oil powder is weakened. However, if the viscosity is too high, it will affect the process performance of the heat-dissipating potting compound during foaming and potting, which is not worth the loss. Therefore, it is not possible to pursue excellent anti-settling properties and perform high filling.


(The picture shows the common physical properties of Haiyang silicon micropowder)
3. Surface properties of the filler.
The surface of the filler is coated with a surface treatment agent to introduce a non-polar oleophilic group on the surface of the powder particles, so that the modified powder has good wettability in the silicone oil, is easy to disperse uniformly, and is not easy to be bonded and aggregated between the particles, and the colloid The anti-settling property is enhanced. At the same time, the surface treatment process can reduce the polarity of the powder and reduce the interfacial tension between the powder and the silicone oil. The compatibility between the two is enhanced, and the colloidal viscosity is lower. Therefore, in the potting compound, the modified filler is better than the ordinary filler in terms of improving its anti-settling ability, and the viscosity does not increase.
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