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1、 What is potting? Potting (potting glue) is to potte polyurethane potting glue, silicone potting glue and epoxy resin potting glue into the devices with electronic components and circuits by equipment or by hand, and to solidify into thermosetting polymer insulation materials with excellent performance under normal temperature or heating conditions, so as to achieve the purpose of bonding, sealing, potting and coating protection.
2、 The main function of potting?
The main functions of potting are: 1) strengthen the integrity of electronic devices, improve the resistance to external shock and vibration; 2) improve the insulation between internal components and lines, which is conducive to miniaturization and lightweight of devices; 3) avoid direct exposure of components and lines, improve the waterproof, dustproof and moisture-proof performance of devices; 5) heat transfer and heat conduction;
3、 Advantages and disadvantages of three kinds of potting adhesives?
1) Epoxy resin potting adhesive
Most of the epoxy resin potting adhesives are hard, almost as hard as stone after curing, which is difficult to remove, with good confidentiality function, but a few are soft. Ordinary temperature resistance is about 100 ℃, heating curing temperature resistance is about 150 ℃, and there are also temperature resistance above 300 ℃. It has the characteristics of fixation, insulation, water-proof, oil-proof, dust-proof, anti-theft, corrosion-resistant, aging resistant, cold and heat shock resistant, etc. Common epoxy potting adhesives are: flame retardant, heat conduction, low viscosity, high temperature resistant, etc.
Advantages: good adhesion to hard materials, excellent high temperature resistance and electrical insulation, simple operation, very stable before and after curing, excellent adhesion to a variety of metal substrates and porous substrates. Disadvantages: weak resistance to cold and heat changes, easy to produce cracks after cold and heat shock, resulting in water vapor infiltrating into electronic components from cracks, poor moisture resistance. And after curing, it is colloidal with high hardness and brittleness, easy to scratch electronic components, unable to open after potting, and poor repairability. Scope of application: epoxy resin potting adhesive is easy to penetrate into the gaps of products, suitable for potting small and medium-sized electronic components under normal temperature conditions without special requirements for environmental mechanical properties, such as automobile and motorcycle ignitor, LED driving power supply, sensor, ring transformer, electric container, trigger, led waterproof lamp, circuit board confidentiality, insulation, moisture-proof (water) potting.
2) Silicone potting adhesive
After curing, most of the silicone electronic potting adhesives are soft, elastic and repairable, which is called soft adhesive for short, with poor adhesion. Its color can be adjusted as needed, transparent or non transparent or colored. Two component silicone potting adhesive is the most common, which includes two types: condensation type and additive type. The general condensation type has poor adhesion to components and encapsulating cavities, and produces volatile low molecular substances in curing process. It has obvious shrinkage rate after curing. Addition molding (also known as silica gel) has a very low shrinkage rate and does not produce volatile low molecular substances in curing process, it can be heated rapidly solidified.
Advantages: strong anti-aging ability, good weather resistance and excellent impact resistance. It has excellent heat and cold resistance and thermal conductivity. It can be used in a wide range of working temperatures. It can maintain elasticity and crack free at the temperature range of -60 to 200 degrees. It can be used at 250 degrees for a long time. It has higher temperature resistance and better insulation performance. The epoxy resin is good and can withstand over 10000v. After potting, it can effectively improve the insulation between internal components and lines, and improve the use stability of electronic components; it has no corrosiveness to electronic components and no by-products are produced in the curing reaction; it has excellent repair ability, and can quickly and conveniently take out the sealed components for repair and replacement; it has excellent heat conductivity and flame retardant ability, and effectively improves the electronic components Heat dissipation capacity and safety factor of components; low viscosity, good fluidity, able to penetrate into small voids and components; room temperature curing or heating curing, good self defoaming, more convenient to use; small curing shrinkage, excellent waterproof performance and seismic resistance.
Disadvantages: high price, poor adhesion. Scope of application: suitable for potting all kinds of electronic components working in harsh environment. What are the advantages of silicone electronic potting adhesive over other potting adhesives? Advantage 1: long-term protection for sensitive circuits or electronic components, and long-term effective protection for electronic modules and devices, whether simple or complex structure and shape. Advantage 2: it has stable dielectric insulation performance and is an effective barrier to prevent environmental pollution. After curing, it forms soft elastomer to eliminate the stress caused by impact and vibration in a large range of temperature and humidity. Advantage 3: able to maintain the original physical and electrical properties in various working environments, able to resist the degradation of ozone and UV, with good chemical stability. Advantage 4: it is easy to clean and remove after potting, so as to repair electronic components, and inject new potting glue into the repaired parts.
3) Polyurethane sealant
Polyurethane potting adhesive is also known as PU potting adhesive. After curing, it is mostly soft and elastic and can be repaired. It is called soft adhesive for short. The adhesion is between epoxy and silicone, and the temperature resistance is general, generally no more than 100 ℃. There are more bubbles after potting. The potting condition must be under vacuum, and the adhesion is between epoxy and silicone.
Advantages: good low temperature resistance, shockproof performance is the best of the three. It has the characteristics of low hardness, moderate strength, good elasticity, water resistance, mould resistance, shock resistance and transparency, excellent electrical insulation and flame resistance, no corrosion to electrical components, good adhesion to steel, aluminum, copper, tin and other metals, as well as rubber, plastic, wood and other materials. Disadvantages: poor heat resistance, solidified surface is slippery and poor toughness, anti-aging ability and UV resistance are very weak, colloid easy to change color. Scope of application: suitable for potting indoor electrical components with low heat generation, which can make the installed and debugged electronic components and circuits free from vibration, corrosion, humidity and dust, etc., and is an ideal potting material for anti humidity and anti-corrosion treatment of electronic and electrical components.
4、 What should be considered when selecting potting materials? 1) Performance requirements after potting: use temperature, cold and hot alternation, internal stress of components, outdoor or indoor use, stress condition, whether flame retardant and heat conduction are required, color requirements, etc.; 2) potting process: manual or automatic, room temperature or heating, complete curing time, setting time of mixed adhesive, etc; 3) Cost: the proportion of potting materials is very different. We must look at the actual cost after potting rather than simply look at the price of materials. According to the functional classification, the adhesives used for potting are heat-conducting potting adhesive, adhesive potting adhesive and waterproof potting adhesive; according to the material classification, polyurethane potting adhesive, silicone potting adhesive and epoxy resin potting adhesive. For the selection of soft adhesive or hard adhesive, at that time, both can be potted, waterproof and insulated. If high-temperature heat-resistant is required, silicone soft adhesive is recommended; if low-temperature resistance is required, silicone soft adhesive is recommended , it is recommended to use polyurethane soft adhesive; if there is no requirement, it is recommended to use epoxy hard adhesive, because the curing time of epoxy hard adhesive is faster than that of silicone. Epoxy resin potting adhesive has a wide range of applications, different technical requirements and a wide variety. There are two types of curing conditions: room temperature curing and heating curing. There are two types of dosage forms: two-component and one-component. There are also two types of room temperature curing epoxy potting adhesive, which is generally two-component. Its advantages are that it can be cured without heating after potting. The equipment requirements are not high, and it is convenient to use. The existing defects are that the operating viscosity of the glue liquid mixture is large, the impregnation is poor, and the application period is short The heat resistance and electrical properties of the cured products are not very high, and they are generally used in the encapsulation of low-voltage electronic devices or in the occasions where they are not suitable for heating and curing.
5、 The quality of potting products is closely related to product design, component selection, assembly and potting materials, and the potting process is also a factor that can not be ignored. There are two kinds of epoxy potting processes, normal and vacuum. Epoxy resin. Amine room temperature curing potting material, generally used for low-voltage electrical appliances, mostly using normal potting. The heat curing potting material of epoxy resin and anhydrides is generally used for the potting of high-voltage electronic devices, and vacuum potting process is often adopted. At present, there are two common ways: manual vacuum potting and mechanical vacuum potting. Mechanical vacuum potting can be divided into two situations: A and B components are mixed and defoaming first and then potting, respectively. There are three methods of operation: the first one is single component electronic potting adhesive, which can be used directly, and can be used for beating or pouring directly; the second one is two-component condensing electronic potting adhesive, with curing agent of 2% - 3% or other proportions, stirring vacuumizing, defoaming and pouring; the third one is adding molding electronic potting adhesive, with curing agent of 1:1 and 10:1; the process flow is as follows: (1) manual vacuum potting process (2) Mechanical vacuum filling process 1) measurement: accurately weigh component A and component B (curing agent). 2) Mixing: mixing components; 3) defoaming: natural defoaming and vacuum defoaming; 4) filling: the rubber material shall be filled within the operation time, otherwise it will affect the leveling; 5) curing: heating or room temperature curing, the potted product shall be cured at room temperature, and the next process can be started after initial consolidation, and the complete curing takes 8-24 hours. If the temperature is high in summer, the curing will be faster; if the temperature is low in winter, the curing will be slower. (3) Precautions: a. the surface of the product to be potted must be cleaned before potting! B. before weighing, fully mix component A and component B to make the pigment (or filler) sunk in the bottom disperse into the glue solution. c. The primer shall not be directly mixed with the rubber material. The primer shall be used first, and then the rubber material shall be filled and sealed after the primer is dry. d. There is a certain relationship between the curing speed and the temperature of the compound. Curing at low temperature will be slower. In contrast, mechanical vacuum potting has the advantages of large equipment investment and high maintenance cost, but it is obviously superior to manual vacuum potting in terms of product consistency and reliability. No matter what kind of filling and sealing method, we should strictly abide by the set process conditions, otherwise it is difficult to get satisfactory products.
Basic principle of automatic glue filling machine (video)
6、 Analysis on the problems and causes of potting products (1) Partial discharge (ARC), line to line ignition or breakdown of high-voltage electronic products such as television, display output transformer, automobile, motorcycle igniter, etc. often occur due to improper filling and sealing process, because the high-voltage coil diameter of such products is very small, generally only 0.02-0.04mm, filling and sealing materials are not fully saturated Turn to turn to leave space between turns. Because the dielectric constant of the gap is far less than that of the epoxy potting material, under the condition of alternating high voltage, the uneven electric field will be generated, which will cause partial discharge at the interface, aging and decomposition of the material, and damage the insulation. From the perspective of technology, there are two reasons for the gap between lines:
1) when filling and sealing, the vacuum degree is not high enough, and the air between them is not completely removed, so that the material cannot be completely infiltrated.
2) before filling, the preheating temperature of glue or product is not enough, and the viscosity cannot be reduced rapidly, which affects the infiltration. For manual filling or first mixing and defoaming followed by vacuum filling, materials are mixed