Potting is to pour the liquid compound into a device equipped with electronic components and circuits by mechanical or manual means, and cure it under normal temperature or heating conditions to become a thermosetting polymer insulating material with excellent performance. It can strengthen the integrity of electronic devices and improve the resistance to external shocks and vibrations; improve the insulation between internal components and circuits, which is conducive to the miniaturization and light weight of the devices; avoid direct exposure of components and circuits, and improve the waterproof and moisture-proof performance of the devices. And improve the performance and stability parameters.
The quality of potting products is closely related to product design, component selection, assembly and potting materials used, and potting process is also a factor that cannot be ignored.
Epoxy potting has two potting processes: normal and vacuum. Epoxy resin. Amine type room temperature curing potting materials are generally used for low-voltage electrical appliances, and normal potting is often used. Epoxy resin. Acid anhydride heating and curing potting materials are generally used for potting high-voltage electronic devices, and vacuum potting processes are mostly used, which is the focus of our research in this section. At present, there are two common methods: manual vacuum potting and mechanical vacuum potting. Mechanical vacuum potting can be divided into two types: A and B components are mixed and defoamed first and then filled and respectively degassed and then mixed and filled. . The process flow is as follows:
(1) Manual vacuum potting process
(2) Mechanical vacuum potting process
Mixing and defoaming before potting process
A and B are respectively degassed and then mixed potting process
In contrast, mechanical vacuum potting requires large equipment investment and high maintenance costs, but it is significantly better than manual vacuum potting in terms of product consistency and reliability. Regardless of the potting method, the given process conditions should be strictly followed, otherwise it is difficult to obtain satisfactory products.
>>>>Common problems and cause analysis of potting products
(1) The initial voltage of partial discharge is low, and the line sparks or breaks through the output transformers of TVs, monitors, and high-voltage electronic products such as automobile and motorcycle igniters. Partial discharges often occur during work due to improper potting processes ( Corona), sparking or breakdown between wires are caused by the small diameter of high-voltage coils of these products, generally only 0.02～0.04mm, and the potting material cannot completely penetrate the turns, leaving gaps between the turns of the coils. Since the dielectric constant of the void is much smaller than that of the epoxy potting compound, under alternating high voltage conditions, an uneven electric field will be generated, causing partial discharge at the interface, aging and decomposing the material, and causing insulation damage.
From the perspective of process analysis, the gap between the lines is caused by the following two reasons:
1) The vacuum is not high enough during potting, and the air between the lines cannot be completely eliminated, so that the material cannot be completely infiltrated.
2) The preheating temperature of the test piece before potting is not enough, and the viscosity of the potting test piece material cannot be reduced quickly, which affects the infiltration.
For manual potting or first mixing and degassing followed by vacuum potting, high material mixing and degassing temperature, long working time or over the material pot life, and the product does not enter the heating and curing process in time after potting, will cause the viscosity of the material to increase. Affect the impregnation of the coil. According to experts from Shanghai Changxiang Industrial Co., Ltd., the higher the starting temperature of the thermosetting epoxy potting material composite, the lower the viscosity, and the faster the viscosity will increase with time. Therefore, in order to make the material have good permeability to the coil, the following points should be paid attention to in operation:
1) The potting compound should be kept within the given temperature range and used up within the applicable period.
2) Before potting, the test piece should be heated to the specified temperature, and the heating and curing program should be entered in time after potting.
3) The vacuum degree of potting must meet the requirements of technical specifications.
(2) Shrinkage, partial depression and cracking on the surface of the potting part. During the heating and curing process, the potting compound will produce two kinds of shrinkage, namely the chemical shrinkage during the phase change from liquid to solid and the physical shrinkage during the cooling process. Further analysis, there are two more processes for the chemical change and shrinkage during the curing process, from the beginning of the chemical cross-linking reaction after potting to the initial formation of the micro-network structure, which we call the gel pre-curing shrinkage. The shrinkage from the gel to the complete curing stage is called post-curing shrinkage. The shrinkage of these two processes is not the same. When the former is transformed from a liquid state to a network structure, the physical state changes suddenly, the consumption of reactive groups is greater than the latter, and the volume shrinkage is also higher than the latter. The epoxy group disappeared in the gel pre-curing stage (75℃/3h) was greater than that in the post-curing stage (110℃/3h). The results of differential thermal analysis also proved this. The curing degree of the sample after 750℃/3h treatment was 53% .
If we take a high-temperature curing for the potting test piece, the two stages in the curing process are too close, and the gel pre-curing and post-curing are almost completed at the same time. This will not only cause excessive exothermic peaks, damage the components, but also The potting parts produce huge internal stress, causing defects in the product's interior and appearance. In order to obtain good parts, we must pay attention to the matching of the curing speed of the potting compound (ie, the gel time of the A and B compound) and the curing conditions when the potting compound formulation design and curing process are formulated. The commonly used method is: according to the nature and purpose of the potting material, the process of solidification in different temperature zones. According to experts, the potting of color TV line output transformers is segmented according to different temperature zone curing procedures and internal heat release curves of the parts. In the gel pre-curing temperature zone, the curing reaction of the potting material proceeds slowly, the reaction heat is gradually released, and the viscosity of the material increases and the volume shrinkage proceeds smoothly. At this stage, when the material is in a fluid state, the volume shrinkage appears as the liquid level drops until it gels, which can completely eliminate the internal stress of the volume shrinkage at this stage. From the gel pre-curing to the post-curing stage, the temperature rise should be gentle. After the curing is completed, the potting parts should be slowly cooled with the heating equipment to reduce and adjust the internal stress distribution of the parts to avoid shrinkage and shrinkage on the surface of the parts. Depression and even cracking.
The formulation of the curing conditions of the potting material should also refer to the arrangement, fullness, size, shape, and single potting volume of the embedded components in the potting product. For a single potting with a large amount and less embedded components, it is completely necessary to appropriately lower the gel pre-curing temperature and extend the time.
(3) Poor surface of the cured product or partial non-curing are also related to the curing process. The main reason is:
1) Failure of metering or mixing device, and operation error of production personnel.
2) The A component has been stored for a long time and precipitates, and it is not fully stirred before use, which causes the actual ratio of resin and curing agent to be out of balance.
3) Component B is stored in open for a long time and loses moisture absorption.
4) The potting parts do not enter the curing process in time in the high humidity season, and the surface of the parts absorbs moisture.
In short, to obtain a good potting product, potting and curing process is indeed a problem worthy of great attention.
>>>>Epoxy resin potting compound and its process and common problems
1. History of packaging technology revolution
There have been two major changes in the field of electronic packaging technology. The first change occurred in the first half of the 1970s, and it was characterized by the transition from pin insertion mounting technology (such as DIP) to quad flat package surface mount technology (such as QFP); the second change occurred in the 1990s In the middle of the decade, its symbol was solder ball array. With the emergence of BGA-type packaging, the corresponding surface mount technology and semiconductor integrated circuit technology will cross the 21st century. With the development of technology, many new packaging technologies and packaging forms have emerged, such as direct chip bonding, potting plastic solder ball array (CD-PBGA), flip chip plastic solder ball array (Fc-PBGA), chip Size package (CSP) and multi-chip module (MCM), etc., in these packages, a considerable part of the use of liquid epoxy material packaging technology. Potting is to pour the liquid epoxy resin compound into a device equipped with electronic components and circuits by mechanical or manual methods, and assimilate it into a thermally isotropic polymer insulating material with excellent performance under normal temperature or heating conditions.
2. Product performance requirements
The potting material should meet the following basic requirements: good performance, long pot life, suitable for mass automatic production line operations; low viscosity, strong impregnation, and can fill the components and lines; in the potting and curing process, fillers and other powders The body components have small sedimentation and no delamination; low curing exothermic peak, small curing shrinkage; excellent electrical and mechanical properties of the assimilation, good heat resistance, good adhesion to a variety of materials, water absorption and linear expansion The coefficient is small; in some occasions, the potting compound is also required to have properties such as flame retardancy, weather resistance, heat conduction, and high and low temperature resistance.
In specific semiconductor packaging, since the material has to be in direct contact with the chip and substrate, in addition to meeting the above requirements, the product must have the same purity as the chip mounting material. In the potting of flip-chip, since the gap between the chip and the substrate is small, the viscosity of the potting compound is required to be extremely low. In order to reduce the stress between the chip and the packaging material, the modulus of the packaging material cannot be too high. Moreover, in order to prevent the penetration of moisture at the interface, the packaging material should have good adhesion properties with the chip and substrate.
3. The main components and functions of potting materials
The function of the potting compound is to strengthen the integrity of electronic devices, improve the resistance to external shocks and vibrations; improve the insulation between internal components and circuits, which is conducive to the miniaturization and light weight of the devices; avoid direct exposure of components and circuits, and improve the performance of the device Waterproof and moisture-proof performance.
Epoxy resin potting compound is a multi-component composite system. It consists of resin, curing agent, toughening agent, filler, etc. The viscosity, reactivity, service life, heat release, etc. of the system need to be in the formulation , Craftsmanship, casting size structure and other aspects to make a comprehensive design to achieve a comprehensive balance.
3.1 Epoxy resin
Epoxy resin potting materials generally use low-molecular liquid bisphenol A epoxy resin, which has low viscosity and high epoxy value. Commonly used are E. 54, E-51, E-44, E-42. In the potting filled under the flip chip, since the gap between the chip and the substrate is very small, the viscosity of the liquid encapsulant is required to be extremely low. Therefore, using bisphenol A epoxy resin alone cannot meet product requirements. In order to reduce product viscosity and meet product performance requirements, we can use combined resins: such as adding low-viscosity bisphenol F epoxy resin, glycidyl ester resin, and alicyclic ring with higher heat resistance, electrical insulation and weather resistance Group epoxides. Among them, the alicyclic epoxide itself also has the function of active diluent.
3.2 Curing agent
The assimilation agent is an important component in the epoxy potting compound formulation, and the performance of the cured product largely depends on the structure of the curing agent.
(1) Room temperature assimilation generally uses aliphatic polyamines as curing agents, but such curing agents are highly toxic, irritating, intensely exothermic, and easy to oxidize during assimilation and use. Therefore, it is necessary to modify polyamines, such as the use of active hydrogen on the amine group of multiple redundant amines, part of which is synthesized with epoxy groups for hydroxyalkylation and part with acrylonitrile for cyanoethylation. The curing agent can achieve a comprehensive modification effect of low viscosity, low toxicity, low melting point, curing at room temperature and certain toughness.
(2) Acid anhydride assimilation agent is the most important assimilation agent for two-component heating curing epoxy potting materials. Commonly used assimilation agents are liquid methyl tetrahydrophthalic anhydride, liquid methyl hexahydrophthalic anhydride, hexahydrophthalic anhydride, methyl nadic anhydride and so on. This kind of curing agent has low viscosity and large dosage. It can play the dual role of assimilation and dilution in the potting compound formulation, relaxes the curing exotherm, and has excellent comprehensive properties of assimilation.
3.3 Curing accelerator
The two-component epoxy-anhydride potting compound generally needs to be heated at about 140°C for a long time to cure. Such curing conditions not only cause energy waste, but also the components and skeleton shells in most electronic devices are unbearable. Adding accelerator components to the formula can effectively reduce the curing temperature and shorten the curing time. Commonly used accelerators are: tertiary amines such as benzyl diamine and DMP-30. Metal salts of imidazole compounds and carboxylic acids, such as 2-ethyl-4-methylimidazole, 2-methylimidazole, etc., can also be used.
3.4 Coupling agent
In order to increase the adhesion between silica and epoxy resin, a silane coupling agent needs to be added. The coupling agent can improve the adhesion and moisture resistance of the material. Commonly used silane coupling agents for epoxy resins are glycidoxypropyltrioxysilane (KH-560), anilinomethyltriethoxysilane, α-chloropropyltrimethoxysilane, and α-mercapto Propyl trimethoxy silane, aniline methyl trimethoxy silane, diethylene diamino propyl trimethoxy silane, etc.
3.5 Reactive diluent
Using epoxy resin alone, the viscosity increases significantly after adding inorganic fillers, which is not conducive to operation and defoaming. It is often necessary to add a certain amount of diluent to increase its fluidity and penetration, and prolong its service life. The diluent is active And inactive. Non-reactive diluent does not participate in the curing reaction. If the amount added is too much, it is easy to increase the shrinkage of the product and reduce the mechanical properties and thermal deformation of the product. The reactive diluent participates in the curing reaction to increase the chain link of the reactant, and has little effect on the performance of the cured product. The potting compound used is the active diluent, and the commonly used ones are: n-butyl glycidyl ether, allyl glycidyl ether, diethylhexyl glycidyl ether, and phenyl glycidyl ether.
The addition of fillers in the potting compound has an obvious effect on improving certain physical properties of epoxy resin products and reducing costs. Its addition can not only reduce the cost, but also reduce the thermal expansion coefficient, shrinkage rate and increase the thermal conductivity of the cured product. Commonly used fillers in epoxy potting materials are silicon dioxide, aluminum oxide, silicon nitride, boron nitride and other materials. Table 1 shows the thermal conductivity of common inorganic fillers. Silica is divided into crystalline, fused angle and spherical silica. Among potting materials for electronic packaging, fused spherical silica is preferred due to product requirements.
In order to solve the problem of bubbles remaining on the surface of the liquid packaging material after assimilation, a defoamer can be added. Commonly used are emulsified silicone oil emulsifiers.
3.8 Toughening agent
Toughening agent plays an important role in potting compound. The toughening modification of epoxy resin mainly improves its toughness by adding toughening agents and plasticizers. Toughening agents are active and inert. Active toughening The agent can participate in the reaction with the epoxy resin to increase the chain link of the reactant, thereby increasing the toughness of the cured product. Generally, carboxyl-terminated liquid nitrile rubber is selected to form a toughened "sea-island structure" in the system to increase the impact toughness and thermal shock resistance of the material.
3.9 Other components
In order to meet the specific technical and process requirements of potting parts, other components can be added to the formula. For example, flame retardants can improve the processability of materials; colorants are used to meet the appearance requirements of the parts.
4 Potting process
Epoxy resin potting has two processes: normal and vacuum
5 Common problems and solutions
5.1 Discharge, inter-line sparking or breakdown phenomenon
Due to improper potting process, the device will produce discharge, inter-line sparking or breakdown during operation. This is because the high-voltage coil wire diameter of this type of product is very small (usually only 0.02mm ~ 0.04mm), and the potting material Failure to completely penetrate the turns, resulting in gaps between the coil turns. Since the dielectric constant of the gap is much smaller than that of the epoxy potting compound, an uneven electric field will be generated under alternating high voltage conditions, causing partial discharge, and aging and decomposition of the material, causing insulation damage. From a process point of view, there are two reasons for the gap between the lines: (1) The vacuum degree is not high enough during potting, and the air between the lines cannot be completely eliminated, so that the material cannot be completely impregnated; (2) The sample is pre-filled before potting. The heat temperature is not enough, the viscosity of the material poured into the test piece cannot be reduced quickly, which affects the infiltration. For manual potting or first mixing and degassing followed by vacuum potting processes, high material mixing and degassing temperature, long working time or over the material pot life, and the product not entering the heating and curing process in time after potting will cause the viscosity of the material to increase, which will affect Impregnation of the coil. For thermo-homogeneous epoxy potting material composites, the higher the starting temperature, the lower the viscosity, and the faster the viscosity increases with time. Therefore, in order to make the material have good impregnation to the coil, attention should be paid to ensure that the potting compound should be kept in a suitable temperature range and used within the applicable period. The test piece should be heated to the specified temperature before potting, and the heating and curing process should be entered in time after potting, and the potting vacuum must meet the technical specifications.
5.2 Shrinkage, partial depression and cracking on the surface of the device
The potting compound will produce two kinds of shrinkage during the process of heating assimilation: the chemical shrinkage during the phase change from liquid to solid and the physical shrinkage during the cooling process. The chemical change and shrinkage in the curing process have two more processes: from the beginning of the chemical cross-linking reaction after potting to the initial formation of the micro-network structure, it is called gel pre-curing shrinkage; from gel to complete curing The shrinkage produced in the stage is called post-curing shrinkage. The amount of shrinkage in these two processes is different. The physical state of the former changes from a liquid state to a network structure. The consumption of reactive groups is greater than the latter, and the volume shrinkage is also higher than the latter. If the potting test piece is cured at a high temperature, the two phases in the curing process are too close, and the gel pre-assimilation and post-curing are almost completed at the same time. This will not only cause excessive exothermic peaks and damage the components, but also make the potting The seal produces huge internal stress, causing defects in the interior and appearance of the product. In order to obtain good parts, it is necessary to focus on the matching of the assimilation speed of the potting compound and the curing conditions when the potting compound formulation design and curing process are formulated. The usual method is to assimilate according to the nature and purpose of the potting material in different temperature zones. In the gel pre-curing temperature zone, the assimilation reaction of the potting material proceeds slowly, the reaction heat is gradually released, and the viscosity of the material increases and the volume shrinkage proceeds smoothly. At this stage, when the material is in a fluid state, the volume shrinkage is manifested as the liquid level drops until the gel, which can completely eliminate the internal stress of the volume shrinkage at this stage. From the gel pre-curing to the post-assimilation stage, the temperature rise should be gentle. After curing, the potting parts should be slowly cooled down with the heating equipment, reducing and adjusting the internal stress distribution of the parts in various ways to avoid shrinkage, depression and even cracks on the part surface phenomenon. The formulation of the curing conditions of the potting material should also refer to the arrangement of the components in the potting device, the degree of fullness, the size and shape of the part, and the amount of single potting. For a single potting with a large amount and less embedded components, it is completely necessary to appropriately lower the gel pre-curing temperature and extend the time.
5.3 The surface of the cured product is poor or partially uncured
The surface defects or partial non-curing of the cured product are also related to the curing process. Experts from the China Epoxy Resin Industry Association said that the main reasons are the failure of the metering or mixing device and the operating errors of the production personnel; the A component has been stored for a long time and has precipitated, and it has not been fully stirred before use, causing the actual ratio of resin and curing agent to be out of balance. Component B is stored in the open for a long time, and the moisture absorption fails; the potting parts do not enter the curing process in time in the high humidity season, and the surface of the object absorbs moisture. In short, to obtain a good potting and curing process is indeed a problem worthy of great attention.