Common processes and problem analysis of epoxy resin sealing
Sealing is the process of mechanically or manually pouring liquid composite materials into devices containing electronic components and circuits, and curing them into high-performance thermosetting polymer insulation materials at room temperature or under heating conditions. Can enhance the overall integrity of electronic devices and improve their resistance to external impacts and vibrations; Improving the insulation between internal components and circuits is beneficial for miniaturization and lightweighting of devices; Avoid direct exposure of components and circuits, improve the waterproof and moisture-proof performance of devices, and enhance their usability and stability parameters.

The quality of sealed products is closely related to product design, component selection, assembly, and the sealing materials used. The sealing process is also an important factor that cannot be ignored.

There are two types of epoxy sealing processes: normal and vacuum sealing. Epoxy resin. Amine room temperature curing potting material, generally used for low-voltage electrical appliances, often used for normal potting. Epoxy resin and anhydride heating curing potting material are generally used for sealing high-voltage electronic devices, and vacuum sealing technology is often used, which is the focus of our research in this section. At present, there are two common methods of vacuum sealing: manual vacuum sealing and mechanical vacuum sealing. Mechanical vacuum sealing can be divided into two situations: A and B components are first mixed and defoamed before sealing, and then separately defoamed before mixed sealing. The process flow is as follows:
(1) Handmade vacuum sealing process
(2) Mechanical vacuum sealing process
Mixing and defoaming before sealing process
A. B first defoams separately and then mixes the sealing process
In contrast, mechanical vacuum sealing requires larger equipment investment and higher maintenance costs, but it is significantly superior to manual vacuum sealing processes in terms of product consistency and reliability. Regardless of the sealing method, the given process conditions should be strictly followed, otherwise it is difficult to obtain satisfactory products.

(1) The starting voltage of partial discharge is low, and high-voltage electronic products such as televisions, display output transformers, car and motorcycle igniters often experience partial discharge (corona), wire to wire ignition or breakdown during operation due to improper sealing technology. This is because the wire diameter of the high-voltage coil of these products is very small, usually only 0.02-0.04mm, and the sealing material has not fully penetrated the turns, leaving gaps between the coil turns. Due to the much lower dielectric constant of the gap compared to epoxy sealant, under alternating high voltage conditions, an uneven electric field will be generated, causing partial discharge at the interface, leading to material aging and decomposition, and causing insulation damage.

From a process perspective, there are two reasons for the gap between lines:
1) The vacuum degree is not high enough during sealing, and the air between the lines cannot be completely eliminated, making it impossible for the material to fully infiltrate.
2) The preheating temperature of the specimen before sealing is insufficient, and the viscosity of the material injected into the specimen cannot be rapidly reduced, which affects infiltration.

For manual sealing or vacuum sealing processes that involve mixing and defoaming materials, high mixing and defoaming temperatures, long operating times, or exceeding the applicable period of the materials, as well as failure to enter the heating and solidification process in a timely manner after sealing, can cause an increase in material viscosity and affect the infiltration of the coil. According to experts from Shanghai Changxiang Industrial Co., Ltd., the higher the starting temperature, the lower the viscosity of the thermosetting epoxy potting material composite. As time goes on, the viscosity increases more rapidly. Therefore, in order to ensure good infiltration of the material into the coil, the following points should be noted during operation:
1) The sealing material composite should be kept within the given temperature range and used up within the applicable period.
2) Before sealing, the specimen should be heated to the specified temperature, and after sealing is completed, it should promptly enter the heating and curing process.
3) The sealing vacuum degree should meet the technical specifications.

(2) During the heating and solidification process of the sealing material, there will be two types of shrinkage: chemical shrinkage during the liquid to solid phase transition and physical shrinkage during the cooling process. Further analysis shows that there are two processes of chemical change shrinkage during the curing process. The shrinkage generated from the heating chemical crosslinking reaction after encapsulation to the initial formation of the micro network structure is called gel pre curing shrinkage. The shrinkage from gel to full curing stage is called post curing shrinkage. The amount of contraction in these two processes is different. During the transformation of the former from a liquid state to a network structure, the physical state undergoes a sudden change, with a higher consumption of reactive groups and a greater volume shrinkage than the latter. The disappearance of epoxy group in the gel pre curing stage (75 ℃/3h) is greater than that in the post curing stage (110 ℃/3h), which is also proved by the results of differential thermal analysis. The curing degree of the sample after 750 ℃/3h treatment is 53%.
Common processes and problem analysis of epoxy resin sealing

If we adopt one high temperature curing for the potting specimen, the two stages in the curing process are too close, and the pre curing and post curing of gel are almost completed at the same time, which will not only cause excessive exothermic peak and damage the components, but also cause huge internal stress in the potting specimen, resulting in internal and external defects of the product. In order to obtain good parts, we must focus on the matching between the curing speed of potting materials (i.e., the gel time of A and B compounds) and the curing conditions when designing the potting material formula and formulating the curing process. The commonly used method is to divide the curing process into different temperature zones according to the properties and uses of the sealing material. According to experts, the encapsulation of output transformers for color televisions is carried out according to different temperature zones and segmented solidification procedures, as well as the internal heat release curve of the components. In the gel pre curing temperature range, the curing reaction of the potting material proceeds slowly, the reaction heat is gradually released, and the viscosity and volume shrinkage of the material increase smoothly. In this stage, if the material is in the flow state, the volume contraction is manifested as the drop of liquid level until gel, which can completely eliminate the internal stress of volume contraction in this stage. From gel pre curing to post curing, the temperature rise should also be gentle. After curing, the potting parts should slow down synchronously with the heating equipment. The stress distribution in the parts should be reduced and adjusted in many ways to avoid shrinkage, depression and even cracking on the surface of the parts.

The formulation of curing conditions for potting materials should also refer to the arrangement and fullness of the embedded components in the potting parts, as well as the size, shape, and single potting amount of the parts. It is absolutely necessary to appropriately reduce the gel pre curing temperature and extend the time for those with a large single potting amount and fewer potting elements.

(3) The phenomenon of poor surface or partial non solidification of cured materials is often related to the curing process. The main reason is:
1) Malfunction of measuring or mixing devices, and operational errors by production personnel.
2) Component A has precipitated after prolonged storage and was not thoroughly stirred before use, resulting in an imbalance in the actual ratio of resin and curing agent.
3) Component B is stored in an open environment for a long time and fails to absorb moisture.
4) During the high tide and wet season, the sealed parts did not enter the curing process in a timely manner, resulting in moisture absorption on the surface of the objects.
In short, to obtain a good potting product, the potting and curing process is indeed a highly valued issue.

1. History of Packaging Technology Transformation
There have been two significant changes in the field of electronic packaging technology. The first transformation occurred in the first half of the 1970s, characterized by the transition from pin insertion mounting technology (such as DIP) to surface mount technology (such as QFP) for four sided flat packaging; The second transformation occurred in the mid-1990s, marked by the emergence of solder ball arrays and BGA type packaging, and the corresponding surface mount technology and semiconductor integrated circuit technology crossed into the 21st century. With the development of technology, many new packaging technologies and forms have emerged, such as direct bonding of chips, encapsulated plastic solder ball array (CD-PBGA), flip chip plastic solder ball array (Fc PBGA), chip size packaging (CSP), and multi chip module (MCM). Among these packaging technologies, a considerable number use liquid epoxy material packaging technology. Encapsulation is the process of mechanically or manually injecting liquid epoxy resin composites into devices containing electronic components and circuits, and assimilating them into high-performance thermal polymer insulation materials at room temperature or under heating conditions.

2. Product performance requirements
The sealing material should meet the following basic requirements: good performance, long applicability, and suitable for large-scale automatic production line operations; Low viscosity, strong infiltration, can fill components and wires; During the filling and curing process, the powder components such as fillers settle less and do not separate into layers; Low exothermic peak during curing and small shrinkage during curing; Assimilates have excellent electrical and mechanical properties, good heat resistance, good adhesion to various materials, low water absorption and linear expansion coefficient; In certain situations, it is also required that the sealing material has properties such as flame retardancy, weather resistance, thermal conductivity, and resistance to high and low temperature fluctuations.

In specific semiconductor packaging, due to the direct contact between the material and the chip or substrate, in addition to meeting the above requirements, the product must also have the same purity as the chip mounting material. In the encapsulation of flip chip, due to the small gap between the chip and the substrate, it is required that the viscosity of the encapsulation material be extremely low. In order to reduce the stress generated between the chip and the packaging material, the modulus of the packaging material cannot be too high. And in order to prevent moisture infiltration at the interface, the packaging material should have good adhesion performance with the chip and substrate.

3. The main components and functions of sealing materials
The function of potting material is to enhance the integrity of electronic devices and improve their resistance to external impacts and vibrations; Improving the insulation between internal components and circuits is beneficial for miniaturization and lightweighting of devices; Avoid direct exposure of components and circuits, and improve the waterproof and moisture-proof performance of devices.

Epoxy resin sealant is a multi-component composite system composed of resin, curing agent, toughening agent, filler, etc. The viscosity, reactivity, service life, heat release, etc. of the system need to be comprehensively designed in terms of formula, process, casting size and structure to achieve comprehensive balance.

3.1 Epoxy resin
Epoxy resin sealant generally uses low molecular weight liquid bisphenol A type epoxy resin, which has low viscosity and high epoxy value. Commonly used ones include E.54 E-51、E-44、E-42。 In the filling process under the flip chip, due to the small gap between the chip and the substrate, it is required that the viscosity of the liquid encapsulation material be extremely low. Therefore, using bisphenol A epoxy resin alone cannot meet the product requirements. In order to reduce product viscosity and meet product performance requirements, we can use composite resins such as adding low viscosity bisphenol F-type epoxy resin, glycidyl ester type resin, and cycloaliphatic epoxides with high heat resistance, electrical insulation, and weather resistance. Among them, cycloaliphatic epoxides themselves also have the function of active diluents.

3.2 Curing agent
Assimilating agent is an important component in the formulation of epoxy sealant, and the properties of the cured material depend largely on the structure of the curing agent.
(1) Room temperature assimilation generally uses aliphatic polyamines as curing agents, but these curing agents are highly toxic, irritating, and exothermic, and are prone to oxidation during assimilation and use. Therefore, it is necessary to modify polyamines, such as using the active hydrogen on the amine group of polyamines, partially combining with epoxy groups to form hydroxyalkylation, and partially combining with acrylonitrile to form cyanoethylation, which can achieve a comprehensive modification effect of low viscosity, low toxicity, low melting point, room temperature curing, and certain toughness of the curing agent.
(2) Acid anhydride assimilators are the most important assimilators for two-component heat cured epoxy potting materials. Common assimilators include liquid methyltetrahydrophthalic anhydride, liquid methylhexahydrophthalic anhydride, hexahydrophthalic anhydride, methyl nadic anhydride, etc. This type of curing agent has low viscosity and a large dosage, which can play a dual role of assimilation and dilution in the formulation of potting materials. The curing heat release is gentle, and the comprehensive performance of assimilates is excellent.

3.3 Curing Accelerator
Two component epoxy anhydride potting material generally needs to be heated for a long time at around 140 ℃ to cure. Such curing conditions not only result in energy waste, but also make it difficult for most electronic components and skeleton shells to withstand. Adding accelerator components to the formula can effectively reduce the curing temperature and shorten the curing time. Common accelerators include tertiary amines such as phenylenediamine and DMP-30. Imidazole compounds and metal salts of carboxylic acids, such as 2-ethyl-4-methylimidazole and 2-methylimidazole, can also be used.

3.4 Coupling agent
In order to increase the adhesion between silica and epoxy resin, silane coupling agent needs to be added. Coupling agents can improve the adhesion and moisture resistance of materials. Common silane coupling agents suitable for epoxy resins include glycidoxypropyltrimethoxysilane (KH-560), phenyltrimethoxysilane, α - chloropropyltrimethoxysilane, α - mercaptopropyltroxysilane, phenylmethyltrimethoxysilane, diethylenediamine propyltrimethoxysilane, etc.

3.5 Reactive Diluents
When using epoxy resin alone and adding inorganic fillers, the viscosity significantly increases, which is not conducive to operation and defoaming. It is often necessary to add a certain amount of diluent to increase its fluidity and permeability, and extend its service life. Diluents can be active or inactive. Non reactive diluents do not participate in the curing reaction, and excessive dosage can easily increase the shrinkage rate of the product, reduce its mechanical properties and thermal deformation. The participation of reactive diluents in the curing reaction increases the chain links of reactants and has a relatively small impact on the properties of the cured material. The active diluents used in the sealing material include n-butyl glycidyl ether, allyl glycidyl ether, diethylhexyl glycidyl ether, and phenyl glycidyl ether.

3.6 Fillers
The addition of fillers in the sealing material has a significant effect on improving certain physical properties of epoxy resin products and reducing costs. Its addition not only reduces costs, but also lowers the thermal expansion coefficient, shrinkage rate, and increases thermal conductivity of the cured material. The commonly used fillers in epoxy potting materials include silicon dioxide, aluminum oxide, silicon nitride, boron nitride, and other materials. Table 1 shows the thermal conductivity of common inorganic fillers. Silicon dioxide is divided into crystalline, fused angle, and spherical types. In electronic packaging potting materials, molten spherical silica is preferred due to product requirements.

3.7 Defoamer
To solve the problem of bubbles remaining on the surface of liquid packaging materials after assimilation, defoamers can be added. The commonly used emulsifiers are silicone oil emulsifiers.

3.8 Toughening agent
Toughening agents play an important role in potting materials. The toughening modification of epoxy resin mainly improves its toughness by adding toughening agents, plasticizers, etc. There are two types of toughening agents: active and inert. Active toughening agents can participate in reactions with epoxy resin to increase the chain links of reactants, thereby increasing the toughness of the cured product. Generally, carboxyl terminated liquid nitrile rubber is chosen to form a toughened "sea island structure" in the system, which increases the impact toughness and thermal impact resistance of the material.

3.9 Other components
To meet the specific technical and process requirements of the sealed parts, other components can also be added to the formula. Flame retardants can improve the processability of materials; Colorants are used to meet the appearance requirements of the product.

4 Sealing process
There are two processes for epoxy resin sealing: normal and vacuum.
5 Common Problems and Solutions

5.1 Discharge, inter wire sparking or breakdown phenomenon
Due to improper sealing process, the device may experience discharge, inter wire sparking or breakdown during operation. This is because the wire diameter of the high-voltage coil of such products is very small (usually only 0.02mm~0.04mm), and the sealing material cannot fully penetrate the turns, resulting in gaps between the turns of the coil. Due to the much lower dielectric constant of the gap compared to epoxy sealant, uneven electric fields will be generated under alternating high voltage conditions, causing partial discharge and leading to material aging and decomposition, resulting in insulation damage. From a process perspective, there are two reasons for the gap between the lines: (1) insufficient vacuum during sealing, which prevents the air between the lines from being completely eliminated, making it impossible for the material to fully infiltrate; (2) The preheating temperature of the specimen before sealing is insufficient, and the viscosity of the material poured into the specimen cannot be rapidly reduced, which affects infiltration. For manual sealing or vacuum sealing processes that involve mixing and defoaming materials, high mixing and defoaming temperatures, long operating times, or exceeding the material's shelf life, as well as failure to enter the heating and solidification process in a timely manner after sealing, can cause an increase in material viscosity and affect the infiltration of the coil. The higher the initial temperature, the lower the viscosity of the thermosetting epoxy encapsulation material composite, and the faster the viscosity increases with time. Therefore, in order to ensure good infiltration of the material into the coil, attention should be paid to maintaining the sealing material composite within a suitable temperature range and using it within the applicable period. Before sealing, the test piece should be heated to the specified temperature. After sealing, it should enter the heating and curing process in a timely manner. The sealing vacuum degree should meet the technical specifications.

5.2 Surface shrinkage, local depression, and cracking of devices

There are two types of shrinkage during the heating and assimilation process of the sealing material: chemical shrinkage during the phase transition from liquid to solid, and physical shrinkage during the cooling process. The chemical change shrinkage in the curing process has two processes: the shrinkage generated from the heating chemical crosslinking reaction after encapsulation to the initial formation of the micro network structure is called gel pre curing shrinkage; The shrinkage from gel to full curing stage is called post curing shrinkage. The shrinkage of these two processes is different. The former undergoes a sudden change in physical state during the transition from liquid to network structure, with a higher consumption of reactive groups and a higher volume shrinkage than the latter. If the potting test piece is cured at a high temperature, the two stages in the curing process are too close, and the gel pre assimilation and post curing are almost completed at the same time, which will not only cause excessive exothermic peaks and damage components, but also cause the potting piece to produce huge internal stress, resulting in internal and external defects of the product. In order to obtain good parts, it is necessary to focus on the matching between the assimilation speed of the potting material and the curing conditions when designing the potting material formula and developing the curing process. The usual method is to assimilate the filling material into different temperature zones according to its properties and uses. In the gel pre curing temperature range, the potting material assimilation reaction proceeds slowly, the reaction heat is gradually released, and the material viscosity increases and the volume shrinkage proceeds smoothly. In this stage, if the material is in the flow state, the volume shrinkage is shown as the liquid level drops until gel, which can completely eliminate the internal stress of volume shrinkage in this stage. The temperature rise should be gentle from the gel pre curing to the post assimilation stage. After curing, the potting parts should slow down with the heating equipment. The stress distribution in the parts should be reduced and adjusted in many ways to avoid shrinkage, depression and even cracking on the surface of the parts. The formulation of curing conditions for potting materials should also refer to the arrangement and fullness of components inside the potting device, as well as the size, shape, and single potting amount of the parts. It is absolutely necessary to appropriately reduce the gel pre curing temperature and extend the time for those with a large single potting amount and fewer potting elements.

5.3 Surface defects or partial non solidification of cured materials
The phenomenon of poor surface or partial non solidification of cured materials is also often related to the curing process. Experts from the China Epoxy Resin Industry Association stated that the main reasons are the malfunction of measuring or mixing devices and operational errors by production personnel; Component A has precipitated after prolonged storage and was not thoroughly stirred before use, resulting in an imbalance in the actual ratio of resin and curing agent. Component B has been stored open for a long time, causing moisture absorption failure; During the high tide and wet season, the sealed parts did not enter the curing process in a timely manner, resulting in moisture absorption on the surface of the objects. In short, obtaining a good sealing and curing process is indeed a highly valued issue.