Common technology and problem analysis of epoxy resin pouring

Potting is to pour liquid compound into the devices with electronic components and circuits by mechanical or manual way, and then solidify into thermosetting polymer insulation materials with excellent performance under normal temperature or heating conditions. It can strengthen the integrity of electronic devices and improve the resistance to external shock and vibration; Improving the insulation between internal components and lines is conducive to the miniaturization and lightweight of devices; Avoid direct exposure of components and circuits, improve the waterproof and moisture-proof performance of devices, and improve the performance and stability parameters.

Encapsulation technology

The quality of potting products is closely related to product design, component selection, assembly and potting materials, and the potting process is also an important factor.

There are two kinds of epoxy potting process: normal and vacuum. Epoxy resin and amine room temperature curing potting materials are generally used for low-voltage electrical appliances, and most of them are normal potting materials. Epoxy resin and anhydride heat curing potting materials are generally used for potting high-voltage electronic devices, and vacuum potting process is the focus of our research in this section. At present, there are two common ways: manual vacuum sealing and mechanical vacuum sealing, and mechanical vacuum sealing can be divided into two situations: A and B components are mixed first and then decarburized and then mixed first and then decarburized respectively. The technological process is as follows:

( one ) Manual vacuum sealing process

( two ) Mechanical vacuum sealing process

Mixing and deaeration before pouring

A. B. separate defoaming followed by mixed pouring process

In contrast, mechanical vacuum potting , The equipment investment is large and the maintenance cost is high, but the consistency and reliability of the product are obviously better than the manual vacuum sealing process. No matter what kind of pouring method, should strictly abide by the given process conditions, otherwise it is difficult to get satisfactory products.

Analysis of problems and causes of potting products

(1) The initial voltage of partial discharge is low, the line to line ignition or breakdown of TV, display line output transformer, automobile, motorcycle igniter and other high-voltage electronic products, often due to improper potting process, partial discharge will occur when working ( corona )、 The phenomenon of line to line ignition or breakdown is due to the small wire diameter of high voltage coil of this kind of products, which is generally only 0 . 02～0 . 04mm, the potting material can not completely soak the turn to turn, so that there is a gap between the turns of the coil. 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 interface partial discharge, aging and decomposition of the material, and insulation damage.

From the point of view of technology, there are two reasons for the gap between lines

one ) The vacuum degree is not high enough, and the air between the lines can not be completely removed, so that the material can not be completely infiltrated.

two ) If the preheating temperature of the specimen is not enough before pouring, the viscosity of the material can not be reduced rapidly, which affects the infiltration.

For manual pouring or vacuum pouring process after mixed defoaming, high temperature of material mixed defoaming, long operation time or beyond the material application period, and the product after pouring does not enter the heating and curing process in time will cause the increase of material viscosity and affect the coil infiltration. The higher the initial temperature is, the lower the viscosity is, and the faster the viscosity increases 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:

one ) The potting compound should be kept in the given temperature range and used up within the applicable period.

two ) Before pouring, the specimen should be heated to the specified temperature. After pouring, the heating and curing procedure should be started in time.

three ) The vacuum degree of pouring should meet the requirements of technical specifications.

(2) There are two kinds of shrinkage on the surface of the potting parts: chemical shrinkage in the process of liquid to solid phase transformation and physical shrinkage in the process of cooling. Further analysis shows that there are two processes of chemical shrinkage in the curing process, namely, the shrinkage from the beginning of the chemical crosslinking reaction after the encapsulation to the initial formation stage of the micro network structure, which we call gel precuring shrinkage. Shrinkage from gel to complete curing is called post cure shrinkage. The shrinkage of these two processes is different. In the process of the former changing from liquid to network structure, the physical state changes suddenly, the consumption of reactive groups is greater than that of the latter, and the volume shrinkage is also higher than that of the latter. Gel precuring stage ( 75℃ / 3h ) The disappearance of epoxy group is greater than that of post curing stage ( 110℃ / 3h ) The results of DTA also prove this point / After 3 h treatment, the curing degree was 53 %。

Common technology and problem analysis of epoxy resin pouring

If we adopt a high temperature curing for the encapsulated specimen, the two stages in the curing process are too close, and the gel pre curing and post curing will be completed at the same time. This will not only cause exothermic peaks, damage elements, but also cause huge internal stress of the sealing parts, resulting in defects in the internal and external appearance of the products. In order to get good parts, we must pay attention to the curing speed of the potting materials when designing the filling material formula and setting the curing process ( Gelation time of A and B Composites ) The matching problem with curing condition. The commonly used method is: according to the nature and use of the potting material, curing process in different temperature zones. According to experts, color TV output transformer potting according to different temperature zone curing procedures and internal heat release curve. The curing reaction of the encapsulated material in the gel precuring zone was slow, the reaction heat gradually released, the viscosity increased and the volume contracted slowly. At this stage, when the material is in the flowing state, the volume shrinkage shows that the liquid level drops until the gel can completely eliminate the shrinkage internal stress at this stage. From the gel pre curing to the post curing stage, the temperature rise should also be gentle. After the curing is completed, the sealing parts should be cooled slowly with the heating equipment at the same time, and the internal stress distribution of the parts can be reduced and adjusted in many ways, so as to avoid shrinkage holes, sunken or even cracking on the surface of the parts.

For the formulation of curing conditions of potting materials, it is necessary to refer to the layout, plumpness, size, shape and single potting amount of the embedded components in the potting parts. It is necessary to reduce the gel precuring temperature and prolong the long time when the single encapsulating quantity is larger and the sealing elements are less.

(3) The phenomenon of poor surface or partial non curing of cured products is also related to the curing process. The main reasons are:

one ) The metering or mixing device fails, and the production personnel operate incorrectly.

two ) The precipitation of component A is deposited for a long time, and it is not stirred evenly before use, which causes the actual proportion of resin and curing agent to be out of balance.

three ) The B component is stored in open for a long time and the moisture absorption fails.

four ) The sealing parts did not enter the curing procedure in time in the high tide and wet season, and the surface of the object was hygroscopic.

In short, in order to obtain a good potting product, the potting and curing process is indeed a problem worthy of high attention.

Epoxy resin filling material, its technology and common problems

1. Packaging technology change history

There have been two major changes in the field of electronic packaging technology. The first change appeared in the first half of 1970s, which is characterized by the transition from pin insertion installation technology (such as DIP) to surface mount technology (such as QFP) of four sided flatpack; the second change occurred in the mid-1990s, which marked the appearance of solder ball array.Bga packaging, which is a cross-cultural 21st century with the corresponding surface mount technology and semiconductor integrated circuit technology. 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 packaging (CSP) and multi chip assembly (MCM). In these packages, liquid epoxy material packaging technology is used in a large part of these packages. Potting is to inject liquid epoxy resin composite into the device with electronic components and circuits by mechanical or manual means, and assimilate it into a thermal polymer insulation material with excellent performance under normal temperature or heating conditions.

2. Product performance requirements

The filling materials shall meet the following basic requirements: good performance, long application period, suitable for large-scale automatic production line operation; small viscosity, strong permeability, and can be filled between components and lines; in the process of filling and curing, the composition of filler and other powder has small settlement and no stratification; low curing exothermic peak and small curing shrinkage; the assimilate has excellent electrical and mechanical properties, good heat resistance, and is suitable for many kinds of materials The material has good adhesion, water absorption and linear expansion coefficient are small; in some cases, it is also required to find that the sealing material has the properties of fire-resistant, weather resistance, heat conduction, high and low temperature alternating resistance.

In the specific semiconductor packaging, because the material is to be directly in 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 pouring of flip chip, the viscosity of the filling material is very low because of the small gap between the chip and the substrate. In order to reduce the stress between chip and packaging material, the modulus of packaging material cannot be too high. In order to prevent the water penetration at the interface, the bonding performance between the packaging material and the chip and substrate should be good.

3. Main components and functions of potting materials

The function of filling material is to strengthen the integrity of electronic devices, improve the resistance to external shock and vibration; improve the insulation between internal components and lines, which is conducive to the miniaturization and lightweight of devices; avoid direct exposure of components and lines, and improve the waterproof and moisture-proof performance of devices.

Epoxy resin filling material is a multi-component composite system. It is composed of resin, curing agent, toughener and filler. For the viscosity, reaction activity, service life and heat release of the system, it is necessary to design comprehensively the formulation, process, casting size and structure, so as to achieve comprehensive balance.

3.1 epoxy

The epoxy resin filling material generally adopts the low molecular liquid bisphenol A epoxy resin, which has low viscosity and high epoxy value. The common ones are e.54, E-51, E-44 and E-42. In the filling process of the flip chip, the viscosity of the liquid packaging material is very low because the gap between the chip and the substrate is very small. Therefore, the use of bisphenol A epoxy resin alone can not meet the product requirements. In order to reduce the viscosity of the product and meet the requirements of product performance, we can use composite resin, such as bisphenol F type epoxy resin with low viscosity, glycidyl resin and aliphatic cyclooxygenate with high heat resistance, electrical insulation and weather resistance. Among them, the aliphatic epoxide itself also has the function of active diluent.

3.2 curing agent

Assimilate is an important component in the formulation of epoxy potting material, and the properties of the curing material depend on the structure of the curing agent to a large extent.

(1) In general, aliphatic polyamines are used as curing agents for room temperature assimilation, but the curing agents are toxic, irritating and exothermic, and are easy to oxidize during assimilation and use. Therefore, it is necessary to modify polyamines, such as the use of active hydrogen on the multiredundant amine group, the partial hydroxylation with epoxy group and partial Cyanoethylation with propylene to Cyanoethylation. The curing agent can achieve the comprehensive modification effect of low viscosity, low toxicity, low melting point, room temperature curing and certain toughness.

(2) The anhydrides are the most important Assimilators of the two-component heating curing epoxy potting materials. The commonly used Assimilators include liquid methyltetrahydrophto-phthalic anhydride, liquid methylhexahydrophto-phthalic anhydride, hexahydrophthalic anhydride, methylnadic anhydride, etc. The curing agent has small viscosity and large amount of coordination, which can play a double role of assimilation and dilution in the filling and sealing material formula. The curing heat release is eased and the comprehensive performance of assimilate is excellent.

3.3 curing accelerator

The two-component epoxy - anhydrides can be cured after heating for a long time at 140 ℃. Such curing conditions not only cause energy waste, but also the components and skeleton shell of most electronic devices are difficult to bear. The addition of accelerator component in the formula can effectively reduce the curing temperature and shorten the curing time. The common accelerators are: Bian diamine, DMP-30 and other tertiary amines. Metal salts of imidazole compounds and carboxylic acids, such as 2-ethyl-4-methylimidazole and 2-methylimidazole, can also be used.

3.4 coupling agent

In order to increase the tightness between silica and epoxy resin, silane coupling agent should be added. Coupling agent can improve the adhesion and moisture resistance of the material. The common silane coupling agents suitable for epoxy resin include glycidyloxypropyltrimethoxysilane (KH-560), aniline methoxysilane, α - chloropropyltrimethoxysilane, α - mercaptopropyltrimethoxysilane, aniline methyl trimethoxysilane, diethylenediamine propyltrimethoxysilane, etc.

3.5 active diluent

The viscosity of the resin is obviously increased after adding inorganic filler, which is not conducive to operation and defoaming. A certain amount of diluent is often added to increase its fluidity and permeability, and prolong the service life. The diluent has the active and inactive components. The non active diluent does not participate in curing reaction, and the amount of people is too much, which will easily cause the increase of product shrinkage and reduce the mechanical properties and thermal deformation of the product. The active diluent increased the chain link of the reactant and had little influence on the properties of the reactant. The active diluent is selected in the filling material, and the commonly used ones are n-butyl glycidyl ether, allyl glycidyl ether, diethylhexyl glycidyl ether and phenylglycidyl ether.

3.6 filler

The addition of filler in the filling material has obvious effect on improving some physical properties and reducing cost of epoxy resin products. The addition of the product can not only reduce the cost, but also reduce the thermal expansion coefficient, shrinkage rate and increase the thermal conductivity of the solidified material. The filler commonly used in epoxy potting materials includes silica, alumina, silicon nitride, boron nitride and other materials. Table 1 shows the thermal conductivity of common inorganic fillers. Silica can be divided into crystalline, melting angle and spherical silica. In the filling material for electronic packaging, the melting spherical silica is preferred due to the product requirements.

3.7 defoamer

In order to solve the problem of bubble on the surface of liquid packaging material after assimilation, antifoaming agent can be added. Emulsifying silicone oil emulsifier is commonly used.

3.8 toughening agent

Toughening agent plays an important role in filling and sealing materials. The toughening modification of epoxy resin mainly improves its toughness by adding toughening agent and plasticizer. Toughening agent has two kinds of active and inert. Active toughener can participate in the reaction with epoxy resin, increase the chain link of reactant, and increase the toughness of the solidified material. Generally, the liquid NBR with carboxygenation agent is selected to form toughened "island structure" in the system, which increases the impact toughness and heat resistance impact performance of the material.

3.9 other components

In order to meet the specific technical and technological requirements of the potting parts, other components can be added to the formula. For example, flame retardant can improve the process of the material, and the colorant is used to meet the appearance requirements of the parts.

4. Filling and sealing process

There are two kinds of technology of epoxy resin filling and sealing, normal and vacuum.

5 common problems and Solutions

5.1 discharge, inter wire ignition or breakdown

Due to improper sealing process, the device will produce discharge, sparking or breakdown during operation, which is because the high-voltage coil wire diameter of this kind of product is very small (generally only 0.02mm-0.04mm), and the sealing material fails to fully soak the turn to turn, which causes the coil turn to remain a gap. Because the dielectric constant of the void is much smaller than that of the epoxy potting material, the uneven electric field will be produced under alternating high voltage, which will cause partial discharge, which will cause the aging and decomposition of the material and cause insulation damage. From the perspective of technology, there are two reasons for the gap between the lines: (1) the vacuum degree is not high enough during the filling and sealing, the air in the line cannot be completely eliminated, so that the material can not be fully soaked; (2) the preheating temperature of the sample before the filling is not enough, the viscosity of the injected sample can not be reduced rapidly, and the impact infiltration is affected. For manual filling or vacuum filling and sealing process after mixing and bubbling, the high temperature of material mixing and deboaming, long operation time or beyond the applicable period of materials and the product not entering the heating and curing procedure in time after filling and sealing will cause the material viscosity to increase and affect the infiltration of coil. The higher the initial temperature, the smaller the viscosity, the faster the viscosity increases with the time. Therefore, in order to make the material have good permeability to the coil, attention should be paid to keep the compound of the filling and sealing material in the proper temperature range and use it in the applicable period. Before filling, the test piece shall be heated to the specified temperature, and the heating and curing procedure shall be timely entered after the filling and sealing, and the vacuum degree of filling and sealing shall meet the requirements of technical specifications.

5.2 shrinkage, local depression and cracking of device surface

There are two kinds of shrinkage in the process of heat assimilation: chemical shrinkage during phase transition from liquid to solid state and physical shrinkage during cooling process. There are two processes in the process of chemical shrinkage in the curing process: the shrinkage from the heating of the chemical crosslinking reaction to the initial formation stage of the micro network structure after encapsulation, which is called gel precuring shrinkage, and the shrinkage from the gel to the complete curing stage is called post consolidation shrinkage. The shrinkage of the two processes is different. The former changes from liquid to mesh structure, and the consumption of reaction group is larger than that of the latter, and the volume shrinkage is higher than that of the latter. If the encapsulated specimen is cured at a high temperature, the two stages in the curing process are too close, and the gel pre assimilation and post curing will be completed at the same time. This will not only cause exorbitant exothermic peaks and damage elements, but also cause the internal stress of the potting parts to cause internal and external defects. In order to obtain good parts, we must pay attention to the matching of assimilation speed and curing conditions when the formulation design and curing process of the potting materials are made. The method is usually used to assimilate in different temperature zones according to the properties and uses of the potting materials. In the gel precuring zone, the assimilation reaction was slow, the reaction heat gradually released, the viscosity of the material increased and the volume contracted slowly. At this stage, when the material is in the flow state, the volume shrinkage shows that the liquid level drops until the gel, which can completely eliminate the internal shrinkage stress in this stage. The temperature rise from gel pre curing to post assimilation stage should be gentle. After curing, the filling parts should be slowly cooled with the heating equipment synchronously, and the internal stress distribution of the parts can be reduced and adjusted in many ways, so as to avoid shrinkage holes, sunken or even cracking on the surface of the parts. The setting of curing conditions of potting materials should also refer to the arrangement, fullness and size, shape and quantity of single potting in the potting device. It is necessary to reduce the gel precuring temperature and prolong the long time when the single encapsulating quantity is larger and the sealing elements are less.

5.3 the surface of the solidified object is poor or the part is not cured

The poor surface of the solidified material or the non curing of the part are also related to the curing process. Experts from China Epoxy Resin Industry Association said the main reasons are the failure of metering or mixing device and improper operation of production personnel; precipitation occurs in long-term storage of component A, and it is not stirred evenly before use, resulting in imbalance of actual proportion of resin and curing agent, long-term open storage of component B, and failure of moisture absorption; sealing parts of high tide and wet season do not enter curing procedure in time, and surface of object is not in time Moisture absorption. In short, to obtain a good filling and curing process is indeed a worthy of great attention.