Potting is the liquid compound with mechanical or manual way into the electronic components, lines of the device, under normal temperature or heating conditions to become excellent performance of the thermosetting polymer insulation material. Can improve the integrity of electronic devices, to improve the external shock, vibration resistance; improve the internal components, line insulation, is conducive to the device miniaturization, lightweight; to avoid components, direct line exposure to improve the device waterproof, moisture- And improve the use of performance and stability parameters.
Potting process
The quality of potting products is mainly related to product design, component selection, assembly and potting materials used, potting process is also a factor that can not be ignored.
Epoxy potting has both normal and vacuum potting process. Epoxy resin. Amine at room temperature curing potting material, generally used for low-voltage electrical appliances, the use of normal potting. Epoxy resin. Acid anhydride curing curing potting material, generally used for high-voltage electronic device potting, the use of vacuum potting process, is the focus of our research section. At present, there are two kinds of manual vacuum potting and mechanical vacuum potting, and mechanical vacuum potting can be divided into A, B components before the first combination of defoaming after potting and respectively after the first defoaming after mixing potting two cases The The process is as follows:
(1) manual vacuum potting process
(2) mechanical vacuum potting process
Mixing technology before mixing
A, B were first defoaming after mixing potting process
In contrast, mechanical vacuum potting, equipment investment, maintenance costs are high, but in product consistency, reliability, etc. is significantly better than manual vacuum potting process. Regardless of what kind of potting methods, should strictly abide by the given process conditions, otherwise it is difficult to get a satisfactory product.
Analysis of the problems and causes of potting products
(1) partial discharge starting voltage is low, the line between the ignition or breakdown of the TV, display line output transformers, cars, motorcycle ignition and other high-voltage electronic products, often due to improper cooking process, work will be partial discharge ( Corona), the line between the ignition or breakdown phenomenon, because these products high-voltage coil diameter is very small, generally only 0.02 ~ 0.04mm, potting material can not completely penetrate the turn, so that the coil turns between the gap. As the void dielectric constant is much smaller than the epoxy potting material, in the alternating high pressure conditions, will produce uneven electric field, causing the interface part of the discharge, the aging of the material decomposition, causing insulation damage.
From the process point of view, resulting in inter-line gap in the following two reasons:
1) potting when the vacuum is not high enough, the line between the air can not be completely ruled out, so that the material can not be completely impregnated.
2) Before the potting of the preheating temperature is not enough, poured into the specimen material viscosity can not be quickly reduced, affecting the infiltration.
For the manual potting or the first mixing after the vacuum potting process, the material mixing defoaming temperature is high, the operation time is longer or more than the material application period, and after the potting product is not timely into the heating curing process, will cause the material viscosity increases, Affect the infiltration of the coil. According to the experts of Shanghai Changxiang Industrial Co., Ltd., heat curing epoxy potting compound, the higher the starting temperature, the smaller the viscosity, with the extension of time, the more rapid growth of viscosity. So in order to make the material on the coil has a good infiltration, the operation should pay attention to the following points:
1) The potting compound should be maintained within a given temperature range and used within the applicable period.
2) before the potting, the specimen should be heated to the specified temperature, potting should be completed into the heating curing process.
3) potting vacuum to meet the technical requirements.
(2) The surface shrinkage, local depression and cracking of the potting body will produce two kinds of shrinkage during the heating and curing process, that is, the chemical shrinkage during the process of liquid to solid phase transformation and the physical shrinkage during the cooling process. Further analysis, the chemical process of curing shrinkage there are two processes, from the potting after heating chemical cross-linking reaction to the initial formation of micro-network structure of the contraction, we call the gel pre-curing shrinkage. From the gel to the complete curing phase of the contraction we call the post cure shrinkage. The shrinkage of these two processes is not the same. The physical state of the former changes from the liquid to the network structure, the reaction group consumption is greater than the latter, the volume shrinkage is also higher than the latter. (75 ℃ / 3h) was more than that in the post-curing stage (110 ℃ / 3h). The results of differential thermal analysis showed that the cure rate was 53% after 750 ℃ / 3h treatment, The
If we take a high-temperature curing of the potting test specimen, the two stages of the curing process are too close, and the gel pre-curing and post-curing are almost simultaneously completed, which not only causes excessive exothermic peaks, So that potting a huge internal stress, resulting in product internal and appearance defects. In order to obtain good parts, we must focus on the curing speed of the potting material (ie, the gel time of A and B complex) and the matching of curing conditions when the design and curing process of the potting formula are formulated. The usual method is: according to the nature of the potting material, the use of different temperature zone curing process. According to experts, color TV line output transformer potting according to different temperature section curing procedures and parts of the internal heat curve. In the gel pre-curing temperature zone, the curing reaction of the potting material is slow, the reaction heat is gradually released, the viscosity of the material increases and the volume shrinkage is carried out smoothly. At this stage of the material in the flow, the volume of contraction showed a liquid level down, until the gel, can completely eliminate the volume of the phase shrinkage stress. From the gel pre-curing to post-curing stage, the temperature should be gentle, curing is completed, potting should be synchronized with the heating equipment, slow down, reduce the number of parts to adjust the stress distribution, to avoid the surface of the workpiece shrinkage, Depression or even cracking phenomenon.
The preparation of curing conditions of the potting material, but also refer to the encapsulation of the sealing parts within the layout, fullness and the size of the pieces, shape, a single potting and so on. It is absolutely necessary to reduce the gel pre-curing temperature and prolong the time when the amount of the encapsulation element is large and the embedding element is small.
(3) cured surface or local non-curing these phenomena are also related to the curing process. The main reason is:
1) metering or mixing device failure, production personnel operation errors.
2) A group of long-term storage of precipitation, before the use of sufficient mixing evenly, resulting in the actual proportion of resin and curing agent imbalance.
3) B group of long-term exposure to storage, moisture failure.
4) high humidity season potting is not timely into the curing process, the surface moisture absorption.
In short, to get a good potting products, potting and curing process is indeed a matter of great importance.
Epoxy resin potting and its process and common problems
1, the history of packaging technology change
There have been two major changes in the field of electronic packaging technology. The first change occurred in the first half of the 1970s, characterized by the transition from pin-insertable installation techniques such as DIP to surface-mount technology (such as QFP) in quad flat-flat packaging; the second transition occurred in the 20th century Mid-term, its logo is the ball array. BGA-type package appears, with the corresponding surface mount technology and semiconductor integrated circuit technology together across the 21st century. With the development of technology, there have been many new packaging technologies and packages, such as chip direct bonding, potted plastic ball array (CD-PBGA), flip chip plastic ball array (Fc-PBGA), chip Size packaging (CSP) and multi-chip components (MCM), etc., in these packages, a considerable part of the use of liquid epoxy packaging technology. Potting, is the liquid epoxy resin composite mechanical or manual way into the electronic components, lines of the device, under normal temperature or heating conditions to assimilate the performance of hot homogeneous polymer insulation materials.
2, product performance requirements
Potting material should meet the following basic requirements: good performance, long application period, suitable for high-volume automatic production line operations; small viscosity, infiltration strong, can be filled with components and lines; in the potting and curing process, Body composition is small, no stratification; curing exothermic peak low, shrinkage shrinkage small; assimilates excellent electrical and mechanical properties, good heat resistance, a variety of materials have good adhesion, water absorption and linear expansion Coefficient is small; in some occasions also requires potting material with flame retardant, weather resistance, thermal conductivity, resistance to high and low temperature alternating performance.
In the specific semiconductor package, because the material to the chip, the substrate in direct contact, in addition to meet the above requirements, but also requires the product must have the same chip and chip material purity. In the flip chip potting, due to the gap between the chip and the substrate is very small, requiring a very low viscosity of the potting material. In order to reduce the stress generated between the chip and the packaging material, the modulus of the package material can not be too high. And in order to prevent the interface at the water penetration, packaging materials and chips, between the substrate should have a good bonding performance.
3, the main components and the role of potting material
The role of potting is to strengthen the integrity of electronic devices to improve the external impact, vibration resistance; improve the internal components, line insulation, is conducive to the device miniaturization, lightweight; to avoid components, the line directly exposed to improve the device Waterproof, moisture-proof performance.
Epoxy resin potting material is a multi-component composite system, its south resin, curing agent, toughener, filler and other components, for the system viscosity, reactivity, use, heat and so on in the formula , Process, casting size structure and other aspects of a comprehensive design, so that a comprehensive balance.
3.1 Epoxy resin
Epoxy resin potting material generally use low molecular weight liquid bisphenol A type epoxy resin, this resin viscosity is small, high epoxy value. Commonly used. 54, E-51, E-44, E-42. In the flip-chip filled potting, the gap between the chip and the substrate is very small, so the viscosity of the liquid package is very low. So the use of bisphenol A type epoxy resin alone can not meet the product requirements. In order to reduce the product viscosity, to achieve product performance requirements, we can use composite resin: such as adding low viscosity bisphenol F epoxy resin, glycidyl ester resin and has high heat resistance, electrical insulation and weather resistance of the alicyclic Family epoxide. Among them, the alicyclic epoxide itself also has the role of reactive diluent.
3.2 curing agent
The assimilator is an important component of the epoxy potting formulation, and the properties of the cured product are highly dependent on the structure of the curing agent.
(1) room temperature assimilation generally use aliphatic polyamine as curing agent, but this type of curing agent toxicity, irritation, exothermic, assimilation and the use of the process of easy oxidation. Therefore, the need for modified polyamines, such as the use of polyamine amine on the active hydrogen, part of the epoxy synthesis of hydroxyalkylation and partial synthesis of propylene with cyanide synthesis of comprehensive modification of So that curing agent to achieve low viscosity, low toxicity, low melting point, room temperature curing and a certain toughness of the comprehensive modification effect.
(2) acid anhydride compatibilizer is a two-component heat curing epoxy potting material the most important assimilation agent. Commonly used assimilants are liquid methyl tetrahydrophthalic anhydride, liquid methyl hexahydrophthalic anhydride, hexahydrophthalic anhydride, methylnadic anhydride and so on. This kind of curing agent viscosity is small, with the amount of large, can be in the potting formula to play assimilation, dilution dual role, curing heat relaxation, assimilate comprehensive performance.
3.3 curing accelerator
Two-component epoxy acid anhydride potting material, generally about 140 ℃ long time to heat to cure. Such curing conditions, not only cause energy waste, and most of the components in the electronic device, the skeleton shell is unbearable. The addition of the accelerator component in the formulation can effectively reduce the curing temperature and shorten the curing time. Commonly used accelerators are: Bian diamine, DMP-30 and other tertiary amines. Also possible are imidazoles and metal salts of carboxylic acids such as 2-ethyl-4-methylimidazole, 2-methylimidazole and the like.
3.4 Coupling agent
In order to increase the adhesion between the silica and the epoxy resin, a silane coupling agent is added. Coupling agent can improve the adhesion and moisture resistance of the material. Typical silane coupling agents suitable for use in epoxy resins are glycidoxypropyltrioxysilane (KH-560), anilidine methyltriethoxysilane, α-chloropropyltrimethoxysilane, α-mercapto Propyltrimethoxysilane, aniline methyltrimethoxysilane, diethylene diaminopropyltrimethoxysilane, and the like.
