Potting is the mechanical or manual method of pouring the liquid compound into the device equipped with electronic components and circuits, and curing 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, 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 performance and stability parameters.
The quality of potting products is closely related to product design, component selection, assembly, and potting materials used. The 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 and then potting process
A and B are respectively degassed and then mixed potting process
In contrast, mechanical vacuum potting requires a large investment in equipment 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 observed, 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 the wires are due to the small diameter of the 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. 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, 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 over 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 potting vacuum degree must meet the 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 shows that there are two more processes for chemical 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 the 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 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. After the sample was treated at 750℃/3h, its curing degree 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, resulting in defects in the interior and appearance of the product. 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 usual method is: according to the nature and purpose of the potting material, it is a process of solidifying in different temperature zones. According to experts, the potting of color TV line output transformers is based on different temperature zones and segmented 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 heat of reaction 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-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 phenomenon.
For the formulation of the curing conditions of the potting material, it is also necessary to refer to the arrangement, fullness, size, shape, and single potting volume of the embedded components in the potting product. For a single potting volume 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, causing 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.