Epoxy resins are used for product encapsulation. Starting from their engineering application cases, the effects of adding reactive diluents, thickeners, thixotropic agents, defoamers, coupling agents and other additives on the actual application of epoxy resins are discussed; Study the thermodynamic properties of epoxy resin (Tg, TG), and apply to the improvement of product packaging technology and process problems.
1 Overview of epoxy resin
Epoxy resin is a general term for low molecular weight substances with two or more epoxy groups in their molecules and their cross-linked cured products. An important category is bisphenol A epoxy resin. Because of its easy availability of raw materials, low cost, and the proportion of output accounting for more than 80% of the world's epoxy resin usage, it is also called a general-purpose epoxy resin.
Epoxy resins, together with phenolic resins and unsaturated polyester resins, are called the three general-purpose thermosetting resins. Epoxy resin has good process operation and bonding characteristics, excellent mechanical and electrical properties, good dimensional stability, and weather resistance and temperature resistance. Epoxy resins are widely used and can be used in the fields of coatings, adhesives, encapsulants, engineering plastics, composite materials and construction materials. In particular, it is widely used in sealing processes such as potting and pouring of products in the electronic and electrical industry.
2 Engineering application of epoxy resin and its additives
Bisphenol A epoxy resin encapsulant is mainly composed of high epoxy value and low viscosity epoxy resin, toughening modifier, filler and other components.
Bisphenol A type epoxy resin encapsulation materials are mainly divided into two categories: Types are amine curing epoxy resins, which are called normal temperature quick-drying type. The normal-temperature surface drying time is 2h to 6h, and the complete curing time is 24h to 48h; the other is the anhydride curing epoxy resin, which must be heated and cured. The required temperature is often as high as 120 ℃ or more. Therefore, compound amine or imidazole accelerators are often added to acid anhydride curing agents to lower the curing temperature.
Now the research and discussion on the application and improvement of bisphenol A epoxy resin in the packaging of electronic components.
Bisphenol A epoxy resin is used for product packaging. Due to product characteristics and specific construction process requirements, many problems will be encountered. Therefore, the additives used to adapt to product characteristics and curing process are particularly important.
2.1 Application of thinner and thickener
Due to the addition of fillers and modifiers, epoxy resins have a higher viscosity, especially amine-cured epoxy resins, whose viscosity changes significantly with ambient temperature. Therefore, the use of thinners and thickeners becomes inevitable.
When the temperature is high, the viscosity of the resin decreases, and the package is prone to sagging. We need to use a certain proportion of thickener; and when the temperature is low, the resin viscosity increases and the fluidity is poor, which affects the construction efficiency. Agent. This is to adjust the viscosity of the epoxy resin and control it within a certain range, so that it can obtain good operability.
The thickener generally uses inorganic fillers such as Al(OH)3 and SiO2, and generally uses powder with a small particle size of 300 mesh or more. The diluent is divided into active diluent and non-reactive diluent. The non-reactive diluent is mainly ketones, esters and other solvents. It is a low boiling point volatile solvent, and it does not participate in the curing reaction itself. Cause the resin performance to decline. Therefore, the use of epoxy resin reactive diluent involved in cross-linking, bifunctional diglycidyl ether has become the mainstream of the dilution system.
Taking a certain electronic product packaging as an example, based on actual experience, combined with the measurement of the rotor viscometer, we found that the viscosity range of room temperature curing epoxy resin HA suitable for operation is 32000mPa·s～36000mPa·s.
According to repeated tests and test demonstrations: at a temperature of 22-25°C, the sealing epoxy resin system can meet this requirement; when ≥26°C, the resin must be added with 0.5-1.5% thickener D16; while ≤22°C, 0.3 ~0.5% active diluent D18 can meet the above requirements. The results prove that the proper ratio of thickener and thinner can control the viscosity of this epoxy resin in the viscosity range suitable for operation.
2.2 Application of thixotropic agents
Thixotropic agents are additives that increase the viscosity of the liquid and make the liquid thixotropic. It shows that when sheared, the consistency becomes smaller; when the shearing stops, the consistency increases again and the property is thixotropy. The thixotropic agent has excellent anti-sagging properties, which can prevent splashing and sagging of fluids such as epoxy resin in the process of packaging and painting, and improve the leveling performance.
Commonly used thixotropic agents are fumed silica, organic bentonite, hydrogenated castor oil, etc. In practical applications, fumed silica is commonly used to improve the rheological properties of epoxy resins.
Fumed silica is a solid aerogel. When the system is subjected to shear force, the hydrogen bond is weak, the network structure is destroyed, the gel function disappears, and the viscosity decreases; when the shear force is removed, the hydrogen bond continues to act. It can be restored to its original shape.
In the production process, we encountered the problem that the operation of the resin was affected by the poor thixotropy. Adding 0.3% to 0.5% of fumed silica can achieve good results. The thixotropy of the resin is greatly improved. It is used in conjunction with the diluent. The resin is smooth when dispensed, and the resin shrinks well at the mouth of the product without flowing. It meets the needs of the production process.
2.3 Application of coupling agent
In epoxy resin materials, coupling agents are particularly important due to the use of inorganic materials such as fillers and the diversity of bonding interfaces.
Coupling agent refers to a class of substances with amphoteric structure, some of which have affinity with organic substances, and the other groups have affinity with inorganic fillers, so that incompatible organic and inorganic substances are tightly bound by this bridging form .
When there are inorganic fillers such as reinforcing agents and thickeners in the epoxy resin, silane coupling agents are commonly used in the system. The silane coupling agent can effectively increase the fusion ability of the filler and the resin, making the "filler (inorganic)-coupling agent-epoxy resin (organic)" tightly connected; at the same time, the silane coupling agent can make the epoxy resin tight The performance is greatly enhanced. In practical applications, in inorganic shell products, the coupling agent can connect the "ceramic shell (inorganic substance)-coupling agent-epoxy resin (organic substance)" and increase its bonding strength.
In the encapsulated epoxy resin, 0.5% silane coupling agent is added. Practice has proved that the coupling agent can effectively increase the bonding strength of the epoxy resin and ceramic inorganic interface.
2.4 Application of defoamer
In the production and formulation of epoxy resin, air is often mixed due to mixing, pouring, flowing, stirring and other processes, which generates a large number of bubbles. These bubbles will form voids or even voids after the epoxy resin is cured. The sealing performance is very harmful.
A class of additives that can effectively prevent the generation of air bubbles or the rapid escape of air bubbles is called defoamer. The low surface tension of the defoamer can make the bubbles burst quickly, thus effectively preventing the generation and continuous existence of bubbles.
Low-molecular-weight alcohols, greases, silicones, etc. are commonly used as defoamers. In the epoxy resin system, one of the most commonly used defoamers is polysiloxane-based defoamers.
A certain product uses 0.3%～0.5% silane defoamer, and the vacuum is used to improve the bad bubble. The defect rate is reduced from 7% to 8% to 0.5%.
3 Tg measurement and improvement of epoxy resin
Epoxy resin Tg, the glass transition temperature. The glass transition temperature refers to the temperature corresponding to the glass transition of an amorphous or semi-crystalline polymer from a rubber state to a glass body during the cooling process. The Tg value of epoxy resin is an important thermodynamic characteristic temperature of epoxy resin. During the production process, we are used to evaluate the baking process, the effect of additives, and the characteristics of cured products under abnormal conditions; sometimes used in combination with thermogravimetric TG, It has an important guiding role for the application of epoxy resin.
Thermal analysis has a significant effect on the improvement of epoxy resin applications. Thermal analysis can analyze information such as Tg and TG of epoxy resin, which has an important role in the practical application of epoxy resin and the improvement of baking process. Based on this test data, the above related items can be summarized and compared as selected for epoxy resin research Standard basis.
In addition, we must consider the use conditions and needs of epoxy resins when designing new products or improving existing product designs and curing processes.
Epoxy resin, as a widely used engineering resin, has unparalleled advantages when used for packaging electronic components. We can select epoxy resins based on comprehensive evaluation of material operability, mechanical properties, electrical properties, flame retardant properties, environmental protection characteristics, and prices in accordance with product design requirements. However, the use of epoxy resin is affected by the environmental temperature and humidity, the product's own characteristics and process requirements. It is often necessary to add some viscosity modifiers (thickeners and thinners), thixotropic agents, coupling agents, defoamers and other additives to Improve operability of epoxy resin, reduce defects and improve curing reliability. In addition, the packaging and baking process of epoxy resin is closely related to its product characteristics and manufacturing process. Through thermal analysis to study the Tg and TG information of epoxy resin and its different additives and different curing processes, an epoxy suitable for the product can be obtained. Resin system and baking process, and improved. On this basis, the comparative study of epoxy resins is of great significance for the engineering application and improvement of epoxy resins.