Piezoresistor with cured silicone oil inner encapsulation layer

Abstract

The invention relates to the technical field of electronic components, in particular to a piezoresistor with a cured silicone oil inner encapsulation layer, and solves the problem that an existing piezoresistor with an inner moisture-proof layer is not suitable for assembly line production or enables the assembly line production efficiency to be extremely low. Electrodes are arranged on the two sides of the chip, pins of the piezoresistor are electrically connected to the electrodes and extend out of the outer encapsulation layer, a cured silicone oil inner encapsulation layer is arranged in the outer encapsulation layer, the chip, the electrodes and part of the pins are wrapped by the cured silicone oil inner encapsulation layer, and the cured silicone oil inner encapsulation layer is formed by heating and rapidly curing a silicone oil mixture. The silicone oil mixture comprises the following components in percentage by mass: 80-95% of vinyl silicone oil, 3-15% of fumed silica and 1-5% of a curing agent.

Description

Technical Field

[0001] This invention relates to the field of electronic components technology, and in particular to a varistor with a cured silicone oil encapsulation layer. Background Technology

[0002] During multiple combined wave impacts (1.2 / 50μs + 8 / 20us), the thermal expansion and contraction of the outer encapsulation layer can easily create gaps between the encapsulation layer and the chip, leading to discharge at the electrode edges and ultimately causing chip breakdown and burnout. Moreover, due to the material properties of the epoxy resin encapsulation material, pores remain after the encapsulation layer cures. When used in high temperature and high humidity environments, moisture can enter the chip side through these pores, causing ion migration and forming a short circuit effect, resulting in premature failure of the varistor.

[0003] To address the aforementioned issues, Chinese Patent Publication No. CN102664082A discloses a varistor and its manufacturing method, which adds a silicone rubber layer between the chip and the outer encapsulation layer. This silicone rubber layer mainly serves a moisture-proof function. However, the single-component silicone rubber used for room temperature curing requires more than 4 hours of natural curing at room temperature. The final curing effect of the silicone rubber is affected by the environment, and the entire process of coating silicone rubber → curing silicone rubber → coating epoxy resin → curing epoxy resin takes more than 5 hours. This excessively long time for any one step limits the production of varistor products on automated production lines or seriously reduces the efficiency of the production line. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a varistor with a cured silicone oil inner encapsulation layer, which overcomes the problem that existing varistors with an inner moisture-proof layer are not suitable for assembly line production or cause extremely low assembly line production efficiency.

[0005] The technical solution adopted by the present invention to solve its technical problem is: a varistor with a cured silicone oil inner encapsulation layer, having an internal chip and an external encapsulation layer, electrodes on both sides of the chip, the pins of the varistor being electrically connected to the electrodes and extending out of the outer encapsulation layer, a cured silicone oil inner encapsulation layer being provided inside the outer encapsulation layer, the cured silicone oil inner encapsulation layer encapsulating the chip, electrodes and part of the pins, the cured silicone oil inner encapsulation layer being formed by heating and curing a silicone oil mixture, the silicone oil mixture comprising 80%-95% by mass of vinyl silicone oil, 3%-15% by mass of fumed silica and 1%-5% by mass of curing agent.

[0006] The principle behind the construction of this invention is as follows: the inner encapsulation layer of the cured silicone oil uses vinyl silicone oil as the base material. Silicone oil consists of linear molecular chains, and there is no chemical connection (crosslinking) between the chains. Since the molecular chains can slide freely, it exhibits a liquid state. The longer the chain, the greater the viscosity. After mixing with fumed silica, it still maintains linear molecular chains, but the chains are connected together through chemical crosslinking points. Heating and curing form a three-dimensional network structure. The crosslinking network restricts the free flow of molecular chains, but the silicon-oxygen bonds themselves remain flexible. Therefore, the material exhibits the characteristics of an elastic solid.

[0007] In terms of moisture protection, the cured silicone oil encapsulation layer, which has a three-dimensional network structure after curing, has a dense texture, few pore defects, good adhesion to chips, electrodes and pins, and excellent toughness and plasticity. Therefore, there are no micro-cracks on the contact surface with chips, electrodes and pins, which prevents moisture penetration and enhances the product's moisture protection capability.

[0008] In terms of lightning protection, the varistor chip will heat up during a lightning strike, but the curing silicone oil encapsulation layer still maintains good insulation, which reduces the electrode discharge effect of subsequent lightning strikes, reduces the increase in impedance of the varistor, and delays the continuous heating and breakdown failure of the varistor.

[0009] The fumed silica in the components controls rheology, reduces the overflow effect of the adhesive, and enhances adhesion and mechanical strength; the curing agent in the components crosslinks with the vinyl silicone oil to form a cured silicone oil encapsulation layer.

[0010] Furthermore, the silicone oil mixture comprises 88%-92% by weight of vinyl silicone oil, 5%-10% of fumed silica, and 2%-3% of curing agent.

[0011] Adding less than 5% fumed silica will result in excessive fluidity, insufficient thixotropy, decreased mechanical properties, increased filler sedimentation and shrinkage; adding more than 10% fumed silica will result in excessive viscosity, poor fluidity, excessive thixotropy, increased brittleness and difficulty in dispersion.

[0012] Therefore, in order to optimize the thixotropic index of fumed silica, the mass percentage of fumed silica in the silicone oil mixture is 5.5%-7.5%.

[0013] Specifically, the curing agent is a hydrogen-containing silicone oil or polymethylhydrosiloxane.

[0014] Specifically, to improve product manufacturing efficiency, the silicone oil mixture heat curing process includes a first curing after the silicone oil mixture coats the chip, electrodes, and some pins, and a second curing process that is carried out together with the outer encapsulation layer.

[0015] Specifically, the heating temperature for the first curing is 140~180℃ and the heating time is 20~40 minutes.

[0016] Specifically, in order to improve the density of the encapsulation layer of the cured silicone oil, the fumed silica is silane-modified hydrophobic fumed silica.

[0017] Specifically, the thickness of the cured silicone oil encapsulation layer is 0.01~0.5mm.

[0018] Furthermore, the thickness of the cured silicone oil encapsulation layer is 0.05~0.3mm.

[0019] Specifically, the thickness of the outer encapsulation layer is 0.1~2mm.

[0020] Furthermore, the thickness of the outer encapsulation layer is 0.3~1.2mm.

[0021] The beneficial effects of this invention are:

[0022] 1. The curable silicone oil encapsulation layer of the varistor of the present invention is a two-component inner layer, which is a rapidly curable inner layer, facilitating the production of the product on an assembly line.

[0023] 2. In the curing silicone oil encapsulation layer of the varistor of the present invention, an appropriate amount of fumed silica is used to control rheology, reduce the overflow effect of the adhesive, and enhance adhesion and mechanical strength. The thickness of the encapsulation layer is uniform, there is no dripping after the adhesive is applied, and the adhesive material adhering to the side of the chip will not be thinned, which reduces the discharge between the electrodes on both sides of the chip and enhances the product's resistance to lightning strikes. Detailed Implementation

[0024] The present invention will now be described in further detail with reference to preferred embodiments.

[0025] A varistor with a cured silicone oil inner encapsulation layer has an internal chip and an external encapsulation layer. Electrodes are provided on both sides of the chip. The leads of the varistor are electrically connected to the electrodes and extend out of the outer encapsulation layer. The cured silicone oil inner encapsulation layer is provided inside the outer encapsulation layer. The cured silicone oil inner encapsulation layer wraps the chip, electrodes and part of the leads. The cured silicone oil inner encapsulation layer is formed by heating and curing a silicone oil mixture. The silicone oil mixture includes 90% vinyl silicone oil, 7% fumed silica and 3% curing agent by mass percentage. The curing agent is polymethylhydrosiloxane. The heating and curing of the silicone oil mixture includes a first curing after the silicone oil mixture wraps the chip, electrodes and part of the leads and a second curing that is carried out together with the outer encapsulation layer. The heating temperature for the first curing is 160°C and the heating time is 30 minutes.

[0026] The fumed silica is hydrophobic fumed silica modified with silane.

[0027] The primary particle size of fumed silica is 7~40nm, the aggregate particle size is 200~500nm, the specific surface area is 100~400m² / g, and the purity is greater than 99.8%.

[0028] The thickness of the inner encapsulation layer of the cured silicone oil is 0.2 mm, and the thickness of the outer encapsulation layer is 0.8 mm.

[0029] In order to verify the performance of the product, this invention tests three aspects: combined wave limit capability, moisture resistance, and encapsulation tensile strength.

[0030] I. Combined Wave Limit Test: Six pieces were taken from each of the same batch of pre-soldered semi-finished products. Six pieces were coated with an inner layer of a mixture of 95% vinyl silicone oil and 5% polymethylhydrosiloxane by mass percentage and cured by heating. An outer layer of epoxy resin was then coated and cured again by heating. These were the Group A products, which did not contain fumed silica in the inner layer and were labeled A1 to A6. Another six pieces were coated with an inner layer of a mixture of 90% vinyl silicone oil, 7% fumed silica, and 3% polymethylhydrosiloxane by mass percentage and cured by heating. An outer layer of epoxy resin was then coated and cured again by heating. These were the Group B products, which contained fumed silica in the inner layer and were labeled B1 to B6. One piece from each group was then taken for a 6KV / 3KA combined wave impact test. The number of times the product could withstand a 6KV / 3KA combined wave impact was recorded, as shown in Table 1.

[0031] Table 1: Test Table for Combined Wave Limiting Capability

[0032]

[0033] Table 1 shows that after comparing the average capabilities of products in Group A and Group B, the varistors with added fumed silica in the inner sealing layer have a 22% higher limit for resisting combined wave impacts than those without added fumed silica [(70.7-57.8) / 57.8=22%], indicating a significant improvement in combined wave capability.

[0034] II. Moisture Resistance Test: Take 13 pieces from each of the other batches of pre-soldered semi-finished products. Coat the inner layer with a mixture of 95% vinyl silicone oil and 5% polymethylhydrosiloxane by mass percentage and heat-cur. Coat the outer layer with epoxy resin and heat-cur again to make Group C products without added fumed silica, and mark them as C1 to C13. Coat the inner layer with a mixture of 90% vinyl silicone oil, 7% fumed silica and 3% polymethylhydrosiloxane by mass percentage and heat-cur. Coat the outer layer with epoxy resin and heat-cur again to make Group D products with added fumed silica, and mark them as D1 to D13. Take one piece from each group and conduct a leakage current test (i.e., double 85 test) after being powered on at 85℃ / 85%RH for 1000 hours, as shown in Table 2.

[0035] Table 2: Leakage Current Test Table for Double 85

[0036]

[0037] Table 2 shows that after the double 85 test of products in groups C and D, the leakage current of the varistor with added fumed silica in the inner sealing layer decreased by 48% compared with that without added fumed silica [(11.45-5.90) / 11.45=48%], indicating that the addition of fumed silica significantly improved the moisture-proof capability of the inner sealing layer.

[0038] 3. Encapsulation layer tensile strength test: Take 5 pieces from each of the other batches of pre-soldered pins. Coat the inner layer with a mixture of 95% vinyl silicone oil and 5% polymethylhydrosiloxane by mass percentage and heat-cur. Coat the outer layer with epoxy resin and heat-cur again to make Group E products without added fumed silica, and mark them as E1 to E5. Coat the inner layer with a mixture of 90% vinyl silicone oil, 7% fumed silica and 3% polymethylhydrosiloxane by mass percentage and heat-cur. Coat the outer layer with epoxy resin and heat-cur again to make Group F products with added fumed silica, and mark them as F1 to F5. Take 1 piece from each group for tensile strength test, as shown in Table 3.

[0039] Table 3: Tensile Strength Test Table

[0040]

[0041] Table 3 shows that the varistor with added fumed silica in the inner encapsulation layer has a 30% greater bonding force between the inner encapsulation layer and the chip, electrode, and pins than the one without added fumed silica [(1.25-0.96) / 0.96=30%], indicating that fumed silica improves the tightness of the inner encapsulation layer.

[0042] The above description is only a specific embodiment of the present invention. Various examples and illustrations do not constitute a limitation on the substantive content of the present invention. Those skilled in the art can make modifications or variations to the above-described specific embodiments after reading the specification without departing from the substance and scope of the invention.

Claims

1. A varistor with a cured silicone oil inner encapsulation layer, comprising an internal chip and an external encapsulation layer, wherein electrodes are provided on both sides of the chip, and the pins of the varistor are electrically connected to the electrodes and extend out of the external encapsulation layer, characterized in that: The outer encapsulation layer contains a cured silicone oil inner encapsulation layer, which encapsulates the chip, electrodes, and some of the pins. The cured silicone oil inner encapsulation layer is formed by heating and curing a silicone oil mixture, which includes 80%-95% vinyl silicone oil, 3%-15% fumed silica, and 1%-5% curing agent by mass percentage.

2. The varistor with a cured silicone oil encapsulation layer according to claim 1, characterized in that: The silicone oil mixture comprises 88%-92% by weight of vinyl silicone oil, 5%-10% of fumed silica, and 2%-3% of curing agent.

3. The varistor with a cured silicone oil encapsulation layer according to claim 1 or 2, characterized in that: The mass percentage of fumed silica in the silicone oil mixture is 5.5%-7.5%.

4. The varistor with a cured silicone oil encapsulation layer according to claim 1, characterized in that: The curing agent is a hydrogen-containing silicone oil or polymethylhydrosiloxane.

5. The varistor with a cured silicone oil encapsulation layer according to claim 1, characterized in that: The heat curing of the silicone oil mixture includes a first curing after the silicone oil mixture coats the chip, electrodes, and part of the pins, and a second curing along with the outer encapsulation layer.

6. The varistor with a cured silicone oil encapsulation layer according to claim 1, characterized in that: The fumed silica is silane-modified hydrophobic fumed silica.

7. The varistor with a cured silicone oil encapsulation layer according to claim 1, characterized in that: The thickness of the cured silicone oil encapsulation layer is 0.01~0.5mm.

8. The varistor with a cured silicone oil encapsulation layer according to claim 7, characterized in that: The thickness of the cured silicone oil encapsulation layer is 0.05~0.3mm.

9. The varistor with a cured silicone oil encapsulation layer according to claim 1, characterized in that: The thickness of the outer encapsulation layer is 0.1~2mm.

10. The varistor with a cured silicone oil encapsulation layer according to claim 9, characterized in that: The thickness of the outer encapsulation layer is 0.3~1.2mm.

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