WO2002019423A1

WO2002019423A1 - Composite

Info

Publication number: WO2002019423A1
Application number: PCT/SE2001/001841
Authority: WO
Inventors: Anna-Liisa LÖNN; Peter Isberg
Original assignee: Abb Ab
Priority date: 2000-08-30
Filing date: 2001-08-30
Publication date: 2002-03-07
Also published as: SE0003050D0

Abstract

An electronic device comprising at least one semiconductor device (11) encapsulated in a capsule comprising a composite that contains gel and a more rigid elastomer, where said gel (14) is situated nearest to said at least one semiconductor component. The composite has good temperature resistance, is chemically stable, has high dielectric and mechanical strength.

Description

Composite

TECHNICAL FIELD

The present invention relates to a capsuled electronic device comprising at least one capsuled semiconductor component. The term electronic device is intended to include all types of semiconductor devices such as diodes, thyristors and IGBTs. More particularly the present invention is particularly applicable to capsuled high-voltage devices, as the problems the invention aims to solve are more pronounced in such devices. High-voltage means devices that are used at voltages of 1 kV or higher. However the invention is not restricted to high-voltage devices.

BACKGROUND OF THE INVENTION

Electronic devices containing at least one semiconductor component often need to be protected from various stresses and strains from their surroundings. Factors affecting the electronic components can be mechanical stresses, air, water and other corrosive and electrically conducting materials for example.

Non-hermetically sealed capsules enclose semiconductor components in a polymer-based material instead of a gas. The polymer-based material is in direct contact with the surrounding atmosphere. A non-cured sealing compound is applied over the semiconductor components. On curing, the sealing compound becomes an elastomer, providing vibration damping around the delicate connecting wires that connect the electronic components to the surroundings.

An elastomer is a polymer material that can regain its shape after deformation. Nearly all elastomers are hydrocarbons i.e. they are organic. Silicone elastomers differ from organic elastomers in that the main chain of their structure does not contain carbon. A silicone elastomer's structure is made up of a polysiloxane chain comprising alternate silicon and oxygen atoms. Elastomers can be hardened by crosslinking their polymer chains.

Silicone elastomer have certain advantages over organic elastomers such as higher heat and chemical stability, high moisture resistance, low toxicity, high flame-resistance, low surface tension and high dielectric strength. A disadvantage with silicone elastomers is their relatively high thermal expansion coefficient combined with high rigidity.

The power handled by semiconductor components varies and this leads to temperature changes in the components. Variations in the temperature give rise to expansion or contraction of the components as well as the material surrounding the components. A difference in the thermal expansion of adjacent materials is problematic as this gives rise to mechanical stresses in the materials. During use, significant mechanical stresses can arise in the material around the electronic components. These mechanical stresses can damage the semiconductor devices, can break the connecting wires or can create microscopic cracks in the insulating elastomer capsule.

Experiments have shown that silicone elastomers do not attain good adhesion with electronic components and with the substrate on which they are mounted. However silicone elastomers provide the required electrical resistance and are much cheaper than silicone gel, which is also used for capsuling electronic devices. Delamination can occur between the elastomer and the electronics due to excessive shear stress. Contamination of the surfaces to which the elastomer is applied, which can occur while assembling the electric device, further worsens the adhesion. Delamination occurs when local shear stresses exceed the adhesion strength. As high stresses can arise at geometrical discontinuities, such as the corners of components, delamination is often initiated in these regions. Delamination can also occur if the elastomer absorbs too much water. During increases in temperature, caused by heat being generated in semiconductor components, any water contained in the elastomer is converted to a saturated steam. The pressure of the steam, coupled with a decrease in the elasticity of the elastomer at high temperatures, can lead to the formation of cracks in the elastomer through which entrapped steam can then escape. The cracks accelerate the transport of water and other undesired substances towards the semiconductor components. Delamination can be minimised by choosing a suitable elastomer. For example an elastomer's wettability can be improved or the elastomer's hygroscopicity can be decreased and adhesion-promors added to improve the elastomer's adhesion to the electronic components (see US 6034174 and US 5977216 for example).

Silicone gel is a cured silicon product whose polymer chain is less crosslinked than that of a silicone elastomer. Silicone gel has therefore a lower viscosity than a silicone elastomer and is not as hard. Encapsulating electronic components in silicone gel prevents water from reaching said components, thus avoiding corrosion. As silicone gel has a low viscosity this means that it is easier to remove air bubbles from the gel. Silicone gel is thermally and chemically stable, it provides good adhesion and excellent electric resistance. It is able to surround geometrically complex shapes and it wets surfaces well. Silicone gel is sufficiently elastic to surround fragile connecting wires and it can easily adapt to thermal movements. The disadvantage with using silicone gel is that it is expensive and has low tensile strength.

The Japanese abstract 11145343 discloses that silicone gel can be mechanically protected using a layer of organic material such as a hard resin, an epoxy resin for example. The resin layer strengthens the silicone gel. However silicone gel has a tendency to move around the resin whereby the adhesion between the gel and the resin is weakened. Poor adhesion can lead to the silicone gel and the resin separating from each other which means that the capsule would no longer be gas- or liquid-tight. Epoxy resin is water-permeable which means that water can reach the electronic components by penetrating the epoxy resin or via capillary action along the connecting wires.

US 5275841 discloses an integrated circuit capsuled in a composite that comprises a silicone gel layer and a silicone elastomer layer where the gel layer of the capsule is arranged nearest to the integrated circuit. The advantages of having silicone gel next to the active electronic components is that better electric insulation is achieved near to the semiconductor components (pure silicone gel can withstand around 55 kV/mm while a silicone elastomer can withstand around 22 kV/mm). Better adhesion between the capsule and the electronic components is obtained with a reduced risk of delamination and air bubbles occurring near to the semiconductor components. The more rigid silicone elastomer protects the connecting wires from vibrations and strengthens the softer silicone gel. One problem with this composite is that the difference in thermal expansion between the gel and elastomer layers is relatively large which can give rise to shearing strain between the two layers, which can damage the composite. SUMMARY OF THE INVENTION

The object of this invention is to provide capsuled electronic device of the type described at the beginning of this document, which at least partly overcomes some of the discussed drawbacks of known capsuled electronic devices.

The invention and it's various preferred embodiments defined in the appended claims give rise to a compact, reliable, chemical- and heat- resistant capsule for electronic devices. It offers good adhesion, high dielectric strength, good mechanical strength and better electric insulation around semiconductor components and counteracts problems caused by differences in thermal expansion between the different materials around the electronic components during variations in temperature and protects connecting wires from vibrations.

The object of the invention is achieved by utilizing an electronic device according to any of claims 1-19 and a method according to claims 11 or 12.

A compact, reliable capsule for electronic components is achieved by utilizing a composite comprising both gel and more rigid elastomer, where the gel in the capsule is arranged adjacent to the semiconductor components and the remaining composite has either gradually or step-wise increasing rigidity in the direction away from the semiconductor components towards the surface of the composite.

A plurality of layers is applied, each layer having a different rigidity and thermal expansion coefficient in order to spread out tensile stresses over a greater volume and to avoid large tensile stresses in a single intermediate layer. In a preferred embodiment of the invention a gradient is created in the gel or the elastomer composition. In another preferred embodiment the softest gel is applied nearest to the semiconductor components and then the rigidity of subsequently applied layers of gel or elastomer is successively increased resulting in a composite having a gradually increasing rigidity from the semiconductor components towards the composite's outer surface i.e. towards the layer that is furthest from the electronic components. In another preferred embodiment layers having different rigidities are applied so that when cracks occur in the more rigid layers, softer gel from adjacent layers flows into and fills the cracks. According to another preferred embodiment the composite's outer layer comprises gel for self-healing purposes.

According to a preferred embodiment of the invention the composite comprises at least three layers, applied in order of increasing rigidity from said at least one semiconductor component, which form an integral unit. According to another preferred embodiment of the invention the composite comprises an outer layer of gel that contains silicon for example. According to yet another preferred embodiment of the invention the composite's outer layer comprises an elastomer that contains silicone for example. It is apparent to a person skilled in the art that even other gels and elastomers are suitable for use in a composite according to the present invention. A primer is advantageous to use in order to improve the adhesion between the electronic components and the gel layer adjacent thereto.

According to a preferred embodiment of the invention additives are added to the composite to modify its properties. Such additives include, for example, colourants, surfactants, adhesion promoters and fillers that modify the composite's electrical or thermal properties such as metal oxides, metal fibres or glass. According to yet another preferred embodiment of the invention the composite is strengthened with metal oxides or other inorganic material in the form of particles, tubes, fibres or whiskers. BRIEF DESCRIPTION OF THE DRAWING

A greater understanding of the present invention may be obtained by reference to figure 1 , which shows a cross-section of part of an electronic device encapsulated in a composite according to a preferred embodiment of the invention, when considered in conjunction with the subsequent description of the preferred embodiments,.

DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 represents a typical geometry for part of an electronic device. The figure depicts a composite having stepwise increasing rigidity. Semiconductor components 11 are situated on a base plate 12 and connecting wires 13 connect the semiconductor components to the surroundings. The semiconductor components 11 and the connecting wires 13 are encapsulated by a composite having a rigidity that increases in the direction from the semiconductor components 11 towards the surface of the composite 17. The composite comprises a thin layer of gel 14 such as pure silicone gel, an adjacent layer 15 of silicone elastomer for example and a more rigid layer 16 adjacent thereto that comprises a more rigid elastomer for example.

The silicone gel layer is produced using conventional methods such as hardening a suitable low viscosity addition-hardening composition around the semiconductor components, the base plate and the connecting wires. A cross-linking agent such as organohydropolysiloxane is subsequently applied on the surface of the silicone gel. The organohydropolysiloxane is hardened to produce an elastomer layer adjacent to the silicone gel.

The gel layer is substantially thinner than the elastomer layer that covers it and the thickness of the gel layer is less than 40% or preferably less than 30% of the total thickness of the composite. The gel layer only needs to cover the semiconductor components.

Claims

1. An electronic device comprising at least one semiconductor device (1 1) encapsulated in a capsule comprising a composite that contains gel (14) and a more rigid elastomer, where said gel is situated nearest to said at least one semiconductor component, characterized in that said composite has a gradually increasing rigidity in the direction from said at least one semiconductor component (1 1) towards the surface of the composite (17).

2. An electronic device comprising at least one semiconductor device (11) encapsulated in a capsule comprising a composite that contains gel and a more rigid elastomer, where said gel (14) is situated nearest to said at least one semiconductor component, characterized in that said composite has a stepwise increasing rigidity in the direction from said at least one semiconductor component (1 1) towards the surface of the composite (17).

3. An electronic device according to claim 2, characterized in that said composite comprises at least three layers (14, 15, 16) that are applied in order of increasing rigidity in the direction from said at least one semiconductor component (11) towards the surface of the composite (17).

4. An electronic device according to claim 3, characterized in that the composite's layers (14, 15, 16) form an integral unit.

5. An electronic device according to claims 3 or 4, characterized in that the composite's outer layer contains gel.

6. An electronic device according to claims 3 or 4, characterized in that the composite's outer layer contains an elastomer.

7. An electronic device according to any preceding claims, characterized in that said gel contains silicon.

8. An electronic device according to any preceding claims, characterized in that said elastomer contains silicon.

9. An electronic device according to any preceding claims, characterized in that said composite comprises at least one of the following: colourant, surfactant, adhesion promoter, fillers that modify the composite's electrical and/or thermal properties.

10. An electronic device according to any preceding claims, characterized in that the composite is strengthened with metal oxide or an inorganic material in the form of particles, tubes, fibres or whiskers.

1 1. Method of encapsulating an electronic device characterized in that it comprises the steps of covering at least one semiconductor component (11 ,), the base plate on which said at least one semiconductor component is mounted (12) and the connecting wires (13) to the said at least one semiconductor component in a sealing compound, and then hardening said sealing compound to provide a first inner layer (14), upon which a further layers (15, 16) of sealing compound are applied and then hardened whereby the said layers (14, 15, 16) have an increasing rigidity from said at least one semiconductor component (1 1) towards the outermost layer.

12. A method according to claim 11 , characterized in that a primer is applied on said at least one semiconductor component (11), the base plate (12) and the connecting wires (13) before the first layer of sealing compound is applied in order to improve the adhesion of the first layer to said at least one semiconductor component, base plate and connecting wires.

13. The use of an electronic device according to any of claims 1 to 10 in devices containing at least one switching component of transistor type, such as an IGBT.

14. The use of an electronic device according to any of claims 1 to 10 in devices that are used at voltages of 1 kV or higher.