DE2855494A1 - Thermally conducting insulating discs for electronic components - comprise ceramic inserts in plastics insulating plate, extending from surface to surface - Google Patents

Abstract

The object of the composite disc is to reduce the danger of damage to the semiconductor assembly, for example an integrated or printed circuit, due to differential expansion between the same and the disc. A disc or plate made of plastics insulation material is pierced by holes which are filled with a ceramic material. The regions of high differential thermal expansion at the ceramic-semiconductor interface are thereby confined to discrete small areas, thus giving low forces tending to rupture the semiconductor. The proportion of ceramic filling of the plate is pref. no less than 80%. The ceramic fillings are with advantage hexagonal in cross section. The ceramic material is typically beryllium oxide, but aluminium oxide may also be employed.

Description

"Electrically insulating heat sink"

The invention relates to an electrically insulating heat sink on the basis of ceramics as a support and / or connection plate for electrical and electronic components Components, integrated circuits and conductor tracks.

It can be used for the construction of vapor-deposited or printed Circuits, but also for the connection between power semiconductor components and / or modules and a larger potential-free heat sink. It understands that the semiconductor components also work together with resistors that are on different electrical potentials are mounted on the support plate could be.

The well-known arrangement is known as a “power semiconductor module” of one or more, preferably two, on a common Mounting plate attached semiconductor elements in the form of provided with electrodes, Edge-insulated or passivated, mostly caseless tablets and optionally with wiring elements in a common housing with cooling device understood, whereby the main power connections and any

existing control current connections of the semiconductor element or elements are led out of the housing on a single side if possible. At acquaintances Different circuit variants are possible for such modules (brochure "Power Semiconductors "from March 1974 vbfl; ' from AEI Semiconductors (Publication 111), Pages 48/49; furthermore data sheet thyristor modules Thy F 75 / G 75 / H 75 from the company Siemens AG, Erlangen). For the construction of converter circuits it is advantageous to that the power semiconductor modules are mechanically and electrically ready-to-connect functional units are made up of one or more discrete semiconductor elements, possibly with additional wiring elements and / or the control electronics. Another advantage of such modules is the Cheaper by avoiding the individual encapsulation and the assembly of individual Semiconductor elements on site.

The disadvantage of the known modules is the high heat flux density because of the high concentration of heat-emitting components, what the cooling difficult and so-the performance of such modules is limited. To the accruing Heat with an acceptable temperature drop to the environment, e.g. via a metal cooler or a heat pipe, it is necessary to have the thermal resistance between the heat-emitting surface of the module and the heat-absorbing surface of the cooler to keep it small.

It is known for modules, for any semiconductor element for attachment an insulating one on a copper base plate Washer off Use oxide ceramics (DE-GM 75 12 573).

It is commonly known as thin ceramic disks as heat sinks from beryllium oxide to use in semiconductor technology. Such ceramic discs In addition to good electrical properties, they also have relatively good thermal properties the thermal conductivity (A12 03: Rç 0.4W / cm.K; BeO: R ~ Y1, GW / cm.K).

For example, the temperature drop over a 1 mm thick A1202-2 Disc at 50 W / cm approx. 12.5 CC, in the case of BeO even only 3.2 "C.

With the help of these materials, the heat loss could be transferred directly to a cooled rail can be dissipated without a transformation of the heat flux density would be required. The problem with these ceramic materials, however, lies in theirs lower thermal expansion coefficients compared to copper and their susceptibility to breakage. Due to the different thermal expansion coefficients of ceramics and Copper (tG Alz = 78.10 1 / ° c, Z Cu = 175.10 7 1 / ° c) are large-area connections of these two materials not to be carried out, because of the resulting thermal shock loads the permissible tensile stress in the ceramic is far exceeded. this leads to Cracks in the ceramic, which in turn drastically reduce the electronic dielectric strength reduce.

Extensive contact is therefore only possible up to a diameter of approx 10 mm feasible.

These problems also apply to the well-known "direct bonding method" (J.F. Burgess and C.A. Neugebauer and G.Flanagan: The Direct Bonding of Metals to Ceramics by the Gas-Metal Eutectic Method, J. Electrochem. Soc .: Solid State Science and Technology, Vol. 122, No. 5, May 1975, pages 688 to 690). It is about to a process with the copper (and other metals) directly on insulating oxide substrates like BeO or A1203 are bonded.

The invention is therefore electrical based on the aforementioned insulating heat sink on the base of ceramics, the task based on their good electrical insulating properties and their good thermal conductivity, the mechanical strength under thermal cycling to improve. It should e.g. the use of power semiconductor components or power semiconductor modules resulting from high thermal shock loads withstand and put as little stress on the brittle semiconductor tablets as possible.

The solution to this problem is that the ceramic by plastic is separated into individual ceramic disc areas and that these areas of disc surface traverse to disk surface.

The thermal resistance of this disk is advantageously obtained in the axial direction, i.e. perpendicular to the plane of the slices, as a parallel connection the thermal resistance of ceramics and plastics. With a preferred degree of filling of over 80% ceramics, it is essentially determined by the ceramics itself, so that a temperature drop of 3.5 to 15 Cc can be achieved depending on the ceramic. This value must be judged from the point of view that it is simultaneous the expansion coefficient can be adjusted via the degree of filling and thus the thermal Expansion of the disc in the radial direction.

The individual ceramic disk areas with close tolerances are useful Arranged area-wide, preferably they are hexagonal. The distance between the neighboring hexagon sides are only needed empirically for the application purpose or to correspond to the maximum thermal expansion to be determined by calculation.

By setting the degree of filling, preferably over 80% is one Adaptation of the expansion coefficient to that of the metal of the heat sink to be connected possible. Different radial expansions only exist between the individual ceramic disc areas and the metal of the heat sink.

2 Large-area contacts well over 1 cm are possible without the above-described cracking occurs.

It is generally known per se to use plastic films for insulation Use semiconductor components. Plastic films have very good electrical properties Properties (insulation resistance, dielectric strength), but very poor -3 thermal (thermal conductivity = <4 .. 10 W / cm. Kj properties. So is the temperature drop in the case of a 0.1 minute thick film in the case of those in question Power densities of 2 50 W / cm approx. 125 OC, which is not acceptable. Makes sense the heat loss via plastic can only be dissipated if the 2 heat flux density is transformed to values o 5 W / cm. This could e.g. be realized with a heat pipe will. The disadvantage, however, is the relatively large volume of a heat pipe.

The possibly obvious combination of good electrical and mechanical Properties of plastics with good electrical and thermal properties of ceramic in the form of a homogeneous composite body made of ceramic and plastic does not lead to the desired success, because such a composite body only its expansion coefficient can largely be adapted to that of copper, however, very close to the high one in terms of its resulting thermal resistance The thermal resistance of the plastic is because it is essentially made up of the sum the individual resistances of the two components results. Even if the volume fraction of the plastic would only be about 30%, such a homogeneous composite body would have still a thermal resistance that is only about a factor of 3 compared to pure Plastic would be degraded.

With the measures according to the invention, the coefficient of thermal expansion Set via the filling level and at the same time achieve a low thermal resistance.

The invention is illustrated below with reference to one in the drawing Embodiment explained in more detail.

The figures show: FIG. 1a a plan view of a heat dissipation disk circular ceramic areas and plastic, Fig. Ib is a plan view of a heat dissipation disk of hexagonal ceramic areas and plastic, Fig. 2 shows a section accordingly the section line II-II in Fig. 1, Fig. 3 shows the temperature drop at the heat sink according to FIG. 1 as a function of the slice thickness and compared with that of a homogeneous one Composite body.

The heat dissipation disk sawn off, for example, from an elongated composite body 1 consists of ceramic areas 2 (in Fig. La - areas 2a) and plastic 3, preferably Epoxy resin.

In the case of a hexagonal shape, the ceramic areas 2b according to FIG. Lb can a higher ceramic filling level can be achieved.

This is only limited by the thermal expansion of the individual Areas to be observed distances of the hexagon sides from each other. It is a degree of filling achievable from close to 100% ceramic.

As FIG. 2 shows, the ceramic disk areas 2a are sufficient from the one disc surface 4 to the other disc surface 5. You should in the state of use to the semiconductor surface on the one hand and to the one to be connected On the other hand, connect the heat sink with minimal thermal contact resistance.

For all geometric shapes of the ceramic disk areas 2, a Aim for a minimum distance from area to area.

With a degree of filling of 80 to 90%, the thermal resistance is heterogeneous Heat sink 1 already close to that of the pure ceramic, so that depending on Ceramic, e.g. beryllium oxide or aluminum oxide, a temperature drop of 3.5 resp.

15 ° C can be achieved.

In Fig. 3, the temperature drop is a function of the slice thickness t is shown at a heat flux density of 50 W / cm for beryllium oxide and aluminum oxide. For comparison, this dependency is also for a homogeneous composite body shown with a fill level of 70% aluminum oxide ceramic.

It has already been explained that the coefficient of expansion extends over the filling level can be set. E.g. 90% beryllium oxide, the remainder epoxy resin, the coefficient of expansion of the heat conducting disk l adapted to that of copper. Different radial expansions only exist between the individual ceramic disk areas 2 (with a circular shape e.g.

from 1 to 5 mm in diameter) and copper. Due to thermal stress The stresses arising in this ceramic disk area 2 are proportional d / t, where d = pulley diameter and t = pulley thickness. So you let yourself by the appropriate choice of the diameter of the ceramic disc areas 2 in Keep boundaries. The lower value of the slice thickness t is determined by the required insulation strength and manufacturability, in their upper value accordingly the thermal requirements. It should be between 0.5 and 1 mm.

The following table shows the thermal resistance again for comparison of electrically insulating panes, based on 10 mm thickness: material thermal resistance ° C / W / cm plastic 250 Al2O3 (99.7%) 2.63 BeO 0.56 homogeneous composite body 75 (70 % A12 03, remainder plastic) Heterogeneous composite body 3.29 (axial) (80% AlaO3 remainder Plastic Heterogeneous composite body 0.69 (axial) (80% BeO, rest plastic) Man can therefore determine that one with the heterogeneous composite body or the heterogeneous Heat dissipation disks 1 reaches values for the thermal resistance that are in the order of magnitude of the values of the ceramic itself, whereas the value of the thermal resistance for the comparatively listed homogeneous composite body is still relatively high.

The homogeneous composite body therefore requires a transformation when used the heat flow density via, for example, a heat pipe 2 to the required values below 5 W / cm, whereas the heterogeneous heat sink 1 without such a transformation the heat current density can be used and thus better application possibilities, especially with regard to the integration of power semiconductors and their components to modules.

Conductor tracks can also be vapor-deposited onto the heterogeneous heat dissipation disk 1 or resistors are dusted on in a manner known per se (DE magazine "Vakuum-Teehnik" 27th year

Issue 5, pages 141 to 146). Because of the possible high heat-2 current density, e.g. 50 W / cm, a 100 W resistor 2 can advantageously stand on a pane surface of 2cm can be accommodated.

A very advantageous application of the heterogeneous heat sink 1 consists in its combination with a bundle contact disk from the disk surface Pre-twisted wires made of silver or copper running through to the disc surface according to patent application P (filed on the same day). Such a bundle contact disk is highly elastic in the direction of the plane of the disk.

If e.g. for a power semiconductor component on one side Copper heat sink is to be connected potential-free, the heterogeneous heat dissipation disk 1 can be inserted between the silicon wafer and the bundle contact wafer; the bundle contact disk connects with its other side to the copper heat sink at. Because the expansion differences between ceramic and copper are not as great as the one between silicon and copper, could even be a relatively thin copper bundle disk Find application. The advantage of this variant would also be that even Ceramic discs (e.g.

100 mm diameter), which is relatively easy by pressing and sintering can be made that could be contacted with copper. In the latter case the copper bundle disk takes over the compensation of the different radial Expansion between copper and ceramic.

Claims (4)

Claims Electrically insulating heat sink on the base of ceramics as a support and / or connection plate for electrical and electronic Components, integrated circuits and conductor tracks, characterized in that the ceramic is separated into individual ceramic disc areas (2) by plastic (3) and that these areas (2) from disk surface (3) to disk surface (4) run through. 2. Heat sink according to claim 1, characterized by a Geometric arrangement of the ceramic disc areas (2) with closely toleranced area-wide Filling. 3. Heat sink according to claim 2, characterized in that the ceramic disc areas (2b) are hexagonal. 4. Heat dissipation disc according to claim 1 or the following, characterized due to a filling level of over 80% ceramic.