Method for mounting electronic components on a substrate.

p0001The invention relates to a method of securing electronic components, in particular large-area power semiconductors, on a substrate by pressure sintering.

p0002. 800 ° C connected to each other, wherein the connection between aluminum and molybdenum as a kind - in the conventional production of large-area power semiconductors such as thyristors and the like are first a substrate of molybdenum, a small plate made of aluminum and a semiconductor body of silicon by heating to about 700 solder joint can be viewed, while the connection between aluminum and silicon is caused by the formation of an eutectic alloy. While this alloy process thereby takes place simultaneously doping the silicon. The high temperature stress resulting from the different thermal expansion coefficients of the individual parts to high voltages in the power semiconductor, which is noticeable in the finished product by corresponding bulges.

p0003In so-called. MOS technology made possible the doping, and in the structure production through the use of photoresist and etching techniques accuracies in the range of microns. However, an application of the MOS technology to the manufacture of large-area power semiconductors was not possible so far, because the high temperatures required for alloying to a substrate would lead to a destruction of the pn junctions and thin aluminum structures. Also, a production of the structures according to the alloying of silicon on the substrate would not be possible because the existing warping would exclude the application of photoresist and etching techniques with accuracies in the range of microns.

p0004From DE-OS 3414065 a method for mounting electronic components on a substrate by pressure sintering is already known, in which one - A metal-made powder, preferably silver powder, and a solvent paste layered applying to the contacting of the component and / or the contact surface of the substrate, - Then, the device is built on the substrate, - The solvent is completely casts and - Then the entire assembly with the simultaneous application of mechanical pressure of 80 - N / cm² heated 90 to sintering temperatures 380-420 ° C.

p0005When applying this known method, in particular small electronic devices could be connected at a relatively low thermal stress with a high mechanical strength with a substrate, wherein the compound layers produced by Durcksintern had extremely low electrical resistances and even very small thermal resistances. However, an application of the known method to the attachment of large-area power semiconductors, on a substrate led to no useful results. The connection layers produced by pressure sintering were always inhomogeneous and permeated by channel-like structures, which correspondingly too low adhesive strength, high electrical contact resistance and high heat resistance resulted.

p0006The invention has for its object to provide a method for mounting electronic components on a substrate by pressure sintering, which is also suitable for large-area power semiconductors. In particular produced by MOS technology electronic components should be able to be fixed at low heat load on a substrate.

p0007This object is achieved by a process of securing electronic components, in particular large-area power semiconductors, on a substrate by pressure sintering, in which one<ul><li>a. one made of metal powder and a solvent, applying the paste coated onto the contacting layer of the device and / or the contact surface of the substrate,</li><li>b. then drying the applied paste,</li><li>c. the device is built on the substrate, and</li><li>d. then the entire assembly is heated under simultaneous exertion of mechanical pressure of mindetens 900 N / cm² to the sintering temperature.</li></ul>

p0008The invention is based on the realization that the existing metal powder and solvent paste must be dried on the substrate prior to installation of the device, otherwise the solvent vapor lifts large components and escapes through a system of forming in the paste channels outwards. On the other hand, the preliminary drying of the paste results in relatively rough surfaces in the contact area, wherein this disadvantage can be compensated by an increase in mechanical pressure during the pressure sintering at least 900 N / cm². Surprisingly it has been found that such high pressures and pressures that are far above lead themselves when pressure sintering of produced in MOS technology power semiconductors to no damage to the delicate structures.

p0009According to a preferred embodiment of the invention, the powder of a noble metal or a noble metal alloy is used as metal powder. The connecting layers then exhibit particularly niedriege electrical resistances. It has proven to be particularly advantageous when used as a metal powder, silver powder or the powder of a silver alloy.

p0010According to a further embodiment of the invention, a mechanical pressure of at least 1000 N / cm² is exerted in the pressure sintering. A further increase in quality of the connection is achieved when the pressure sintering a mechanical pressure of at least 1500 N / cm² is exerted. It has proved to be advantageous if, when pressure sintering a mechanical pressure is exerted which corresponds at least to the intended operating pressure of a pressure-contacted component.

p0011Through the connection of component and substrate-related mechanical stresses can be further reduced in that the component and the substrate are heated without pressure at a temperature below the sintering temperature of at least 100 ° C before the pressure sintering. A largely tension-free during the operation execution is eventually achieved in that the component and the substrate are pressure-heated to a temperature which corresponds approximately to the intended operating temperature of a device.

p0012To achieve a particularly low heat load, pressure sintering may already be carried out at a sintering temperature of at least 150 ° C. Even better connections are however achieved when the pressure sintering is performed at a sintering temperature between 180 ° C and 250 ° C. Sintering temperature of 250 ° C and less are especially particularly advantageous if the elements for life setting with electrons or protons are irradiated, since higher temperatures would counteract the effect of this radiation again.

p0013According to a further preferred embodiment of the method the pressure sintering is performed in a press with heated punches. The heating to the sintering temperature at the same time exerting the necessary mechanical pressure is then particularly easy, further wherein the suitability should be emphasized for mass production. When such a press may then be protected by a deformable intermediate layer when pressure sintering the top of the device. In addition, may then be placed even when pressure sintering on the intermediate layer, a second intermediate layer made of a hard material with a low thermal expansion. This second rigid interlayer then ensures that thermal expansion of the press ram are not transferred to the device or its fine structures.

p0014Experiments have shown that a further significant increase in the mechanical strength of the connection can be achieved in that after pressure sintering a pressure-free resintering is performed. Through this non-pressure sintering that can be referred to as annealing, sintering times can also be shortened in the press and are therefore achieved at a higher mass production cycle times. It has been found to be advantageous if the pressure sintering sintering times in the region of one minute and for the re-sintering sintering times in the region of several minutes are selected.

p0015An embodiment of the invention is illustrated in the drawing and will be described in more detail below. while the thicknesses of individual intermediate layers were shown differently from the rest of scale greatly exaggerated to illustrate the layer structure.

p0016Show it:<ul><li>Fig. 1 is a substrate in cross-section,</li><li>Fig. 2 shows a large-area power semiconductors, in cross section,</li><li>Fig. 3 shows the power semiconductor according to FIG. 2 in top view,</li><li>Fig. 4 shows the application of a paste on the substrate according to FIG. 1,</li><li>FIG. 5 shows the placement of the power semiconductor on the substrate of FIG shown in FIGS. 2 and 3. 4 and</li><li>Fig. 6 shows the arrangement of FIG. 5 when pressure sintering in a press.</li></ul>

p0017Fig. 1 shows a section through a substrate S, which bears on all sides a contact surface KO. When the substrate S is, for example, a 1.5 mm thick plate of molybdenum with a diameter of 29.6 mm. The electroplated contact surface KO is about 2 to 3 micrometers thick and is made of silver.

p0018Figs. 2 and 3 show an electronic component B in the cross-section or in plan view. In this component B is a large-area thyristor, on top of an aluminum structures Ignition Zk and a cathode Ka can be seen. On the underside of the silicon body is a layer sequence, which layer in the individual from an approximately 1 micron thick aluminum layer, an approximately 100 nm thick titanium layer, a 500 nm thick center and a 200 nm thick silver layer is , In the drawing, merely serving as the contact surface KO silver layer can be seen.

p0019Referring to FIG. 4, a paste P coated on the contact surface of the substrate S KO. The application of the paste P in a thickness between 10 and 100 .mu.m, preferably from about 20 .mu.m is made by screen printing. As a starting material for the production of the paste P silver powder is mixed with platelet-shaped powder particles, a particle size of ≦ 15 microns and a bulk density of about 1.9 g / ml is used. This silver powder is then in cyclohexanol as a solvent in a weight ratio of about 2: 1 suspension. Subsequently, the screen-printable paste P in this manner is degassed under vacuum in order to prevent later during drying or sintering, a void generation.

p0020After application of the paste P this is dried to completion by driving off the solvent. The time required for this drying is at room temperature for about 30 minutes and at elevated temperatures only a few minutes.

p0021After drying the paste the substrate P S and the component B are together or separately heated to a temperature at least 100 ° C but still below the sintering temperature. This pressure-free heating can already be made in a later to be explained Press.

p0022At the latest after preheating the component B is then placed with its contact-KS to completely dried paste P of the substrate S in FIG. 5. The entire assembly is then placed in a press, of which in Fig. 6, the upper punch St1 and St2 of the lower punch can be seen. successively a 80 .mu.m thick first intermediate position Z1 of aluminum and a very stiff about 1.5 mm thick intermediate layer of molybdenum Z2 disposed between the top of the component B and the underside of the upper punch St2, the arrangement of these parts in Figure is shown. 6 for better recognition in exploded state.

p0023The two stamp ST1 and ST2 of the press are heated so that the desired sintering temperature is achieved in the structure arranged in between. This sintering temperature is for example 230 ° C is then maintained for about 1 minute, at the same time on the stamp ST1 and ST2, a mechanical pressure of 1500 N / cm² is exerted on the entire assembly. However, it is expressly noted that even at sintering times of a few seconds, good results can be achieved and that the mechanical pressure if necessary to 1 - can be increased 2 tons per square centimeter. Furthermore, it should be emphasized that the pressure sintering is performed in a normal atmosphere, that is an application of protective gas or forming gas is not required.

p0024After the pressure sintering the entire assembly is removed from the press and resintered to further increase the mechanical strength of the connection. This pressureless sintering, which could also be referred to as annealing is, for example, carried out at a temperature of about 250 ° C, which produces a time period of 5 minutes leads to a considerable improvement of the mechanical strength.

p0025In the above preparation of a compound of the thyristor semiconductor body and the substrate is made by pressure sintering, the sintering process should always go as solid state reaction without the occurrence of liquid phases on. The described method is particularly suitable for large area and manufactured in MOS technology power semiconductor, however, significant advantages can be obtained also in the production of other electronic components.