EP2994265A1

EP2994265A1 - Silver-composite sintering pastes for low-temperature sintering-bonding

Info

Publication number: EP2994265A1
Application number: EP14716310.9A
Authority: EP
Inventors: Andrea Feiock; Steffen Orso; Bernd Hohenberger
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2013-05-07
Filing date: 2014-04-09
Publication date: 2016-03-16
Also published as: DE102013208387A1; WO2014180620A1; US20160121431A1

Abstract

The present invention relates to a liquid-phase sintering composition containing low-molecular organic auxiliary agents, at least one silver salt, silver particles and one further metal solid material, characterized in that the further solid material is particulate and comprises tin.

Description

Description title

Silver composite sintered pastes for low temperature sintered joints

The invention relates to a liquid-phase sintered composition comprising low molecular weight organic auxiliaries, at least one silver salt, silver particles and another metallic solid material, characterized in that the further solid material is particulate and comprises tin.

PRIOR ART Modern power electronics are used in many areas of technology and are subject to ever higher quality requirements within these applications. In particular, the demands on reliability and service life of the elements have increased significantly in recent years. If one also considers that the required current intensities are shifted to higher and higher ranges in some applications, this results in a special thermal load situation of the components. This is particularly critical if the use of high current intensities causes significant self-heating of the components and, in addition, further unfavorable environmental conditions, such as, for example, due to changing temperatures in the component environment, are present. By way of example, at this point control devices in the automotive sector are to be named, which are arranged directly in the engine or in the gear compartment. These controllers are exposed to a constant temperature change, which can reach a temperature difference of up to 200 ° C on the component. These stresses must be taken into account when selecting the joining technique of the components.

Furthermore, a higher thermal flexibility is also desired in the design and construction of multi-chip power packages (MCPP), wherein the power semiconductors are currently soldered directly onto Cu stamped grid or Cu heat sink. The lead-free solders used here typically have one

Melting point in the range of 220-260 ° C, which in total the Einsatztem- limited the temperature of the solder joint. It would be desirable here to use higher temperatures (Tjunction) in order to increase the efficiency of the components.

These circumstances lead to the design requirement that the electronic components and their connection points with each other (or a carrier substrate, respectively) must be designed to be thermally stress-resistant and fail-safe. Usually, a connection of the electronic components takes place through a connection layer. As such a bonding layer solder joints are known, for example, lead-free solder joints of tin-silver or tin-silver-copper. At higher operating temperatures, lead-containing soldered joints can be used. However, lead-containing solder joints are severely limited by legal regulations for reasons of environmental protection in terms of their permissible technical applications. Alternatively, lead-free brazing alloys are suitable for use at elevated or high temperatures, in particular above 200 ° C. Lead-free brazing alloys generally have higher levels

Melting point than 200 ° C on.

Another way to achieve higher operating temperatures in the field of power electronics offers the use of silver-sintered compounds. These silver-sintered compounds can be used for joining electronic components and, theoretically, reach an operating temperature up to the melting point of the silver (961.8 ° C). These compounds are characterized by their high electrical and thermal conductivity and allow electronic components to be operated at higher temperatures, since the compounds do not melt and at the same time more heat from the

System can be derived.

On the other hand, a disadvantage of the pure silver sintered compounds is that quite high sintering temperatures and / or sintering pressures are required in order to dense the structure tightly and the material costs are significantly higher than in the other joining techniques. These factors increase process costs and, of course, impact component costs.

One possible technical solution, for example, US 2008 0073776 A1 ready. This document describes a heat dissipating composition for microelectronic devices comprising a microelectronic device, a heat spreader and a thermal interface material (TIM). which connects the microelectronic device and the heat distributor and the TIM has a sintered metallic nanopaste.

In another document, WO 2009 012450 A1, on the other hand, a method for fixing a semiconductor component on a substrate by means of a sintering paste is described, which has coated nanoparticles of Ag, Au, Cu, Ni, Pd, Fe or their alloys and thus to a Reduction of the sintering temperature can contribute.

Another possibility is the US 2009 0230172 A1. Herein, a semiconductor device is mounted on a substrate by means of a sintered paste. The sintering paste can consist of one or a combination of several metal powders with a certain purity and a defined size distribution. By using, for example, Au, Ag, Pt or Pd powder, a reduction of the required sintering temperatures can be achieved.

Furthermore, DE 10 2009 000192 A1 relates to a sintered material with metallic structural particles provided with an organic coating. According to the invention, it is disclosed within the document that non-organically coated, metallic and / or ceramic auxiliary particles which do not degas during the sintering process are provided.

It is the object of the present invention to provide a sintering composition and a method for joining power electronic components, which eliminate the disadvantages of the prior art and allow cost-effective and reproducible production of mechanically and thermally stable joints. This object is achieved by a sintered composition having the features of claim 1 and a sintering method according to claim 11. The dependent claims, however, indicate preferred embodiments.

Disclosure of the invention

Surprisingly, it has been found that a liquid-phase sintered composition containing low molecular weight organic auxiliaries, at least one Silver salt, silver particles and another solid metal material, characterized in that the further solid material is particulate and comprises tin, in comparison to the prior art sintered compounds has significantly improved properties. Compared to the pure silver sintered compounds, which may have particulate silver, the use of another particulate solid material comprising tin results in that the material cost of the sintered compound can be significantly reduced. Furthermore, the process costs can be significantly reduced, since using tin-containing solid material lower processing temperatures sufficient to produce a dense sintered connection. The melting point of the tin is well below the silver melting point. The silver and the tin-containing solid material can partially melt under heat and form a eutectic compound by the partial incorporation of the tin-containing particles in the silver particle matrix. This reactive sintering step involves a drastic reduction in the melting temperature. In this way melting temperature reductions down to 221 ° C can be realized. These sintered compounds also have a higher electrical and thermal conductivity, in particular in comparison to the solder joints mentioned in the prior art. The sintering composition according to the invention also provides a connection between components which, due to their high melting temperature, enables substantially higher temperatures of use. As a result, a higher efficiency of the components can be made possible.

Further advantages over pure silver-sintered compounds can additionally result from:

• the possibility that the particles can locally and partially melt, so that a liquid-phase sintering process is available. This can reduce the process temperature and provide high microstructure densities at already relatively low sintering pressures. Optionally, this composition can also be sintered without pressure, which leads to a lower expenditure on equipment.

• The only insignificant reduction in thermal and electrical conductivity of the sintered composition compared to standard solder joints, since only part of the silver is exchanged. • the possibility to adapt the CTE-Missmatch, possibly also by the addition of further particles.

• the material and process-related cost reduction in comparison to the use of pure silver-sintered compounds.

A liquid-phase sintered composition in the context of this invention is a composition which has metallic solid particles which partially melt in the course of a sintering process, under the effect of temperature and optionally under pressure. By using different metallic Vollmateria- lien can lead to the formation of mixed phases, which is a lower

Have melting temperature. The liquid, metallic phase can also lead to the formation of a particularly intimate joint between different components within the sintering process. In this way, particularly dense and mechanically / thermally stable connections between the components can be obtained.

The sintering composition may contain low molecular weight organic adjuvants. These organic auxiliaries can be present both in liquid form, in the form of organic solvents, or solid, for example as coating material for the particulate solid materials. The liquid organic solvents can thereby contribute to the cohesion of the compound in the form of a paste or to a better flow of the composition in the context of the sintering process. In addition, these agents can improve the adhesion of the composition to the devices. Other low molecular weight organic compounds can be used, for example, in the form of low molecular weight organic compounds as coating material. By way of example, fatty acids may be mentioned at this point. Further possible embodiments of these auxiliaries are listed, for example, in DE 10 2010 042 702 A1 and DE 10 2010 042 721 A1. In particular, low molecular weight in the context of this invention are considered organic compounds whose molecular weight is less than or equal to 1000 g / mol.

Silver salts in the sense of the invention are ionic compounds which contain the silver in cationic form. Possible usable silver salts are compounds containing anions from the group of carbonates, oxides, hydroxides or organic anions. Silver particles are in particular particles which differ from a surrounding medium through a solid phase interface. The particles comprise metallic silver and are optionally provided with a coating layer. The particles preferably have a size greater than or equal to 0.01 μηη and less than or equal to 1000 μηη. The geometry of the silver particles may be spherical, sparse, platy or wholly irregular. Preferably, the core of the silver particles, without consideration of a surface coating, consists of elemental silver. A metallic solid material in the sense of the invention is a material which, in the

Substantially comprises a non-salt particulate metal. The solid material may have a hole content of less than or equal to 5% by volume in the interior and optionally be coated on the surface by further metallic or non-metallic constituents.

Particulate solid material comprising tin corresponds to the definition of a solid material in the above-mentioned sense, with the proviso that the metallic solid material is in the form of particles and comprises tin in elemental form or in the form of an alloy. The proportion by weight of the tin on the tin-containing particulate solid material is variable and can be greater than or equal to 2.5

Wt .-%, preferably greater than or equal to 5 wt .-% and particularly preferably greater than or equal to 7.5 wt .-% amount. The weight fractions of the metals in the solid state can be determined either wet-chemically or via a suitable calibration by means of an X-ray analysis (energy dispersive X-ray spectroscopy (EDX)).

In a preferred embodiment, the particulate solid material comprising tin in the liquid-phase sintering composition may be spherical, sparse or plate-shaped. These geometries of the tin-comprising solid material have proven to be particularly suitable for obtaining a dense sintered compound. Without being bound by theory, this may in particular result from a simplified alloying of these particle geometries between the silver and the tin-comprising solid material in the context of the sintering process.

In an alternative embodiment of the liquid-phase sintering composition, the proportion of the particulate solid material comprising tin on the Sinter composition greater than or equal to 0.1% by volume and less than or equal to 50% by volume. This volume fraction of the tin-comprising solid material in the total sintered composition has proved to be particularly advantageous. In particular, this proportion results in a significant cost reduction of the sintered compound due to the material savings and the reduction of the sintering temperature to be used. This, while largely maintaining the mechanical and thermal properties of the resulting sintered compound. Smaller proportions lead to an insufficient formation of an elektischen mixture, whereas higher proportions can reduce the thermal stability of the resulting sintered compound too much. The volume share of

Tin full material results from the weight fraction taking into account the particle density.

The subject of a further preferred embodiment is a liquid-phase sintering composition, wherein the maximum extent of the particulate,

Tin full material greater than or equal to 0.1 μηη and less than or equal to 1000 μηη. This size of the tin-comprising solid material has proved to be particularly advantageous for obtaining a dense sintered compound with intimate mixing of the tin-comprising solid material with the silver particles. Smaller maximum expansions can lead to mechanically unstable and larger maximum expansions to inhomogeneous sintered connections. The maximum expansion of the particles results from the greatest distance between two surface points of the same particle. The particulate solid material comprising tin is not necessarily monodisperse. There may also be a size distribution of the tin-comprising solid material, wherein the above-mentioned magnitudes in the case of a size distribution may correspond to an average particle diameter in the sense of a D ₅₀ value. The order of magnitude of the expansion of the tin-comprising solid material can be determined conventionally by an optical method (microscope) or by scattering methods (MALS). To receive the

Size distribution, the results of the scattering methods can be interpreted to a first approximation by means of a spherical geometry of the particles.

In a further embodiment of the invention, the liquid phase sintered composition may have a composition, wherein the proportion

the organic auxiliaries greater than or equal to 0.1% by weight and less than or equal to 10% by weight, the silver salt is greater than or equal to 5% by weight and less than or equal to 20% by weight,

- The silver particles greater than or equal to 40% by weight and less than or equal to 94% by weight and

- of the particulate solid material comprising tin, greater than or equal to 0.1% by weight and less than or equal to 50% by weight

and the sum of the individual shares corresponds to 100% by weight. This composition of the liquid-phase sintered composition according to the invention has proved to be particularly advantageous from a process-economical and procedural point of view. The available sintered compounds have a good mechanical strength, good thermal conductivity, long service life of the components and, compared to the listed in the prior art compositions without tin comprehensive solid material, lower costs.

In a further embodiment of the liquid-phase sintering composition, the composition may contain further metallic or non-metallic particles. It is particularly expedient if the other metallic or non-metallic particles are formed in such a way that they combine with the solid material comprising silver and / or tin during the sintering process. For this purpose, the further metallic or non-metallic particles may, for example, have a sinterable surface which can be realized for example by means of a suitable coating. Furthermore, it is also possible to add inert particles which are enclosed only by the sinter material and have no direct connection to the metallic solid material. It is also possible to select the further metallic or non-metallic particles in such a way that they diffuse into the solid material comprising tin. This can positively influence the structure and density of the sintered connection. As non-metallic particles, for example, ceramic particles such as, in particular, aluminum oxide (also doped), aluminum nitride, beryllium oxide, silicon oxide and silicon nitride can be used. In order not to deteriorate the electrical conductivity too much by the addition of non-metallic particles, it is also possible to use electrically conductive ceramics, such as, for example, boron carbide or silicon carbide. For example, metallic particles may be selected from the group consisting of silver, copper, gold, platinum, palladium or a mixture of the aforementioned particles. These further metallic particles can be used, for example, to adapt the CTE mismatch or to realize certain microstructure densities of the sintered joint. The proportion by weight of the other metallic or non-metallic particles may be greater than or equal to 0.1% by weight to less than or equal to 10% by weight, preferably greater than or equal to 0.5% by weight to less than or equal to 5% by weight and particularly preferably greater than or equal to zero , 5 wt .-% to less than or equal to 2.5 wt .-%, amount.

In a further embodiment according to the invention, the particulate solid material comprising tin can be present in the liquid-phase sintering composition in the form of a tin-containing alloy or mixed phase. To adapt the melting and alloying behavior of the tin, the full material comprising tin can have further metallic constituents. Preferably, these other metallic constituents are not spatially separated, but are present in the form of mixed phases or alloys within the particle. As a result, in particular the melting behavior of the particles comprising tin and the further alloy formation with the solid silver material can be influenced in terms of process engineering. Furthermore, this can include the mechanical hardness of the tin

Full material to be adjusted.

In a preferred embodiment, the particulate solid material comprising tin in the liquid-phase sintered composition may additionally contain copper. Copper as a further constituent of the tin-comprising solid material may be particularly preferred, since copper / tin alloys can lead to a reduction of the CTE compared to pure tin. For example, the CTE of bronze is about 17E-6 1 / K, whereas Ag has a CTE of 19E-6 1 / K and Sn has one of 23E-6 1 / K. By admixing copper, the CTE be adapted to the respective joining situation.

Furthermore, in an additional embodiment, the proportion of copper in the particulate solid material comprising tin in the liquid-phase sintered composition may be greater than or equal to 85 mol% and less than or equal to 95 mol%. This tin-bronze solid material together with the silver particles can lead to a significant reduction in material costs as well as to dense sinter layers compared to the solid materials mentioned in the prior art. Especially the favorable alloy formation of the tin bronze with the silver particles can contribute to a fast process control at low sintering temperatures. Furthermore, this type of sintered connections a, compared to sintered compounds with pure silver particles, only insignificantly worse thermal conductivity.

In a further embodiment variant of the liquid-phase sintered composition, the tin material comprising tin may consist of tin. To obtain the lowest possible melting full particles, the tin-comprising solid material can be made entirely of tin. This means that the tin content of the solid particle can be greater than or equal to 98% by weight. This proportion of tin in the solid particle can facilitate the process by allowing the use of lower process temperatures. In addition, a positive effect on the

Set sintering time.

Furthermore, according to the invention, a method for integrally joining electronic components comprising the method steps

a) providing two components with surfaces to be joined

b) introducing a liquid-phase sintering composition between at least part of the surfaces of the components to be joined from step a) c) sintering the liquid-phase sintered composition

characterized in that the liquid-phase sintered composition in step b) contains particulate solid material comprising tin.

With regard to the method for cohesive joining electronic components exist two different applications. On the one hand, two electronic components can directly contact one another or one or more components can be joined to a carrier. The electrically conductive connection of the individual electronic components can then take place via the carrier. Accordingly, carriers also represent electronic components in the sense of step a). The surfaces of the electronic components are preferably made of metal. The electronic components may belong to the classes of electronic components which are standard within the field of power electronics, consumer electronics, and similar fields use. In particular, these include carrier materials and base plates such as populated circuit carriers, housings, DCB, AMB, IMS, PCB, LTCC, standard ceramic substrates, lead frames and leadframes.

In step b), as a function of the viscosity of the liquid-phase sintered composition, the sintering composition may be wholly or partially applied to one or more of the components both surfaces of the components are applied. For example, by brushing, sprinkling, spraying, knife coating, printing or applying in the form of a foil, a wire or a plate. The two components are either positioned beforehand at the desired distance from each other or this is then, after the application of the liquid phases

Sintered composition, mechanically adjusted.

In step c), the liquid-phase sintered composition is sintered by means of a heating step with or without application of static pressure. In this case, the full particles present in the liquid-phase sintered composition are completely or partially melted and thus the surfaces of the electronic components are joined in a materially cohesive manner. The process can be carried out in several temperature steps or ramps or at a constant temperature.

In an additional embodiment of the method for integrally joining electronic components, the tin-comprising solid material of the sintered composition in step b) can consist of tin or bronze. Tin and bronze solid materials are perfect for silver because of their properties

Melting point-lowering mixing phase to go in particular to the

Use within the joining method according to the invention. Through the use of tin, respectively the tin / copper alloy can be obtained mechanically and thermally very stable sintered layers. In addition, the melting points of the compounds allow economical process management with the use of only low sintering temperatures. The solid material made of tin or bronze may optionally be equipped with a further metallic or non-metallic surface coating.

According to a further embodiment of the method according to the invention for the integral joining of electronic components, the temperature during

Sintering the liquid-phase sintered composition in step c) be greater than or equal to 200 ° C and less than or equal to 500 ° C. Due to the use of solid material comprising tin, these low sintering temperatures can be sufficient to provide a sufficient degree of joining quality of the electronic components through the sintering process. Preferably, the temperature during

Sintering the liquid-phase sintered composition in step c) larger or equal to 200 ° C and less than or equal to 400 ° C, further preferably greater than or equal to 230 ° C and less than or equal to 350 ° C.

In a further embodiment of the method according to the invention for the integral joining of electronic components, in step c) the joining parts may additionally be subjected to a joining pressure greater than or equal to 0.1 MPa and less than or equal to 150 MPa. The process pressure may preferably be at most 150 MPa, preferably less than 100 MPa, more preferably less than 50 MPa. In a further preferred embodiment of the invention, the joining process can be carried out completely without pressure. This can contribute to a significant cost reduction of the process, since complex hydraulic devices to achieve an otherwise necessary contact pressure are not required. Furthermore, the presented embodiments of the method according to the invention for joining power electronic components can be used. Examples of fields of application are the joining of electronic components for: power output stages of electric power steering systems, power output stages of universal inverter units, control electronics, in particular on the starter and / or generator, press-in diodes on generator shields, high-temperature-stable semiconductors, such as silicon carbide, or also sensors which are operated at high temperature and a Sensor-near evaluation need. It is also possible to use semiconductor diodes and modules for inverters, in particular photovoltaic systems.

Claims

claims

1 . Liquid-phase sintered composition containing low molecular weight organic auxiliaries, at least one silver salt, silver particles and another metallic solid material, characterized in that the further solid material is particulate and comprises tin.

2. A liquid-phase sintering composition according to claim 1, wherein the particulate solid material comprising tin is spherical, sparse or plate-shaped.

The liquid phase sintering composition according to any one of the preceding claims, wherein the content of the particulate solid material comprising tin in the sintering composition is greater than or equal to 0.1% by volume and less than or equal to 50% by volume.

4. The liquid-phase sintering composition according to claim 1, wherein the maximum extent of the particulate solid material comprising tin is greater than or equal to 0.1 μm and less than or equal to 1000 μm.

5. Liquid-phase sintering composition according to one of the preceding claims, wherein the proportion

the organic auxiliaries greater than or equal to 0.1% by weight and less than or equal to 10% by weight,

the silver salt is greater than or equal to 5% by weight and less than or equal to 20% by weight,

and the sum of the individual shares corresponds to 100% by weight.

6. A liquid-phase sintering composition according to any one of the preceding claims, wherein the composition contains further metallic or non-metallic particles.

A liquid-phase sintering composition according to any one of the preceding claims, wherein the particulate solid material comprising tin is in the form of a tin-containing alloy or mixed phase.

The liquid-phase sintering composition according to any one of the preceding claims, wherein the particulate solid material comprising tin additionally contains copper.

9. The liquid phase sintering composition according to claim 8, wherein the copper content of the particulate solid material comprising tin is greater than or equal to 85 mole% and less than or equal to 95 mole%.

10. A liquid-phase sintering composition according to any one of claims 1-6, wherein the solid material comprising tin is tin.

1 1. Method for integrally joining electronic components comprising the method steps

a) providing two components with surfaces to be joined b) introducing a liquid-phase sintering composition between at least part of the surfaces of the components to be joined from step a)

c) sintering the liquid-phase sintered composition

12. A method for integrally joining electronic components according to claim 1 1, wherein the tin-comprising solid material of the sintered composition in step b) consists of tin or bronze.

13. A method for integrally joining electronic components according to claim 11 or 12, wherein the temperature during sintering of the liquid-phase sintered composition in step c) is greater than or equal to 200 ° C and less than or equal to 500 ° C.

14. A method for cohesive joining electronic components according to claim 1 1 - 13, wherein in step c) the joining parts additionally with a Fu be applied greater than or equal to 0.1 MPa and less than or equal to 150 MPa.

15. Use of the method according to any one of claims 1 1 -14 for joining power electronic components.