Method of pressureless sintering for connecting electronic components

Abstract

To provide a method of pressureless sintering of metal for connecting electronic components in a reliable manner.SOLUTION: A method for connecting electronic components includes: (a) providing a sandwich structure including at least (a1) an electronic component 1, (a2) an electronic component 2 and (a3) a metal sinter-prepared object positioned between metal contact surfaces of the electronic components 1, 2 and (b) pressureless sintering the sandwich structure. The method is carried out in an ambient gas atmosphere, in which the ambient gas atmosphere is replaced in the course of a pressureless sintering process of metal.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a pressureless sintering method for connecting (bonding) electronic components, in which a metal sintered preparation is used as a connecting material and the ambient gas atmosphere is exchanged during the sintering process.

[0002] As used herein, the term "pressureless" means "without applying mechanical pressure".

[0003] Sintering should be understood to mean connecting electronic components by heating without the metal of the metal sintered preparation reaching the liquid phase. As is well known to those skilled in the art, heating and sintering are carried out in a furnace.

[0004] As used herein, the term "electronic component" refers to a substrate, as well as active and passive components, used within an electronic device. Like conventional electronic components, they have metal contact surfaces. Unless the electronic components are made of metal, their contact surfaces are covered by a metal layer or a metallization layer. In one embodiment, the metal contact surface can be made of a non-precious metal such as, for example, copper, nickel, or aluminum. In another embodiment, the metal contact surface can be made of a precious metal such as silver or gold.

[0005] Examples of substrates include IMS (insulated metal substrate), AMB substrate (active metal brazed substrate), metal ceramic substrate such as DCB (direct copper bonded substrate), ceramic substrate, PCB (printed circuit board), and lead frame.

[0006] Examples of active components include diodes, LEDs (light emitting diodes), dies, IGBTs (insulated gate bipolar transistors), MOSFETs (metal oxide semiconductor field effect transistors), and ICs (integrated circuits).

[0007] Examples of passive components include sensors, base plates, heat sinks, resistors, capacitors, inductors, antennas, and connection elements (e.g., clips).

[0008] As used herein, the term "ambient gas atmosphere" indicates that the atmospheric sintering method for connecting the electronic components disclosed herein is not carried out in a vacuum, but rather in a gas atmosphere, particularly in a gas atmosphere at standard pressure.

[0009] It has been disclosed that a silver sintered joint can be formed on a bare copper surface without applying mechanical pressure (i.e., at normal pressure) during the sintering process by using a sintering paste based on silver nanoparticles or a sintering paste based on a mixture of silver microparticles (typically in the size range of 1 to 10 μm) and silver nanoparticles (typically in the size range of 1 to 100 nanometers). Examples of such disclosures include H. Miyoshi, K. Endoh, S. Kurita, "Application of Silver Nano Particles to Pressureless Bonding onto a Copper Surface - Consideration of Substitute Material for Lead Solder", CIPS conference 2014; H. Zheng, D. Berry, K. D. T. Ngo, G. Lu, "Chip - Bonding on Copper by Pressureless Sintering of Nanosilver Paste Under Controlled Atmosphere", IEEE Transactions on Electronic components, Packaging and Manufacturing Technology, Vol. 4, No. 3, March 2014; H. Zheng, J. Calata, K. Nog, S. Luo, G. Lu, "Low - pressure (<5MPa) Low - temperature Joining of Large - area Chips on Copper Using Nanosilver paste", CIPS conference 2012; and T. Watanabe, N. Nakajima, M. Takesue, "Material Design and Process Conditions of Pressureless Sintered Silver for 200 / 40°C Thermal Cycling Reliability", PCIM Europe 2017.

[0010] In addition, many patent documents, such as International Publication No. 2018 / 206267 (A1) and International Publication No. 2018 / 206162 (A1), disclose a normal-pressure metal sintering connection process. Other patent documents disclose metal sintered preparations having compositions that enable use in a metal sintering connection process or explicitly enable use in a normal-pressure metal sintering connection process. Examples of such patent documents include International Publication No. 2022 / 214228 (A1), International Publication No. 2022 / 128177 (A1), International Publication No. 2021 / 073803 (A1), and International Publication No. 2020 / 057806 (A1).

[0011] An object of the present invention is to provide a normal-pressure metal sintering method for connecting electronic components in a reliable manner. This method shall be feasible using actually any metal sintered preparation, i.e., without the need to use a metal sintered preparation specifically adapted for the connection of electronic components by normal-pressure metal sintering. As already mentioned, such a metal sintering connection is effected via the metal contact surfaces of the electronic components. In particular, the found normal-pressure metal sintering method is well-suited for reliably making a normal-pressure connection of electronic components at least one of which has a non-noble metal contact surface such as a copper contact surface. A reliable connection must be formed even on the unpretreated non-noble metal contact surface of the electronic component during normal-pressure metal sintering.

[0012] Thereby, it has been found that the exchange of the ambient gas atmosphere taking place during the process of the normal-pressure metal sintering process for the connection of electronic components has an unexpectedly surprising positive effect on the strength of the connection between the electronic components.

[0013] The present invention relates to a method for connecting electronic components, comprising: (a) providing a sandwich configuration comprising at least (a1) an electronic component 1, (a2) an electronic component 2, and (a3) a metal sintered preparation positioned between the metal contact surface of the electronic component 1 and the metal contact surface of the electronic component 2; and (b) sintering the sandwich configuration at normal pressure. The overall method of the present invention is carried out in an ambient gas atmosphere, and the exchange of the ambient gas atmosphere is carried out during the process of the normal-pressure metal sintering process, i.e., during step (b).

[0014] The metal sintering process of step (b) comprises three consecutive, or in particular three directly consecutive stages or sub-steps (b1) to (b3), namely a heating sub-step (b1), a sub-step (b2) at the peak temperature (the highest body temperature, i.e., the highest temperature of the sandwich configuration), and a cooling sub-step (b3). The sub-steps (b1), (b2), and optionally the sub-step (b3) are also carried out in a furnace.

[0015] For the present invention, it is essential that the exchange of the ambient gas atmosphere takes place during the process of the atmospheric pressure metal sintering process, i.e., during the metal sintering step (b) in which the sandwich configuration is sintered at atmospheric pressure. More precisely, the exchange of the ambient gas atmosphere can take place either in sub-step (b2) or at the start of sub-step (b3). "In sub-step (b2)" means "after at least 5 minutes of the duration of sub-step (b2) have elapsed", or can mean "substantially in the middle of sub-step (b2)", i.e., after 40 to 60% of the duration of sub-step (b2) have elapsed. Preferably, the exchange of the ambient gas atmosphere takes place at the start of sub-step (b3).

[0016] The expression "exchange of the ambient gas atmosphere" means the exchange or replacement of a first ambient gas or gas mixture by a second ambient gas or gas mixture of a different composition, or alternatively, obtaining a second ambient gas mixture by a change in the composition of the first ambient gas or gas mixture, i.e., the first ambient gas (mixture) and the second ambient gas (mixture) have different compositions. The first ambient gas (mixture) may be an oxidizing gas or a non-oxidizing gas (mixture), and thus, the second ambient gas (mixture) is a non-oxidizing gas or an oxidizing gas (mixture). The exchange of the ambient gas atmosphere, i.e., the exchange of the ambient non-oxidizing gas atmosphere by the ambient oxidizing gas atmosphere, or vice versa, i.e., the exchange of the ambient oxidizing gas atmosphere by the ambient non-oxidizing gas atmosphere, is carried out at least once. The exchange of the ambient gas atmosphere may be carried out more frequently than once, but typically and preferably, it is carried out only once. The exchange of the ambient gas atmosphere can be carried out by purging the interior of the furnace, i.e., purging the interior of the furnace with a subsequent gas type, but in another embodiment, the exchange of the ambient gas atmosphere is not to completely purge the interior of the furnace, but by injecting a gas or gas mixture of a different composition into the furnace, only partially discharging the ambient gas atmosphere in the furnace, or more precisely, by injecting a gas or gas mixture of a different composition into the furnace so as to obtain a second ambient gas mixture, i.e., a second ambient gas mixture of the desired composition, and only partially discharging the first ambient gas or gas mixture in the furnace. Purging is a preferred gas exchange method. Examples of non-oxidizing gas atmospheres include a reducing gas atmosphere free of oxygen such as a hydrogen / nitrogen or formic acid / nitrogen gas mixture, and an inert gas atmosphere free of oxygen composed of at least one inert gas, such as nitrogen, carbon dioxide, a noble gas (e.g., argon). A gas atmosphere free of oxygen should be understood to mean a gas atmosphere having an oxygen content of 300 volume ppm (ppm per volume) or less, preferably 200 volume ppm or less, and even more preferably 100 volume ppm or less.Examples of an oxidizing gas atmosphere include a gas mixture of at least one inert gas containing 10 to 50% by volume (percent per volume) of oxygen, preferably 10 to 30% by volume of oxygen, and air is an example of a preferred oxidizing gas atmosphere.

[0017] Connecting electronic components should be understood to mean mounting the electronic component on a second electronic component. In this context, "on" simply means that the metal contact surface of the first electronic component is connected to the metal contact surface of the second electronic component regardless of the relative arrangement of the two electronic components or regardless of a sandwich configuration comprising at least two electronic components.

[0018] Regarding the electronic components and their metal contact surfaces, reference is made to what was disclosed above. The method of the present invention is particularly advantageously used when the electronic component 1 and / or the electronic component 2 have a non-noble metal contact surface, in particular a copper contact surface. Thus, in a particular advantageous embodiment of the method of the present invention, at least one of the metal contact surfaces of the electronic component 1 and the metal contact surface of the electronic component 2 to be connected is copper, and the copper may or may not be pretreated.

[0019] The connected electronic component 1 and electronic component 2 may be the same or different electronic components. As already described, the present invention not only promotes the connection between the noble metal contact surface of electronic component 1 and the noble metal contact surface of electronic component 2, but in particular, promotes the connection of a non-noble metal contact surface, particularly a copper contact surface, to the noble metal contact surface, or even promotes the connection of two non-noble metal contact surfaces of electronic component 1 and electronic component 2 in such a manner as to provide a reliable connection at room temperature and high temperatures such as 200 to 260 °C. For example, connections of aluminum-nickel, aluminum-copper, aluminum-silver, aluminum-aluminum, aluminum-gold, copper-silver, copper-nickel, copper-copper, copper-gold, silver-nickel, silver-gold, silver-silver or gold-gold can be formed. The term "metal contact surfaces of aluminum, copper, nickel, silver and gold" includes metal contact surfaces made of alloys of the said metals. Similar to the case of contact surfaces of silver, gold or other noble metals, even in the case of non-noble metal contact surfaces such as the surfaces of copper, nickel or aluminum, it is not necessary to pretreat them before performing the sintering step (b), and in particular, a reduction treatment before performing the sintering step (b) is not necessary.

[0020] The term "room temperature" as used in this specification refers to a temperature in the range of 20 to 35 °C.

[0021] Therefore, when two electronic components, namely electronic component 1 and electronic component 2, are connected to each other, the metal sintering preparation is disposed between the metal contact surfaces of electronic component 1 and electronic component 2 before performing the sintering process of step (b). On the other hand, it is also conceivable to connect three or more electronic components to each other. For example, three electronic components, namely electronic component 1, electronic component 2 and electronic component 3, can be connected to each other in a suitable manner such that electronic component 2 is located between electronic component 1 and electronic component 3. In this case, the metal sintering preparation is disposed both between electronic component 1 and electronic component 2 and between electronic component 2 and electronic component 3, or more precisely, between these metal contact surfaces. In this scenario, electronic component 2 naturally has two metal contact surfaces.

[0022] Preferably, first, the metal sintered preparation of the present invention is provided on the metal contact surface of the electronic component 1 or 2. Accordingly, the other electronic component 2 or the electronic component 1 is disposed by its metal contact surface on the metal sintered preparation applied to the metal contact surface of the electronic component 1 or the electronic component 2.

[0023] Metal sintered preparations for connecting electronic components are well known to those skilled in the art. By way of example, metal sintered preforms, in particular metal sintered pastes, may be mentioned. A metal sintered preform is a foil-like piece of solidified, for example dried, unsintered or only partially sintered metal sintered paste. A metal sintered preform can be produced by applying a metal sintered paste on a flat carrier, drying the metal sintered paste so applied without sintering it or not completely sintering it, and then peeling off the metal sintered paste. A metal sintered paste contains metal particles, an organic solvent, and optionally but typically at least one additive. Examples of additives include metal precursors, wetting additives, dispersants, surfactants, defoamers, viscosity control (rheology) agents, and sintering agents. Any such conventional metal sintered preparation can be used in the method of the present invention. The metal sintered preparation need not be specially adapted for connecting electronic components by atmospheric pressure metal sintering. However, the present invention may of course be specially adapted for connecting electronic components by atmospheric pressure metal sintering. As the metal sintered preparation, a copper sintered preparation, in particular a silver sintered preparation, is preferred, and as the metal sintered preparation, a copper sintered paste, in particular a silver sintered paste, is most preferred. Examples of patent documents disclosing copper sintered preparations and silver sintered preparations, and their compositions, include, but are only a few examples, International Publication No. 2020 / 057806 (A1), International Publication No. 2011 / 026623 (A1), International Publication No. 2016 / 028221 (A1), US Patent Application Publication No. 2017 / 0243849 (A1), and US Patent Application Publication No. 2018 / 0056449 (A1).

[0024] When implementing the method of the present invention using a metal sintering paste, the application of the metal sintering paste to the metal contact surface of electronic component 1 or electronic component 2 can be carried out by conventional methods. Examples include dispensing, screen printing, stencil printing, pin transfer, and dipping. Preferably, the thickness of the wetting layer of the metal sintering paste between the connected electronic component 1 and electronic component 2 is in the range of 50 to 150 μm. In this context, the thickness of the wetting layer should be understood to mean the thickness of the layer before drying, if any, and before the sintering step (b). The preferred thickness of the wetting layer depends on the method selected for applying the metal sintering paste. When the metal sintering paste is applied, for example, by screen printing or dispensing, the thickness of the wetting layer can preferably be in the range of 50 to 80 μm. When the metal sintering paste is applied by stencil printing, the preferred thickness of the wetting layer can be in the range of 50 to 100 μm. After the metal sintering paste is applied, an optional drying step can be introduced before the sintering step (b), that is, the organic solvent is removed from the applied metal sintering paste. The drying step can be carried out before forming the sandwich configuration, that is, after applying the metal sintering paste to the metal contact surface of electronic component 1 and before connecting it to the metal contact surface of electronic component 2, or after assembling the sandwich configuration. According to a preferred embodiment, the fraction of the organic solvent in the dried metal sintering paste is, for example, 0 to 5% by weight relative to the original fraction of the organic solvent in the metal sintering paste, that is, the metal sintering paste ready for application. In other words, according to this preferred embodiment, for example, 95 to 100% by weight of the organic solvent originally present in the metal sintering paste is removed during drying. The drying temperature can be in the range of 100 to 150 °C, for example. The typical drying time is in the range of 5 to 45 minutes, for example.

[0025] When implementing the method of the present invention using a metal sintering preform, the application of the metal sintering paste to the metal contact surface of electronic component 1 or electronic component 2 can be carried out by simply placing the metal sintering preform between the metal contact surface of electronic component 1 and the metal contact surface of electronic component 2 that are connected to each other.

[0026] In the sandwich configuration of at least two electronic components 1 and 2 and a metal sintered preparation positioned between their metal contact surfaces, finally, a metal sintering process is carried out in step (b), and in this process, the sandwich configuration is sintered at normal pressure. Step (b) (i.e., at least sub-steps (b1) and (b2)) is typically carried out in a furnace, which may be a stationary furnace or a conveyor furnace. In the case of a stationary furnace, the sandwich configuration is placed in the furnace and undergoes a heating process until it reaches the peak temperature, i.e., the highest object temperature typically in the range of, for example, 200 to 250 °C. After the peak temperature stage, a cooling stage follows. In the case of a conveyor furnace, the sandwich configuration is transported through the furnace and undergoes a heating process until it reaches the peak temperature in the above-mentioned range of, for example, 200 to 250 °C during the transportation process. The furnace temperature may be constant during the transportation of the sandwich configuration passing through the furnace, or it may increase continuously or step by step.

[0027] As already disclosed, the metal sintering process in step (b) includes three consecutive, or in particular three directly consecutive stages or sub-steps (b1) to (b3), namely a heating sub-step (b1), a sub-step (b2) at the peak temperature within the above-mentioned range of, for example, 200 to 250 °C, and a cooling sub-step (b3). The cooling sub-step (b3) is typically carried out as passive cooling, i.e., without taking active cooling means. These sub-steps can be characterized by the following features.

[0028] [Table 1]

[0029] Regardless of the type of the metal contact surfaces of the connected electronic components 1 and 2, the actual normal pressure sintering process can proceed at the peak temperature in the above-mentioned range of, for example, 200 to 250 °C, regardless of the presence or absence of pretreatment of the noble metal or non-noble metal contact surfaces of the electronic components 1 and 2. Such a normal pressure sintering mechanism may already start in the heating stage.

Example

[0030] Heraeus Electronics mAgic (registered trademark) DA295A silver sintering paste was used in the example. Both the nitrogen atmosphere and the air atmosphere each represented the ambient gas atmosphere at standard pressure in the sense of the present disclosure.

[0031] Application and atmospheric pressure sintering The silver sintering paste was applied by pouring it onto the copper surface of a lead frame made of a copper / iron alloy with a high copper content (96 wt% copper, 4 wt% Fe) to form a wet layer 50 μm thick. Subsequently, the applied silver sintering paste was brought into contact with the silicon chip through its 2 mm × 2 mm silver metal contact surface without prior drying. Subsequent atmospheric pressure sintering was carried out according to the temperature profile shown in Table 1. The nitrogen atmosphere contained oxygen at 200 ppm by volume or less. The sandwich configuration including the contact site was continuously heated at a uniform heating rate to 250°C over 60 minutes and then maintained at 250°C for a selected duration. Subsequently, the sandwich configuration thus formed was continuously cooled to 30°C over 50 minutes. The ambient gas atmosphere was exchanged by purging according to Table 1.

[0032] After sintering, the bonding strength was determined by a shear test. In this context, the electronic component was sheared using a shear chisel at a speed of 0.3 mm / second at room temperature and 260°C. The force was measured by a load cell (DAGE 4000 plus device made by DAGE, Germany). Table 2 shows the results obtained using the temperature profiles of Examples 1 to 5.

[0033]

Table 2

[0034]

Table 3

Claims

1. A method for connecting electronic components, comprising: (a) providing a sandwich configuration including at least (a1) an electronic component 1, (a2) an electronic component 2, and (a3) a metal sintered preparation positioned between a metal contact surface of the electronic component 1 and a metal contact surface of the electronic component 2; (b) sintering the sandwich configuration at atmospheric pressure, the method being carried out in an ambient gas atmosphere, and the exchange of the ambient gas atmosphere being carried out during the process of the atmospheric pressure metal sintering process.

2. The method according to claim 1, wherein step (b) includes three consecutive sub-steps (b1) to (b3), namely a heating sub-step (b1), a sub-step (b2) at the peak temperature, and a cooling sub-step (b3), and at least sub-steps (b1) and (b2) are carried out in a furnace.

3. The method according to claim 1, wherein the electronic component 1 and / or the electronic component 2 has a contact surface of a non-noble metal, and the non-noble metal is copper, nickel or aluminum.

4. The method according to claim 1, wherein the metal sintered preparation is a metal sintered preform or a metal sintered paste.

5. The method according to claim 2, wherein the exchange of the ambient gas atmosphere is carried out either in sub-step (b2) or at the start of sub-step (b3).

6. The method according to claim 1, wherein the exchange of the ambient gas atmosphere means exchanging an ambient non-oxidizing gas atmosphere for an ambient oxidizing gas atmosphere, or vice versa.

7. The method according to claim 1, wherein the exchange of the ambient gas atmosphere is carried out once or more than once.

8. The method according to claim 2, wherein the exchange of the ambient gas atmosphere is carried out by purging the interior of the furnace, or by partially discharging only the ambient gas atmosphere in the furnace by injecting a gas or a gas mixture of different composition into the furnace.

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