Atmospheric pressure sintering method 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 sintering 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 sintering 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 in 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 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 connecting elements (e.g., clips).

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

[0009] It is disclosed that a silver sintered bond can be formed on a bare copper surface without applying mechanical pressure during the sintering process (i.e., at atmospheric pressure) using a sintered paste based on silver nanoparticles, or a sintered 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, KDTNgo, 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" One example is "For 200 / 40°C Thermal Cycling Reliability," PCIM Europe 2017.

[0010] Furthermore, numerous patent documents, such as International Publication Nos. 2018 / 206267(A1) and International Publication Nos. 2018 / 206162(A1), disclose atmospheric pressure metal sintering bond processes. Other patent documents disclose metal sintered preparations having compositions that enable, or explicitly enable, use in metal sintering bond processes. Examples of such patent documents include International Publication Nos. 2022 / 214228(A1), International Publication Nos. 2022 / 128177(A1), International Publication Nos. 2021 / 073803(A1) and International Publication Nos. 2020 / 057806(A1).

[0011] The object of the present invention is to provide a method for atmospheric pressure metal sintering for connecting electronic components in a reliable manner. The method is to be practically applicable using any metal sintering preparation, i.e., without the need to use a metal sintering preparation specifically adapted for connecting electronic components by atmospheric pressure metal sintering. As already mentioned, such metal sintering connections are made via the metal contact surfaces of the electronic components. In particular, the atmospheric pressure metal sintering methods found are well suited for reliably connecting electronic components, at least one of which has non-precious metal contact surfaces such as copper contact surfaces, at atmospheric pressure. Reliable connections must be formed during atmospheric pressure metal sintering, even on the untreated non-precious metal contact surfaces of the electronic components.

[0012] This revealed that the exchange of the ambient gas atmosphere during the atmospheric pressure metal sintering process for connecting electronic components has an unexpected and surprising positive effect on the strength of the connections between electronic components.

[0013] The present invention relates to a method for connecting electronic components, comprising (a) 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 surfaces of electronic component 1 and electronic component 2, and (b) a method for sintering the sandwich configuration at atmospheric pressure. The entire method of the present invention is carried out in an ambient gas atmosphere, and the exchange of the ambient gas atmosphere is carried out in the course of the atmospheric pressure metal sintering process, i.e., in the course of step (b).

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

[0015] For the present invention, it is essential that the ambient gas atmosphere is exchanged during the atmospheric pressure metal sintering process, i.e., during the metal sintering step (b) in which the sandwich structure is sintered at atmospheric pressure. More precisely, the ambient gas atmosphere may be exchanged in substep (b2) or at the start of substep (b3). "In substep (b2)" means "after at least 5 minutes of the duration of substep (b2) has elapsed," or "approximately halfway through substep (b2)," i.e., after 40-60% of the duration of substep (b2) has elapsed. Preferably, the ambient gas atmosphere is exchanged at the start of substep (b3).

[0016] The expression "exchange of ambient gas atmosphere" means the exchange or substitution of a first ambient gas or gas mixture with a second ambient gas or gas mixture of a different composition, or it may mean obtaining the second ambient gas mixture by changing the composition of the first ambient gas or gas mixture, i.e., the first ambient gas or gas mixture and the second ambient gas or gas mixture have different compositions. The first ambient gas (mixture) may be an oxidizing gas or a non-oxidizing gas (mixture), and therefore the second ambient gas (mixture) is a non-oxidizing gas or an oxidizing gas (mixture). The exchange of ambient gas atmosphere, i.e., the exchange of an ambient non-oxidizing gas atmosphere with an ambient oxidizing gas atmosphere, or vice versa, i.e., the exchange of an ambient oxidizing gas atmosphere with an ambient non-oxidizing gas atmosphere, is performed at least once. The exchange of ambient gas atmosphere may be performed more frequently than once, but typically and preferably, it is performed only once. The ambient gas atmosphere can be changed by purging the inside of the furnace, that is, by purging the inside of the furnace with a subsequent gas type. In another embodiment, the ambient gas atmosphere can be changed by partially purging the ambient gas atmosphere inside the furnace by injecting a gas or gas mixture of a different composition into the furnace, rather than completely purging the inside of the furnace, or, more precisely, by partially purging the first ambient gas or gas mixture inside the furnace by injecting a gas or gas mixture of a different composition into the furnace to obtain a second ambient gas mixture, i.e., a second ambient gas mixture of a desired composition. Purge is a preferred gas exchange method. Examples of non-oxidizing gas atmospheres include oxygen-free reducing gas atmospheres such as hydrogen / nitrogen or formic acid / nitrogen gas mixtures, and oxygen-free inert gas atmospheres composed of at least one inert gas, such as nitrogen, carbon dioxide, or a noble gas (e.g., argon). An oxygen-free gas atmosphere should be understood to mean a gas atmosphere with an oxygen content of 300 volume ppm (ppm per volume) or less, preferably 200 volume ppm or less, and more preferably 100 volume ppm or less.Examples of oxidizing gas atmospheres include a gas mixture of at least one inert gas containing 10 to 50 volume% (percent per volume) of oxygen, preferably 10 to 30 volume% of oxygen, and air is an example of a preferred oxidizing gas atmosphere.

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

[0018] With respect to electronic components and their metal contact surfaces, refer to those disclosed above. The method of the present invention is particularly advantageous when electronic component 1 and / or electronic component 2 have non-precious metal contact surfaces, in particular copper contact surfaces. Accordingly, in certain advantageous embodiments of the method of the present invention, at least one of the metal contact surfaces of the connected electronic component 1 and electronic component 2 is copper, and the copper may be pre-treated or not.

[0019] The connected electronic components 1 and 2 may be the same or different electronic components. As already stated, the present invention not only facilitates the connection between the precious metal contact surface of electronic component 1 and the precious metal contact surface of electronic component 2, but also facilitates the connection between non-precious metal contact surfaces, particularly copper contact surfaces, to the precious metal contact surface, or even the connection between two non-precious metal contact surfaces of electronic component 1 and electronic component 2 in a manner that provides a reliable connection at room temperature and high temperatures such as 200-260°C. For example, 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 connections can be formed. The term "metallic contact surfaces of aluminum, copper, nickel, silver, and gold" includes metallic contact surfaces made of alloys of the said metals. Similar to contact surfaces of silver, gold, or other precious metals, even in the case of non-precious metal contact surfaces such as copper, nickel, or aluminum surfaces, there is no need to pre-treat them before carrying out the sintering step (b), and in particular, no reduction treatment is required before carrying out the sintering step (b).

[0020] As used herein, the term "room temperature" refers to a temperature 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 sintered preparation is placed between the metal contact surface of electronic component 1 and the metal contact surface of 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 an appropriate manner such that electronic component 2 is positioned between electronic component 1 and electronic component 3. In this case, the metal sintered preparation is placed 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, and in this scenario, electronic component 2 naturally has two metal contact surfaces.

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

[0023] Metal sintered preparations for connecting electronic components are well known to those skilled in the art. Examples include metal sintered preforms, and in particular metal sintered pastes. A metal sintered preform is a foil-like piece of solidified, e.g., dried, unsintered, or partially sintered metal sintered paste. A metal sintered preform can be made by applying a metal sintered paste onto a flat carrier, drying the thus applied metal sintered paste without sintering or without complete sintering, and then peeling off the metal sintered paste. A metal sintered paste comprises metal particles, an organic solvent, and optionally, but typically, at least one additive, examples of which 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 does not have to be specifically adapted for connecting electronic components by atmospheric pressure metal sintering. However, the present invention may, of course, be specifically adapted for connecting electronic components by atmospheric pressure metal sintering. As the metal sintered preparation, copper sintered preparations, particularly silver sintered preparations, are preferred, and as the metal sintered preparation, copper sintered paste, particularly silver sintered paste, is most preferred. Examples of patent documents disclosing copper sintered preparations and silver sintered preparations, and their compositions, include, but only a few, International Publication Nos. 2020 / 057806(A1), International Publication Nos. 2011 / 026623(A1), International Publication Nos. 2016 / 028221(A1), U.S. Patent Application Publication Nos. 2017 / 0243849(A1), and U.S. Patent Application Publication Nos. 2018 / 0056449(A1).

[0024] When the method of the present invention is carried out 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 immersion. Preferably, the thickness of the wet 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 wet 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 wet layer depends on the method selected for applying the metal sintering paste. If the metal sintering paste is applied by, for example, screen printing or dispensing, the thickness of the wet layer may preferably be in the range of 50 to 80 μm. If the metal sintering paste is applied by stencil printing, the preferred thickness of the wet layer may be in the range of 50 to 100 μm. Once the metal sintering paste is applied, an optional drying step can be introduced before the sintering step (b), i.e., the organic solvent is removed from the applied metal sintering paste. The drying step can be performed before forming the sandwich configuration, i.e., 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 organic solvent in the metal sintering paste after drying is, for example, 0 to 5% by weight relative to the original fraction of organic solvent in the metal sintering paste, i.e., in 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, for example, in the range of 100 to 150°C. A typical drying time is, for example, in the range of 5 to 45 minutes.

[0025] When the method of the present invention is carried out using a metal sintered preform, the application of the metal sintered paste to the metal contact surface of electronic component 1 or electronic component 2 can be performed by simply placing the metal sintered 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 located between their metal contact surfaces, finally, a metal sintering process is performed in step (b), in which 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 body 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 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 may increase continuously or stepwise.

[0027] As already disclosed, the metal sintering process in step (b) includes three consecutive, or particularly 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 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 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 50-μm-thick wet layer. Then, 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 performed according to the temperature profile shown in Table 1. The nitrogen atmosphere contained oxygen at 200 volume ppm 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. Then, 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 at room temperature and 260°C at a speed of 0.3 mm / second using a shear chisel. 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 is provided, comprising: (a) 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) the sandwich configuration being sintered at atmospheric pressure, wherein step (b) comprises three consecutive substeps (b1) to (b3), namely a heating substep (b1), a substep at peak temperature (b2), and a cooling substep (b3), wherein at least substeps (b1) and (b2) are performed in a furnace. The method is carried out in an ambient gas atmosphere, wherein the exchange of the ambient gas atmosphere is performed either in substep (b2) or at the start of substep (b3), and the exchange of the ambient gas atmosphere means exchanging an ambient non-oxidizing gas atmosphere for an ambient oxidizing gas atmosphere, or vice versa.

2. The method according to claim 1, wherein electronic component 1 and / or electronic component 2 have contact surfaces of a non-precious metal, and the non-precious metal is copper, nickel, or aluminum.

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

4. The method according to claim 1, wherein the exchange of the ambient gas atmosphere is performed once or twice or more.

5. The method according to claim 1, wherein the exchange of the ambient gas atmosphere is performed by purging the inside of the furnace, or by partially discharging the ambient gas atmosphere inside the furnace by injecting a gas or gas mixture of a different composition into the furnace.

6. The method according to any one of claims 1 to 5, wherein the exchange of the ambient gas atmosphere is an exchange from an ambient non-oxidizing gas atmosphere to an ambient oxidizing gas atmosphere, and is performed once either in substep (b2) or at the start of substep (b3).

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