Pressureless sintering method for connecting electronic components

Abstract

The invention relates to a method for connecting electronic components, in which method (a) an interlayer arrangement is provided, which comprises at least (a1) an electronic component 1, (a2) an electronic component 2 and (a3) a metal sintering preparation located between the metal contact surfaces of the electronic component 1 and the electronic component 2, and in which method (b) the interlayer arrangement is sintered without pressure, wherein the method is carried out in an ambient gas atmosphere, and wherein the exchange of the ambient gas atmosphere is carried out during a pressureless metal sintering process.

Description

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

[0002] As used herein, the term "pressureless" means "no mechanical pressure is applied".

[0003] Sintering should be understood to mean connecting electronic components by heating, without the metal of the metal sintering preparation reaching the liquid phase. It goes without saying for those skilled in the art that the heating and sintering are carried out in an oven.

[0004] As used herein, the term "electronic component" refers to a substrate and active and passive components used in electronic devices. As is conventional for electronic components, they have metal contact surfaces. In the case where the electronic components are not made of metal, their contact surfaces are covered with a metal or metallized 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 brazing substrate), cermet substrate, such as DCB (direct copper bonding 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, baseplates, heat sinks, resistors, capacitors, inductors, antennas and connecting elements (e.g., clamps).

[0008] As used herein, the term "ambient gas atmosphere" means that the method of pressureless sintering for connecting the electronic components disclosed herein is of course not carried out in a vacuum; rather, it is carried out in a gas atmosphere, particularly in a gas atmosphere at standard pressure.

[0009] It has been disclosed that a sintering paste based on a mixture of silver nanoparticles or silver microparticles (usually with a particle size in the range of 1 μm to 10 μm) and silver nanoparticles (usually with a particle size in the range of 1 nanometer to 100 nanometers) can be used to form a silver sintered joint on a bare copper surface during the sintering process without applying mechanical pressure (i.e., pressureless). 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 (<5 MPa) 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] There are also many patent documents that disclose pressureless metal sintering connection methods, such as WO 2018 / 206267A1 and WO2018 / 206162 A1. Other patent documents disclose metal sintered preparations that have compositions that allow the metal sintered preparations to be used in metal sintering connection methods or even specifically in pressureless metal sintering connection methods. Examples of such patent documents include WO 2022 / 214228A1, WO 2022 / 128177 A1, WO 2021 / 073803 A1, and WO 2020 / 057806A1.

[0011] An object of the present invention is to provide a pressureless metal sintering method for connecting electronic components in a reliable manner. The method should be applicable to almost any metal sintered preparation, i.e., it is not necessary to use a metal sintered preparation that is particularly suitable for connecting electronic components by pressureless metal sintering. As already mentioned, such metal sintering connections occur via the metal contact surfaces of the electronic components. Specifically, the discovered pressureless metal sintering method should be very suitable for reliably pressurelessly connecting electronic components, where at least one electronic component has a non-precious metal contact surface such as a copper contact surface. During pressureless metal sintering, reliable connections should be able to be formed even on the unpretreated non-precious metal contact surfaces of the electronic components.

[0012] It has now been found that the exchange of the ambient gas atmosphere during the pressureless metal sintering process for connecting electronic components has an unexpected and surprisingly positive effect on the strength of the connection between the electronic components.

[0013] The present invention relates to a method for connecting electronic components, in which (a) a sandwich arrangement is provided that at least includes (a1) electronic component 1, (a2) electronic component 2, and (a3) a metal sintered preparation located between the metal contact surfaces of the electronic component 1 and the electronic component 2, and in which (b) the sandwich arrangement is sintered without pressure. The entire method of the present invention is carried out in an ambient gas atmosphere, where the exchange of the ambient gas atmosphere occurs during the pressureless metal sintering process, i.e., during step (b).

[0014] The metal sintering process of step (b) includes three consecutive or especially 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 maximum temperature of the sandwich arrangement), and a cooling sub-step (b3). The sub-step (b1), the sub-step (b2), and optionally also the sub-step (b3) are carried out in an oven.

[0015] It is important for the present invention that the exchange of the ambient gas atmosphere takes place during the pressureless metal sintering process, i.e., during the metal sintering step (b), in which the sandwich arrangement is sintered without pressure. More precisely, the exchange of the ambient gas atmosphere can take place within sub-step (b2) or at the start of sub-step (b3). "Within sub-step (b2)" means "after at least 5 minutes of the duration of sub-step (b2) has elapsed" or it can mean "near the middle of sub-step (b2)", i.e., after 40% to 60% of the duration of sub-step (b2) has 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 that the first ambient gas or gas mixture is exchanged or replaced by a second ambient gas or gas mixture of different composition, or it can mean that the composition of the first ambient gas or gas mixture is changed in order to obtain a second ambient gas mixture, i.e., the first ambient gas or gas mixture and the second ambient gas or gas mixture are different in composition. The first ambient gas (mixture) can be an oxidizing or non-oxidizing gas (mixture), and thus, the second ambient gas (mixture) is then a non-oxidizing or oxidizing gas (mixture). The exchange of the ambient gas atmosphere occurs at least once, i.e., the exchange of the ambient non-oxidizing gas atmosphere with the ambient oxidizing gas atmosphere, or vice versa, i.e., the exchange of the ambient oxidizing gas atmosphere with the ambient non-oxidizing gas atmosphere. The exchange of the ambient gas atmosphere can occur more than once; however, it usually and preferably occurs only once. The exchange of the ambient gas atmosphere can be carried out by purging the interior of the oven, i.e., by purging the interior of the oven with a subsequent gas type; however, in another embodiment, the exchange of the ambient gas atmosphere can be carried out by: not completely purging the interior of the oven, but by injecting a gas or gas mixture of different composition into the oven to only partially displace the ambient gas atmosphere in the oven, or more precisely by injecting a gas or gas mixture of different composition into the oven to only partially displace the first ambient gas or gas mixture in the oven in order to obtain a second ambient gas mixture, i.e., a second ambient gas mixture having a desired composition. Purging is the preferred gas exchange method. Examples of non-oxidizing gas atmospheres include an oxygen-free reducing gas atmosphere, such as a hydrogen / nitrogen or formic acid / nitrogen mixture, and an oxygen-free inert gas atmosphere consisting of at least one inert gas (e.g., nitrogen, carbon dioxide, noble gas (e.g., argon)). An oxygen-free gas atmosphere should be understood to mean a gas atmosphere whose oxygen content does not exceed 300 vol.ppm (volume ppm), preferably does not exceed 200 vol.ppm, and even more preferably does not exceed 100 vol.ppm. Examples of oxidizing gas atmospheres include gas mixtures of at least one inert gas that includes 10 vol.% to 50 vol.% (volume percentage) of oxygen, preferably 10 vol.% to 30 vol.% of oxygen; air is a preferred example of an oxidizing gas atmosphere.

[0017] Connecting an electronic component should be understood to mean attaching the electronic component to a second electronic component. In this context, "on" merely 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 these two electronic components or the sandwich arrangement including the at least two electronic components.

[0018] Regarding the electronic component and its metal contact surface, reference is made to the content already disclosed above. The use of the method according to the invention is particularly advantageous when the electronic component 1 and / or the electronic component 2 has a non-noble metal contact surface, in particular a copper contact surface. Thus, in a particular and advantageous embodiment of the method according to the invention, the metal of at least one of the metal contact surfaces of the electronic components 1 and 2 to be connected is copper, which may or may not be pretreated.

[0019] The electronic components 1 and 2 to be connected may be the same or different electronic components. As mentioned above, the invention is not only advantageous for connecting the noble metal contact surfaces of the electronic components 1 and 2, but is particularly advantageous for connecting a non-noble metal contact surface (in particular a copper contact surface) to a noble metal contact surface or even for connecting two non-noble metal contact surfaces of the electronic components 1 and 2 in such a way that a reliable connection is produced at room temperature and at high temperatures (such as 200 °C to 260 °C). For example, an aluminum-nickel connection, an aluminum-copper connection, an aluminum-silver connection, an aluminum-aluminum connection, an aluminum-gold connection, a copper-silver connection, a copper-nickel connection, a copper-copper connection, a copper-gold connection, a silver-nickel connection, a silver-gold connection, a silver-silver connection or a gold-gold connection can be formed. The term "aluminum, copper, nickel, silver and gold metal contact surfaces" includes metal contact surfaces made of alloys of said metals. As in the case of silver, gold or other noble metal contact surfaces, even in the case of non-noble metal contact surfaces such as copper, nickel or aluminum surfaces, there is no need to pretreat them before performing the sintering step (b), and in particular no reduction pretreatment is required before performing the sintering step (b).

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

[0021] Thus, if two electronic components (i.e., the electronic component 1 and the electronic component 2) are to be connected to each other, the metal sintering preparation is located between the metal contact surfaces of the electronic component 1 and the electronic component 2 before the sintering process of step (b) is carried out. On the other hand, it is conceivable to connect more than two electronic components to each other. For example, three electronic components (i.e., the electronic component 1, the electronic component 2 and the electronic component 3) can be connected to each other in a suitable manner such that the electronic component 2 is located between the electronic component 1 and the electronic component 3. In this case, the metal sintering preparation is located between both the electronic component 1 and the electronic component 2 and between the electronic component 2 and the electronic component 3, or more precisely, between their metal contact surfaces, where, in this case, the electronic component 2 of course has two metal contact surfaces.

[0022] Preferably, first, the metal contact surface of the electronic component 1 or the electronic component 2 is provided with the metal sintering preparation of the present invention. Therefore, then the other electronic component 2 or electronic component 1 is placed on the metal sintering preparation applied to the metal contact surface of the electronic component 1 or the electronic component 2 through its metal contact surface.

[0023] Metal sintering preparations for connecting electronic components are well known to those skilled in the art; examples include metal sintering preforms, and especially metal sintering pastes. A metal sintering preform is a foil-like sheet of a cured (e.g., dried) and unsintered or only partially sintered metal sintering paste. It can be made by applying the metal sintering paste to a flat carrier, drying the so-applied metal sintering paste without sintering it or without fully sintering it, and then peeling it off. The metal sintering paste contains metal particles, an organic solvent, and optionally but usually also includes at least one additive, examples of which may include metal precursors, wetting additives, dispersants, surfactants, defoamers, viscosity control (rheology) agents, and sintering agents. Any such conventional metal sintering preparation can be used in the method of the present invention. The metal sintering preparation does not need to be particularly suitable for connecting electronic components by pressureless metal sintering. However, it can of course be particularly suitable for connecting electronic components by pressureless metal sintering. Preferably as the metal sintering preparation is a copper sintering preparation, and especially a silver sintering preparation; most preferably as the metal sintering preparation is a copper sintering paste, and especially a silver sintering paste. Examples of patent documents disclosing copper sintering preparations and silver sintering preparations and their components include WO 2020 / 057806 A1, WO2011 / 026623A1, WO2016 / 028221A1, US2017 / 0243849A1, and US2018 / 0056449A1, to name just a few.

[0024] In the case of using a metal sintering paste for the method of the present invention, its application on the metal contact surface of the electronic component 1 or the electronic component 2 can be carried out by means of conventional methods. Examples include dispensing, screen printing, stencil printing, needle board transfer, and dip coating. Preferably, the thickness of the wet layer of the metal sintering paste between the electronic components 1 and 2 to be connected is in the range of 50 μm to 150 μm. In this context, the thickness of the wet layer should be understood to mean the layer thickness before drying (if any) and before the sintering step (b). The preferred wet layer thickness depends on the method selected for applying the metal sintering paste. If, for example, the metal sintering paste is applied by means of screen printing or dispensing, the wet layer thickness can preferably be in the range of 50 μm to 80 μm. If the metal sintering paste is applied by means of stencil printing, the preferred wet layer thickness can be in the range of 50 μm to 100 μm. Once the metal sintering paste has been applied, an optional drying step can be introduced before the sintering step (b), i.e., then removing the organic solvent from the applied metal sintering paste. The drying step can be carried out before forming the sandwich arrangement, i.e., after applying the metal sintering paste to the metal contact surface of the electronic component 1 and before connecting to the metal contact surface of the electronic component 2, or after assembling the sandwich arrangement. According to a preferred embodiment, the fraction of the organic solvent in the metal sintering paste after drying is, for example, 0 wt% to 5 wt% relative to the initial fraction of the organic solvent in the metal sintering paste (i.e., in the metal sintering paste ready for application). In other words, according to the said preferred embodiment, for example, 95 wt% to 100 wt% of the organic solvent initially present in the metal sintering paste is removed during drying. The drying temperature can be in the range of, for example, 100 °C to 150 °C. The usual drying time is in the range of, for example, 5 minutes to 45 minutes.

[0025] In the case of using a metal sintering preform for the method of the present invention, its application to the metal contact surface of the electronic component 1 or the electronic component 2 can be carried out by simply placing it between the metal contact surfaces of the electronic components 1 and 2 to be connected to each other.

[0026] A sandwich arrangement of at least two electronic components (electronic component 1 and electronic component 2) and a metal sintered preparation located between their metal contact surfaces are finally subjected to a metal sintering process in step (b), during which the sandwich arrangement is sintered without pressure. Step (b), i.e., at least sub-steps (b1) and (b2), is usually carried out in an oven, which can be a stationary oven or a conveyor oven. In the case of a stationary oven, the sandwich arrangement is placed in the oven 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 °C to 250 °C. After the peak temperature stage, a cooling stage follows. In the case of a conveyor oven, the sandwich arrangement is conveyed through the oven, and during this conveying process, the sandwich arrangement undergoes a heating process until the sandwich arrangement reaches its peak temperature in the aforementioned range of, for example, 200 °C to 250 °C, where during the conveyance of the sandwich arrangement through the oven, the oven temperature can be constant or can increase constantly or step by step.

[0027] As already disclosed, the metal sintering process of step (b) includes three consecutive stages or especially three directly consecutive stages or sub-steps (b1) to (b3), namely a heating sub-step (b1), a sub-step (b2) at the peak temperature in the aforementioned range of, for example, 200 °C to 250 °C, and a cooling sub-step (b3). The cooling sub-step (b3) is usually carried out as passive cooling, i.e., no active cooling measures are taken. The characteristics of these sub-steps can lie in the following features:

[0028]

[0029] Regardless of the type of metal contact surfaces of the electronic components 1 and 2 to be connected, the actual pressureless sintering process can be carried out at the peak temperature in the aforementioned range of, for example, 200 °C to 250 °C, with or without pretreatment of the noble metal or non-noble metal contact surfaces of the electronic components 1 and 2. The pressureless sintering mechanism itself can already start in the heating stage. Example:

[0030] Heraeus Electronics was used in the example DA295A silver sintering paste. In the sense of the present invention, both a nitrogen atmosphere and an air atmosphere represent the ambient gas atmosphere under standard pressure.

[0031] Pressureless Sintering and Pressure Application :

[0032] The silver sintering paste is applied to the copper surface of a lead frame made of a copper / iron alloy rich in copper (96 wt% copper, 4 wt% Fe) by dispensing to produce a wet layer 50 μm thick. Then, the applied silver sintering paste is brought into contact with the silicon chip via its 2 mm × 2 mm silver metal contact surface without prior drying. The subsequent pressureless sintering is carried out according to the temperature profile shown in Table 1. The nitrogen atmosphere contains ≤200 vol.ppm of oxygen. The sandwich arrangement including the contact site is steadily heated to 250 °C at a uniform heating rate over a period of 60 minutes and then held at 250 °C for a selected duration. Thereafter, the sandwich arrangement thus formed is steadily cooled to 30 °C over a period of 50 minutes. According to Table 1, the exchange of the ambient gas atmosphere is carried out by purging.

[0033] After sintering, the bond strength is determined by shear testing. In this context, a shear chisel is used to cut the electronic component at room temperature and at 260 °C at a rate of 0.3 mm / s. The force is measured by a load cell (DAGE4000plus device manufactured by DAGE, Germany). Table 2 shows the results obtained using the temperature profiles of Examples 1 to 5.

[0034] Table 1

[0035]

[0036] Table 2

[0037]

Claims

1. A method for connecting electronic components, in which (a) a sandwich arrangement is provided, the sandwich arrangement comprising at least (a1) an electronic component 1, (a2) an electronic component 2, and (a3) a metal sintered preparation located between the metal contact surfaces of the electronic component 1 and the electronic component 2, and in which (b) the sandwich arrangement is sintered without pressure, wherein the method is carried out in an ambient gas atmosphere, and wherein the exchange of the ambient gas atmosphere is carried out during the pressureless metal sintering process.

2. The method according to claim 1, wherein step (b) comprises 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), wherein at least the sub-step (b1) and the sub-step (b2) are carried out in an oven.

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

4. The method according to claim 1, 2, or 3, wherein the metal sintered preparation is a metal sintered preform or a metal sintered paste.

5. The method according to claim 2, 3, or 4, wherein the exchange of the ambient gas atmosphere is carried out within the sub-step (b2) or at the start of the sub-step (b3).

6. The method according to any one of the preceding claims, wherein the exchange of the ambient gas atmosphere means an exchange between an ambient non-oxidizing gas atmosphere and an ambient oxidizing gas atmosphere, or vice versa.

7. The method according to any one of the preceding claims, wherein the exchange of the ambient gas atmosphere occurs once or more than once.

8. The method according to any one of claims 2 to 7, wherein the exchange of the ambient gas atmosphere is carried out by purging the interior of the oven, or by injecting a gas or gas mixture of different components into the oven to only partially displace the ambient gas atmosphere in the oven.

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