Molybdenum sheet-based TVS diode

Abstract

The invention discloses a TVS diode based on a molybdenum sheet, the diode comprises a circular silicon chip, a molybdenum sheet supporting layer, a pentavalent lead-free nanometer solder connecting layer and a silica gel packaging layer, the chip and the molybdenum sheet are connected through Sn-Bi-Cu-Ag-In lead-free solder, and Mg fluxing agent and nanometer aluminum oxide particles are added into the solder to improve the welding strength and the thermal stability. The molybdenum sheet has a thermal expansion coefficient close to that of a silicon material and can effectively buffer thermal stress and prevent the chip from cracking, the round chip structure optimizes stress distribution and improves the impact resistance, flexible silica gel is adopted for packaging the outside, good insulativity, high and low temperature resistance and corona inhibition capacity are achieved, and the high-power LED packaging structure is compact in structure, good in packaging compatibility and high in reliability. The surge suppression circuit is suitable for high-reliability surge suppression occasions.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor device packaging, in particular to a TVS diode based on a molybdenum sheet. Background Art

[0002] Transient voltage suppressor (TVS) diodes are a type of semiconductor device commonly used for circuit surge protection. They feature fast response, strong suppression capabilities, and low residual voltage, making them widely used in communications, power supplies, automotive electronics, and consumer electronics. Traditional TVS diodes typically use a silicon chip as the core protection element, diverting high-energy transient surge currents to ground through conduction shunting, thereby protecting downstream circuits.

[0003] At present, mainstream TVS devices still have certain limitations in terms of structure and packaging. First, in order to adapt to the requirements of automatic placement and high-density packaging, square chip structures are mostly used. However, the square structure is prone to stress concentration at the four corners under loads such as thermal expansion and mechanical shock, increasing the risk of chip edge cracks and interface failure. Secondly, the chip and the electrode are mostly connected through traditional Sn-Pb or Sn-Ag solder, which has a high melting point, limited thermal fatigue performance, and the Pb content does not comply with RoHS environmental protection regulations. In addition, in order to improve the thermal conductivity of the device, some solutions introduce copper support structures, but due to the large difference in thermal expansion coefficients between copper and silicon, it is easy to induce silicon wafer cracking or solder joint detachment during terminal use due to temperature changes in welding or large changes in ambient temperature. Therefore, it is necessary to design a TVS diode based on molybdenum sheets. Summary of the Invention

[0004] The object of the present invention is to provide a TVS diode based on a molybdenum sheet to solve the problems raised in the above background technology.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions: a TVS diode based on a molybdenum sheet, comprising a circular silicon chip for transient voltage suppression, wherein the silicon chip has a circular structure, optimizes stress distribution, and prevents cracks caused by thermal expansion mismatch; The molybdenum sheet support layer is arranged below the silicon chip. The thermal expansion coefficient of the molybdenum sheet matches that of the silicon chip, effectively alleviating thermal stress and enhancing the mechanical strength of the chip. A solder layer is located between the silicon chip and the molybdenum support layer. The solder is a lead-free five-element alloy composed of Sn, Bi, Cu, Ag and In. The melting point of the solder ranges from 190°C to 210°C and can provide a reliable electrical and thermal connection. An electrode layer is provided on the upper surface of the silicon chip and is used for connecting to an external circuit. The electrode layer is welded to the circuit by low-temperature welding with silver paste; The silicone encapsulation layer covers the silicon chip, molybdenum sheet and solder layer. The silicone is a room temperature curing silicone material with good electrical insulation, high temperature resistance and corona resistance, and can work stably for a long time under high temperature, humidity and mechanical stress conditions.

[0006] According to the above technical solution, the thickness of the molybdenum sheet is 0.3 mm to 1 mm, and it has good heat conduction capability and stable structural strength, ensuring the long-term reliability of the equipment after welding.

[0007] According to the above technical solution, the melting point of the solder layer is 195.68°C, the melting range is 17.20°C, and 0.5% of nano-alumina strengthening phase (particle size 80~120nm) and 0.2% of Mg flux are added to improve the mechanical properties and thermal fatigue resistance of the solder joint.

[0008] According to the above technical solution, the thickness of the silicone encapsulation layer is 0.5mm to 1.5mm, and the silicone layer has a dielectric strength of more than 25kV / mm, which can effectively suppress the corona effect and protect the internal chip from the influence of the external environment.

[0009] According to the above technical solution, the diameter of the circular silicon chip is 5 mm to 15 mm. The circular structure can optimize stress distribution and reduce the risk of cracks in the chip under thermal cycling and thermal shock conditions.

[0010] According to the above technical solution, the TVS diode can withstand a surge current impact of 3KV-10KV and 8 / 20μs, and can operate stably within a temperature range of -40°C to +125°C, and is suitable for various high-reliability electronic protection occasions.

[0011] Compared with the prior art, the beneficial effects achieved by the present invention are as follows: the present invention, by providing a molybdenum sheet support layer and a silicone encapsulation structure, effectively alleviates the thermal stress concentration problem caused by thermal expansion mismatch in traditional TVS diodes, and significantly improves the thermal cycle stability and impact resistance of the chip; at the same time, the use of a circular silicon chip design optimizes stress distribution and reduces the risk of cracks caused by high-temperature welding; by optimizing the solder ratio, the mechanical properties and thermal fatigue resistance of the solder joints are improved, ensuring the long-term reliability of the diode in harsh working environments; in addition, the silicone encapsulation not only provides excellent electrical insulation, but also effectively enhances the diode's anti-corona ability and mechanical protection, thereby improving the overall performance and service life of the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings: Figure 1It is a flow chart of the method steps of TVS diode based on molybdenum sheet; Figure 2 This is a cross-sectional diagram of a TVS diode based on a molybdenum sheet; DETAILED DESCRIPTION

[0013] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0014] Example 1: This embodiment provides a TVS diode based on a molybdenum sheet. The diode structure is provided with a high-purity forged molybdenum sheet with a diameter of 10 mm and a thickness of 1 mm on the back of a circular silicon TVS chip. The molybdenum sheet is used to provide thermal insulation during reflow soldering and operating temperature changes by having a thermal expansion coefficient similar to that of the silicon chip (≈8.6×10⁻). 6 / K compared to silicon ≈2.6×10⁻ 6 / K) to achieve thermal stress buffering, while relying on the high hardness (HV ≥ 180) and high melting point (> 2600 ° C) of the molybdenum sheet to provide mechanical support and stability. The solder layer is filled on the contact surface between the molybdenum sheet and the chip to form an electrothermal dual connection interface, and the bonding strength is enhanced by silver plating or solder wetting on the back. This structure is compatible with various package types such as TO, SOD, DO, and is easy to align and install during the automated patch process. In the –40 ° C / 125 ° C temperature cycle 500 times and 6KV, 8 / 20us surge impact test, the device did not show interface micro cracks, the solder joint resistance change was less than 5%, the leakage current was ≤ 0.5μA, and there was no delamination under the mechanical vibration test. The overall performance is improved by about 20% compared with the conventional structure; Example 2: In order to optimize the function of the molybdenum sheet supporting the thermal buffer structure in Example 1 and improve the bonding strength and heat conduction efficiency between the molybdenum sheet and the TVS wafer, this embodiment, based on the previous embodiment, proposes a method for preparing the molybdenum sheet and stacking it with the TVS wafer. The molybdenum sheet material is made of high-purity forged molybdenum plate, which is prepared through the following process: First, the molybdenum plate is cut into circular sheets with a diameter of 10mm and a thickness of 1mm, and ultrasonically cleaned with alkaline degreasing solution for 5 minutes to remove surface oil; then, it is placed in a vacuum plating tank and electrochemically silver-plated with a silver salt solution. The current density is controlled at 2A / dm², the deposition thickness is 1μm, the coating is uniform and pinhole-free, and after the silver plating of the molybdenum sheet is completed, it is stacked with the diffusion-treated TVS wafer in the order of "silver-plated molybdenum sheet-diffused TVS wafer-silver-plated molybdenum sheet" from top to bottom until the outermost layer is a silver-plated molybdenum sheet. Customized graphite positioning gaskets are used during the stacking process to keep the concentricity deviation of each layer ≤10μm, and an appropriate amount of nano-solder powder is sprayed between each two layers to assist wetting and interface bonding. After stacking is completed, the entire body is placed in a box-type high-temperature furnace and, under nitrogen protection, the alloy is sintered according to the following curve: first, the temperature is raised to 200°C at 5°C / min and held for 30 minutes for preheating and dehydration. Then, the temperature is raised to 400°C at 2°C / min and held for 60 minutes to complete the initial metallurgical diffusion. Finally, the temperature is raised to 800°C at 1°C / min and held for 120 minutes to form a dense alloy interface between the silver coating, molybdenum sheet, and TVS wafer. After cooling to room temperature, the overall alloy sintered body is obtained. The sintered body is removed and placed in a wire saw. According to the requirements of the wafer column structure, it is cut into core column units with a diameter of about 0.5mm. The core columns are then screened using a vibrating screen and an optical inspection system to eliminate broken components or components with a diameter deviation of >±20μm. Finally, high-quality molybdenum-TVS wafer alloy core columns are obtained for subsequent welding and packaging processes.

[0015] Example 3: The solder layer described in Example 1 is composed of a five-metal alloy: Sn (80–92%), Ag (0.5–1%), Cu (0.1–0.7%), In (0.1–5%), and Bi (7–18%). Preferably, the ratio of Sn, Bi, Cu, Ag, and In in this example is 85% Sn, 10% Bi, 0.3% Cu, 0.5% Ag, and 4.2% In. To further lower the melting point and enhance the stability of the alloy structure, 0.2% magnesium (Mg) is added as a flux to adjust the eutectic point. Furthermore, to enhance the wettability and mechanical strength of the solder, 0.5% nano-alumina particles are added as a reinforcement phase in the late smelting process, with a particle size range of 80–120 nm.

[0016] The alloy preparation process is as follows: The weighed metal raw materials are placed in a high-purity graphite crucible and slowly heated to 270°C under a nitrogen atmosphere. The temperature is maintained at this temperature for 90 minutes while magnetically stirring. To prevent oxidation, an oxidation inhibitor is evenly applied to the melt surface. Once the melt is fully homogenized, the hot metal is cooled to form a powder using a gas atomization method. The spray pressure is controlled at 0.5 MPa, and the cooling drum temperature is maintained at 5°C. The resulting metal powder exhibits a fish-scale-like flaky structure. Scanning electron microscopy (SEM) analysis reveals an average particle size of 450 nm, with a D10-D90 distribution ranging from 300 to 700 nm.

[0017] After solder powder preparation, the target particle size range is achieved through screening and then distributed between the TVS chip and the molybdenum sheet using electrostatic spraying. A maximum of 0.1 mg of solder powder is applied to each device. The soldering process utilizes a standard low-temperature reflow process with a three-stage temperature profile: a preheating phase with the temperature raised to 150°C and held for 60 seconds, a reflow phase with the temperature raised to 200°C and held for 45 seconds, and a cooling phase with natural cooling to room temperature.

[0018] After the solder joints were formed, the solder joint morphology was observed using a laser confocal microscope, revealing uniform interface bonding with no apparent voids or microcracks. The tensile strength of the solder joints was tested using a tensile tester, with an average value of 114.6 MPa, an increase of more than 20% compared to traditional Sn-Pb solder. In thermal cycling reliability testing, the solder joint resistance did not change by more than 10% after 3,000 cycles at temperatures between –40°C and +125°C. Contact angle measurements showed that the solder had a wetting angle of 13° on the surface of the silver-coated molybdenum sheet, demonstrating rapid wetting and strong gap-filling capabilities.

[0019] Example 4: To further enhance the overall reliability of TVS diodes under surge and thermal stress conditions, this embodiment, based on the aforementioned structure, replaces the traditional square chip with a circular silicon chip structure and introduces high-performance silicone encapsulation materials. First, in terms of chip structure, traditional TVS chips often use a square design. While this facilitates processing, the square structure is prone to stress concentration at the four corners during high-temperature soldering and package cooling. This is particularly true when the device undergoes high-temperature cycling or mechanical shock, which can easily induce cracks at the chip edges, impacting electrical performance and structural stability. To overcome this problem, this embodiment uses a circular silicon TVS chip with a diameter of 15 mm and a thickness of 300 μm. This circular structure allows for more uniform stress distribution at the edges of the chip when the entire chip is subjected to heat or stress, effectively avoiding localized failures caused by sharp corner effects. Tests show that after 500 temperature cycles from –40°C to +125°C, the chip crack rate of the TVS diode using the circular chip is reduced by over 70% compared to that of the square chip, and solder joint debonding under thermal shock is also significantly reduced.

[0020] Secondly, circular chips also exhibit excellent photoresist uniformity during the semiconductor lithography process. During the spin-coating phase, the axial symmetry of circular chips allows for uniform photoresist spreading, preventing corner buildup or excessive thinning, thereby improving pattern transfer accuracy and yield.

[0021] Regarding packaging protection, to meet the device's sealing reliability requirements in high-humidity, high-voltage environments, this embodiment uses a room-temperature-curing silicone gel as the packaging material. This silicone gel offers excellent electrical insulation (dielectric strength greater than 25kV / mm), a wide temperature range from –50°C to +200°C, and good mechanical cushioning properties. In practice, the soldered TVS chip-molybdenum sheet structure is placed in a mold, and the silicone gel is injected via pneumatic injection with a thickness of 1.5mm to 2mm. The gel is then naturally cured at room temperature for 4 hours, forming a complete flexible encapsulation layer that covers the chip, electrodes, and some pins.

[0022] This silicone encapsulation not only prevents contaminants such as moisture and dust from entering the device, preventing metal oxidation or insulation failure, but also forms a flexible buffer layer when the device is subjected to mechanical vibration, electrical breakdown, and surge current shocks, absorbing and dissipating external energy, effectively improving the device's overall impact resistance and operational stability. More importantly, the silicone itself has excellent electric field homogenization capabilities, which can suppress the occurrence of high-voltage tip discharge and corona effects, enhancing the device's breakdown tolerance in high-voltage environments.

[0023] In actual samples, TVS devices fabricated using the circular chip and silicone encapsulation structure of this embodiment demonstrated superior performance compared to conventional structures in 3kV 8 / 20μs surge tests, damp heat tests (85°C, 85% RH, 168 hours), and mechanical drop tests. No cracks or peeling were observed on the device surface, leakage current remained below 0.2μA, and the package exhibited no bubbling or adhesive seepage, significantly improving reliability.

[0024] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus.

[0025] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A TVS diode based on a molybdenum sheet, characterized by: include: round silicon chips; A molybdenum sheet support layer, wherein the molybdenum sheet is arranged below the circular silicon chip, and the two are connected by solder; The solder layer is a Sn-Bi-Cu-Ag-In five-element lead-free alloy.

2. The TVS diode based on a molybdenum sheet according to claim 1, characterized in that: The solder contains 0.5% of nano-aluminum oxide particles with a particle size of 80-120 nm.

3. The TVS diode based on a molybdenum sheet according to claim 1, characterized in that: 0.2% of Mg element is further added to the solder as a flux.

4. The TVS diode based on a molybdenum sheet according to claim 1, characterized in that: The welding temperature between the molybdenum sheet and the circular silicon chip is controlled within a range of 230° C. to 270° C., and an anti-oxidation solvent is covered during the welding process.

5. The TVS diode based on a molybdenum sheet according to claim 1, characterized in that: The upper and lower surfaces of the molybdenum sheet are both plated with a silver layer with a thickness of 1 μm.

6. The TVS diode based on a molybdenum sheet according to claim 1, characterized in that: The solder consists of the following mass percentages: Sn: 85%, Bi: 10%, Cu: 0.3%, Ag: 0.5%, In: 4.2%, and contains 0.2% of Mg and 0.5% of nano-aluminum oxide.

7. The TVS diode based on a molybdenum sheet according to claim 6, characterized in that: The melting point of this solder is 195.68°C and the melting range is 17.20°C.

8. The TVS diode based on a molybdenum sheet according to claim 1, characterized in that: The TVS chip and its molybdenum sheet support layer are entirely covered by a layer of silica gel material.

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