A thermopile heat flux sensor

Abstract

The present invention proposes a thermopile heat flux density sensor (HFS) based on a printed circuit board (PCB) process for monitoring the heat flux density of a chip on a board. The FR4 substrate of the PCB is used as a thermal resistance layer. After processing multiple pairs of parallel staggered copper wires on both sides of the PCB substrate, nickel is plated on one side of the center line of the copper wire to form a nickel-clad copper wire to form multiple thermocouples. The thermocouples are connected in series through vias to form a thermopile evenly distributed on both sides of the thermal resistance layer. The starting and ending positions of the nickel layer of the nickel-clad copper wire on both sides of the substrate constitute a hot junction and a cold junction respectively. Since there is a temperature gradient on both sides of the FR4 where the hot and cold junctions of the thermopile composed of the copper wire and the copper-plated nickel wire are located, according to the Seebeck effect, the thermopile will generate a thermoelectric potential proportional to the temperature difference on both sides of the FR4. The thermoelectric potential can be obtained by a signal acquisition circuit to realize heat flux density sensing. The thermopile heat flux density sensor has the potential to be conveniently and flexibly embedded in an electronic system, and completes the measurement of the heat flux density of electronic devices such as chips under the premise of minimizing the influence on the temperature field of the electronic system.

Description

Technical Field

[0001] The invention belongs to the technical field of heat flux measurement. Background Art

[0002] With the development of digital chips towards low process and high integration and the application of third-generation semiconductor material GaN in the manufacture of high-power and high-heat flux density chips, heat flux density, as an important parameter of the heat transfer process, is becoming increasingly important in the design of electronic systems for its precise measurement and heat dissipation control. As the requirements for electronic system functions and data throughput continue to increase in industrial sites, the collaborative work of multiple high-power and high-heat flux density chips is also being applied in more and more electronic systems, which further increases the overall temperature of the electronic system and makes the temperature field distribution more complicated. As the temperature of electronic devices such as chips increases, the stability and reliability of the electronic system will gradually decrease. Therefore, the temperature measurement of electronic devices such as chips has been widely studied.

[0003] In electronic devices such as chips, most of the electrical energy is dissipated in the form of heat energy. Heat flux sensors are used to measure the heat flux released by electronic devices such as chips to analyze the power dissipation of electronic systems and the working status of the system. Zhang et al. proposed a method for measuring the heat loss of power electronic devices using commercial thermoelectric modules as heat flux sensors (Local Heat Flux Measurement in a Permanent Magnet Motor at No Load. IEEE Transactions on Industrial Electronics, Vol 60, No 11, 4852-4860, Nov 2013). The heat flux measurement range is 0-6W / cm 2. This method is based on the Seebeck and Peltier thermoelectric effects. Its advantage is that it uses active control technology to minimize the net thermal impedance of the heat flux sensor, thereby reducing the impact of the sensor on the operating temperature of the device under test. Rencz et al. designed a heat flux density sensor for detecting the thermal characteristics of chip packages (Determination of the entropy change profile of a cylindrical lithium-ion battery by heat flux measurements, Journal of Power Sources, Vol 330, 61-69, Oct312016). By doping metal elements on both sides of the silicon wafer substrate to form thermocouples connected in series in opposite phases, the output thermoelectric potential is increased. The output potential is proportional to the heat flux density passing through the silicon substrate. Its biggest advantage is that it can form a heat flux density sensor array to accurately measure the horizontal distribution of the local heat flux density of the chip package. Demetrio et al. designed a heat flux density sensor (Heat flow sensor created by printed circuit board manufacturing processes, Proceedings of International Conference on Electronics Packaging, 459-464, Apr 2018) that can be used to measure chip switching losses. It uses a temperature sensor to measure the heat flux density through an alumina substrate. Its advantage is that the thermoelectric module is introduced to absorb the heat of electronic devices such as chips, keeping the equipment the same as the room temperature environment, thereby reducing convective heat transfer and improving the accuracy of measuring heat flow. On this basis, Demetrio et al. also proposed a method for measuring the heat loss value of a chip using a PATA (Proportional to absolute temperature) COMS sensor (Powerelectronics loss measurement using new heat flux sensor based on thermoelectric device with active control, IEEE Transactions on Industry Applications, Vol 50, No 6, 4098-4106, Dec 2014). By measuring its heat flow, the switching loss and conduction loss of the device under dynamic repetitive conditions can be measured.Cherry et al. (Cherry RJ, Cirenza CF, and Diller TE, Heat Flux Gage, US Patent 10,393,598, Aug 2019) obtained a patent for a heat flux density sensor. This heat flux density sensor is a thermocouple sensor, and the thermocouple constituting the thermocouple is composed of two different metals connected in series. Because a series of thermocouples are used, it cannot be processed using a PCB process. The processing of this heat flux density sensor can be achieved by bonding or depositing two different metal wires on a FR-4 substrate and connecting them in series to form a thermocouple. The metal wire attached to the FR4 substrate will form a protrusion, which seriously affects the reliable contact between the heat flux density sensor and the measuring surface. Although it is mentioned in the patent of this invention that FR-4 material can be used as a substrate for a heat flux density sensor, and FR-4 material is the preferred material for a PCB substrate, the processing and manufacturing of this heat flux density sensor cannot be achieved according to the PCB process. Elvira et al. proposed and studied the application of a gradient heat flux sensor in the measurement of local heat flux in wind power electronic systems (Application of a Heat Flux Sensor in Wind Power Electronics, Energies, Vol 9, No 6, 1-14, Jun 2016). The heat flux sensor was used to measure the thermal power consumption of the insulated gate field effect tube (IGBT) in real time online, so as to analyze the working condition of the system. However, the above-mentioned sensors have the disadvantages of complex processing technology or affecting the original temperature field of the chip. Patent document CN101819074 B discloses a method for manufacturing a thin-film heat flux sensor, which requires a vacuum ion sputtering method, has high process requirements and high cost. Patent document CN104359574A discloses a "fuel cell internal temperature-heat flux joint measurement sensor" which has five layers of material and requires a vacuum evaporation coating process. Not only is the structure complex, but the processing technology is also demanding. Summary of the invention

[0004] In view of the problems existing in the above-mentioned prior art, the present invention proposes a thermopile heat flux density sensor based on PCB process and a preparation method thereof, which is used for heat flux density monitoring of electronic devices such as chips in electronic systems, so that the sensor has excellent embeddability and can be easily integrated into the electronic system, and can minimize the impact on the original temperature field distribution in the electronic system while completing the online real-time measurement task.

[0005] The technical solution of the present invention is as follows:

[0006] A thermopile type heat flux density sensor, which uses the FR4 substrate of the PCB as the thermal resistance layer, processes multiple pairs of parallel and staggered copper wires on both sides of the PCB substrate, nickel-plates the half-segment of each copper wire to form nickel-clad copper wires to form multiple thermocouples, and connects the thermocouples on both sides of the FR4 substrate in series through vias to form a thermopile evenly distributed on both sides of the thermal resistance layer, wherein the starting positions of the nickel layers of the nickel-clad copper wires on both sides of the FR4 substrate constitute hot nodes and cold nodes respectively. The signal line of the entire sensor is led out from a pad placed on the PCB substrate and connected to the copper wire.

[0007] For the heat flux density sensor with the above structural design, since the thickness of the FR4 substrate is much smaller than its length and width, its edge effect and thus its edge heat dissipation can be ignored, and it is assumed that the heat flux is one-dimensionally conducted along the thickness direction of the sensor. Consider loading a uniform heat flux perpendicular to the surface of the heat flux density sensor. As the one-dimensional heat transfer process tends to be stable, it can be seen from the one-dimensional Fourier heat transfer law that a temperature gradient proportional to the heat flux density is formed on the upper and lower sides of the FR4 substrate. Since there is a temperature gradient on both sides of the FR4 where the hot and cold nodes of the thermopile composed of copper wires and copper-plated nickel wires are located, according to the Seebeck effect, the thermopile will generate a thermoelectric potential proportional to the temperature difference on both sides of the FR4. The heat flux density sensing can be achieved by acquiring the thermoelectric potential through the signal acquisition circuit.

[0008] In the present invention, the copper wires are designed to be arranged in parallel and staggered. The parallel spacing of the copper wires on the same surface and the staggered spacing on different surfaces should be as small as possible while ensuring that the copper wires are not short-circuited. Such a setting will improve the resolution and accuracy of the thermopile type heat flux density sensor.

[0009] In the thermopile heat flux density sensor of the present invention, the sensitivity of the heat flux density sensor is improved by increasing the thickness d of the thermal resistance layer FR4, or increasing the via distance l, or the nickel plating thickness.

[0010] In the present invention, the calculation formula of the heat flux q sensed by the thermopile heat flux sensor is:

[0011]

[0012] Among them, ρ Ni and ρ Cu are the resistivities of copper and nickel, S Ni and S Cu are the cross-sectional areas of copper and nickel respectively, w is the line width of the copper conductor, h is Ni is the thickness of the electroplated nickel layer, h Cu is the thickness of the copper wire, U is the output thermoelectric potential of the heat flux density sensor, N is the logarithm of the thermocouples that make up the thermopile, d is the thickness of the thermal resistance layer formed by the FR4 substrate, λ is the thermal conductivity of the FR4 substrate, and S is the Seebeck coefficient of the thermocouple composed of the selected metal.

[0013] By selecting the Seebeck coefficient S of the metal thermocouple, the width w of the copper wire, the via distance l, and the thickness h of the nickel plating Ni The probe coefficient C can be adjusted by the thickness d of the FR4 thermal resistance layer and the number N of thermocouples that make up the thermopile.

[0014]

[0015] The sensitivity k of the thermal pile heat flux density sensor is expressed as

[0016]

[0017] The smaller the probe coefficient, the higher its output sensitivity.

[0018] The present invention further provides a method for preparing a thermopile type heat flux density sensor, which comprises the following steps:

[0019] (a) A double-sided copper-clad board with the same geometric dimensions as the heat flux density sensor structure model designed by EDA software is selected, where FR4 is the insulating substrate;

[0020] (b) drilling holes at the via positions designed in the heat flux density sensor structure model to connect the upper and lower surfaces of the substrate;

[0021] (c) depositing a layer of chemical copper on the surface of the drilled hole wall to achieve electrical connection between the two sides of the FR4 substrate;

[0022] (d) applying a dry anti-corrosion film on the surface of the FR4 substrate by hot pressing;

[0023] (e) using the original film output by the EDA software as a mask for exposure;

[0024] (f) obtaining a photosensitive dry film that undergoes polymerization;

[0025] (g) developing;

[0026] (h) etching the copper-clad board, and then washing away the resist layer to obtain the non-copper-plated structure of the sensor designed in the EDA software;

[0027] (i) Protect the parts that do not require nickel plating;

[0028] (j) A uniform and dense nickel layer is plated on the copper wire.

[0029] The present invention proposes that the above-mentioned thermopile type heat flux density sensor structure based on PCB technology can accurately sense heat flux density and has the potential to be conveniently and flexibly embedded in electronic systems. It can complete the measurement of heat flux density of electronic devices such as chips under the premise of minimizing the impact on the temperature field of the electronic system. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG1(a) is a schematic diagram of the sensing principle and structure of a thermopile type heat flux density sensor based on PCB technology;

[0031] FIG1( b ) is a schematic diagram of the structure of a thermopile type heat flux density sensor based on PCB technology;

[0032] Figure 2(a) shows the structural parameters of the thermopile heat flux density sensor based on PCB technology;

[0033] FIG2( b ) is a top view of a thermopile type heat flux density sensor based on PCB technology;

[0034] FIG2(c) is a nickel-plated cross-sectional view of a thermopile type heat flux density sensor based on PCB technology;

[0035] Figure 3 This is the processing flow chart of the thermal pile type heat flux density sensor based on PCB process;

[0036] Figure 4(a) is the layout diagram of the heat flux density sensor based on PCB technology (front view)

[0037] Figure 4(b) is the layout diagram of the heat flux density sensor based on PCB technology (back side)

[0038] Figure 5 This is the curve of the change of thermoelectric potential of the heat flux density sensor based on PCB technology with the heat flux density

[0039] Markings in the figure: 1-thermopile type heat flux density sensor, 2-FR4 substrate, 3-copper wire on the board, 4-nickel-clad copper wire on the board, 5-via, 5-cold junction, 6-hot junction, 7-copper wire under the board, 8-cold junction, 9-nickel-clad copper wire under the board, 10-soldering pad, 11-nickel-clad copper via. DETAILED DESCRIPTION

[0040] The present invention is further described in detail below with reference to the accompanying drawings.

[0041] Example 1. Structure of a thermopile heat flux density sensor

[0042] The structure of the thermopile heat flux density sensor 1 based on PCB technology proposed by the present invention is shown in Figures 1(a) and 1(b). The heat flux density sensor uses the FR4 substrate 2 of the PCB as the thermal resistance layer. A plurality of pairs of parallel and staggered copper wires 3 are processed on both sides of the FR4 substrate, and then nickel is plated on one side of the midpoint of the copper wire to form a nickel-clad copper wire 4, forming a plurality of thermocouples. The nickel-clad copper wires 4 on both sides of the FR4 substrate are connected in series through the vias 5 and the nickel-clad copper vias 11 in the PCB to form a thermopile evenly distributed on both sides of the thermal resistance layer, thereby realizing heat flux density sensing. Among them, the starting positions of the nickel layers of the nickel-clad copper wires 4 on both sides of the substrate respectively constitute hot junctions 7 and cold junctions 8. The thermoelectric potential generated by the thermopile is output through the electrode 10 to realize heat flux density sensing.

[0043] In the above structure, since the thickness of the FR4 substrate is much smaller than its length and width, its edge effect and edge heat dissipation can be ignored, and it is assumed that the heat flow is one-dimensionally conducted along the thickness direction of the sensor. Consider loading a uniform heat flow perpendicular to the surface of the heat flux density sensor. As the one-dimensional heat transfer process tends to be stable, it can be seen from the one-dimensional Fourier heat transfer law that a temperature gradient proportional to the heat flux density is formed on the upper and lower sides of the FR4 substrate. Since there is a temperature gradient on both sides of the FR4 where the hot and cold nodes of the thermopile composed of copper wires and copper-plated nickel wires are located, according to the Seebeck effect, the thermopile will generate a thermoelectric potential proportional to the temperature difference on both sides of the FR4. The heat flux density sensing can be achieved by acquiring the thermoelectric potential through the signal acquisition circuit.

[0044] Figure 2(a) and 2(b) The structural parameters and top view of the thermal pile type heat flux density sensor are shown respectively. Since the copper wire constituting the thermocouple can be etched on the FR4 substrate through the PCB process, but nickel cannot be directly electroplated on the FR4 substrate, the copper wire is first processed on the FR4 substrate, and then the nickel layer is electroplated on it. The heat flux density sensor after nickel plating and its nickel-plated cross-section are shown in Figure 2(c), 4-1 is the nickel layer of the nickel-clad copper wire, and 4-2 is the copper wire of the nickel-clad copper wire.

[0045] The heat flux density q sensed by the heat flux density sensor is

[0046]

[0047] Where ρ Ni and ρ Cu are the resistivities of copper and nickel, S Ni and S Cu are the cross-sectional areas of copper and nickel respectively, w is the line width of the copper conductor, h is Ni is the thickness of the electroplated nickel layer, h Cuis the thickness of the copper wire. Where T1 and T0 are the temperatures on both sides of the thermal resistance layer formed by the FR4 substrate of the heat flux density sensor, d is its thickness, λ is the thermal conductivity of the FR4 substrate, and the negative sign indicates that the direction of heat transfer is opposite to the direction of temperature rise. Where S is the Seebeck coefficient of the thermocouple composed of copper wire and nickel wire.

[0048] Therefore, when the geometric structure, size parameters and manufacturing materials of the heat flux density sensor are determined, the heat flux density to be measured is proportional to the output thermoelectric potential.

[0049] The probe coefficient C of the thermal pile type heat flux density sensor based on PCB technology refers to the unit thermoelectric potential generated when the heat flux with a heat flux density of q (only its value is taken) is vertically loaded on the temperature sensing surface of the heat flux density sensor. According to formula (4), C can be expressed as

[0050]

[0051] The sensitivity k of the thermal pile heat flux density sensor based on PCB technology can be expressed as

[0052]

[0053] From the above analysis, the smaller the probe coefficient of the heat flux density sensor, the higher its output sensitivity. At the same time, the factors affecting the sensor probe coefficient include the Seebeck coefficient S of the selected metal thermocouple, the width w of the copper wire, the via distance l, the thickness h of the nickel plating, and the Ni And the thickness d of the FR4 thermal resistance layer and the number N of thermocouples that make up the thermopile.

[0054] Example 2. Preparation process of heat flux density sensor:

[0055] Based on theoretical analysis and simulation results, the present invention uses EDA software to design a thermopile type heat flux density sensor based on PCB technology. The processing flow of the thermopile type heat flux density sensor based on PCB technology is as follows: Figure 3 As shown:

[0056] (a) Select a double-sided copper-clad board with the same geometric dimensions (length, width, and height) as the heat flux density sensor structure model in the EDA software, where FR4 is the insulating substrate.

[0057] (b) Drill holes at the via positions on the board to connect the upper and lower surfaces of the substrate.

[0058] (c) A layer of chemical copper is deposited on the surface of the drilled hole wall by means of chemical copper (PTH) or the like to achieve electrical connection on both sides of the FR4 substrate.

[0059] (d) Apply a dry anti-corrosion film on the copper surface by hot pressing.

[0060] (e) The original film output by the EDA software is used as a mask and exposed to UV light. The portion covered by the mask does not undergo polymerization, while the portion not covered by the mask undergoes polymerization.

[0061] (f) Photosensitive dry film that undergoes polymerization.

[0062] (g) Using alkaline solution (such as K2CO3) to wash away the photosensitive dry film that has not undergone polymerization reaction is called development.

[0063] (h) The copper-clad board is etched with an etchant (such as CuCl2). The portion not covered by the anti-etching layer will be reacted. After etching, the anti-etching layer is washed away with a strong base (such as NaOH). At this point, the uncopper-plated structure of the sensor designed in the EDA software is completed.

[0064] (i) The parts that do not need to be electroplated are protected by wax sealing or other methods, and the parts that need to be nickel-plated do not need to be treated.

[0065] (j) A uniform and dense nickel layer is plated on the copper wire by electroplating.

[0066] The front and back sides of the layout of the thermal pile type heat flux density sensor based on the PCB process processed according to the above steps are as follows Figure 4(a) and 4(b) shown.

[0067] Example 3. Verification experiment

[0068] The calibration and testing experiments of the processed heat flux density sensor mainly test the relationship between the output thermoelectric potential of the thermopile heat flux density sensor based on the PCB process and the heat flux density loaded on its surface. The experimentally measured curve of the thermoelectric potential of the thermopile heat flux density sensor based on the PCB process and the change of heat flux density is shown in the figure below. Figure 5 As shown in Figure 2, the nonlinear error of the sensor is 1.59%. The smaller nonlinear error indicates that the relative deviation between the output value of the heat flux density sensor and the fitting value is smaller, and the measurement linearity of the sensor is higher.

[0069] It can be seen from the above embodiments that the thermopile heat flux sensor (Heat flux sensor, HFS) based on the printed circuit board (PCB) process and which can be used for monitoring the heat flux density of the chip on the board proposed by the present invention has the following characteristics:

[0070] 1. The thermopile heat flux density sensor can realize heat flux density sensing.

[0071] 2. The thicker the FR4 thermal resistance layer, the greater its sensitivity, but its response time increases; the sensor sensitivity increases with the increase of the via distance of the PCB; the thicker the nickel plating thickness, the greater its sensitivity, but due to the thickening of the nickel plating layer, the temperature of the thermocouple hot junction is degraded, and the sensitivity is slightly reduced.

[0072] 3. Since the heat flux density sensor is all processed by PCB preparation technology, it can be easily and flexibly embedded in the electronic system, and can measure the heat flux density released by the chip to the PCB with little impact on the chip thermal field. The experimental results show that the heat flux density sensor can measure the heat flux density transmitted from the chip to the PCB and then obtain the heat transferred from the chip to the PCB.

[0073] 4. Since the thermopile heat flux density sensor is manufactured using PCB technology, it has the characteristics of low cost, simple processing technology, mass production and compatibility with existing electronic devices. It can be widely used in thermal management of electronic devices such as chips.

Claims

1. A thermopile heat flux density sensor, characterized in that: The thermopile heat flux density sensor is used for monitoring the heat flux density of chips in electronic systems. The FR4 substrate of the PCB is used as the thermal resistance layer. A plurality of pairs of parallel and staggered copper wires are processed on both sides of the FR4 substrate by using the PCB preparation process. The half-segment of each copper wire is nickel-plated to form a nickel-clad copper wire to form a plurality of thermocouples. The thermocouples on both sides of the FR4 substrate are connected in series through vias to form a thermopile evenly distributed on both sides of the thermal resistance layer. The starting positions of the nickel layers of the nickel-clad copper wires on both sides of the FR4 substrate respectively form a hot junction and a cold junction. Since there is a temperature gradient on both sides of the FR4 where the hot and cold junctions of the thermopile formed by the copper wire and the nickel-clad copper wire are located, it can be known from the Seebeck effect that the thermopile will generate a thermoelectric potential proportional to the temperature difference on both sides of the FR4. The heat flux density sensing can be realized by acquiring the thermoelectric potential through a signal acquisition circuit. The signal line of the entire sensor is led out from a pad placed on the FR4 substrate and connected to the copper wire.

2. The thermopile heat flux density sensor according to claim 1, characterized in that: The effect of the parallel staggered copper wires is equivalent to winding a copper wire around two sides of the FR4 substrate to sense the temperature of the two sides of the FR4 substrate to obtain an induced potential, thereby obtaining heat flux density information of the entire FR4 substrate surface.

3. The thermopile heat flux density sensor according to claim 1, characterized in that: The sensitivity of the thermopile heat flux density sensor can be improved by increasing the thickness d of the thermal resistance layer FR4, or increasing the via distance l, or the nickel plating thickness.

4. The thermopile heat flux density sensor according to claim 1, characterized in that: The calculation formula of the heat flux q sensed by the thermopile heat flux sensor is: Among them, ρ Ni and ρ Cu are the resistivities of copper and nickel, S Ni and S Cu are the cross-sectional areas of copper and nickel respectively, w is the line width of the copper conductor, h is Ni is the thickness of the electroplated nickel layer, h Cu is the thickness of the copper wire, U is the output thermoelectric potential of the heat flux density sensor, N is the logarithm of the thermocouples that make up the thermopile, d is the thickness of the thermal resistance layer formed by the FR4 substrate, λ is the thermal conductivity of the FR4 substrate, and S is the Seebeck coefficient of the thermocouple composed of the selected metal.

5. The thermopile heat flux density sensor according to claim 1, characterized in that: By selecting the Seebeck coefficient S of the metal thermocouple, the width w of the copper wire, the via distance l, and the thickness h of the nickel plating Ni The probe coefficient C is adjusted according to the thickness d of the FR4 thermal resistance layer and the number N of thermocouples that make up the thermopile. The sensitivity k of the thermal pile heat flux density sensor is expressed as The smaller the probe coefficient, the higher its output sensitivity.

6. The method for preparing the thermopile heat flux density sensor according to any one of claims 1 to 5, characterized in that: The steps include: (a) A double-sided copper-clad board with the same geometric dimensions as the heat flux density sensor structure model designed by EDA software is selected, where FR4 is the insulating substrate; (b) drilling holes at the via positions designed in the heat flux density sensor structure model to connect the upper and lower surfaces of the substrate; (c) depositing a layer of chemical copper on the surface of the drilled hole wall to achieve electrical connection between the two sides of the FR4 substrate; (d) applying a dry anti-corrosion film on the surface of the FR4 substrate by hot pressing; (e) using the original film output by the EDA software as a mask for exposure; (f) obtaining a photosensitive dry film that undergoes polymerization; (g) developing; (h) etching the copper-clad board, and then washing away the resist layer to obtain the non-copper-plated structure of the sensor designed in the EDA software; (i) Protect the parts that do not require nickel plating; (j) A uniform and dense nickel layer is plated on the copper wire.

7. Application of the thermopile heat flux density sensor according to any one of claims 1 to 5 in chips and electronic devices.

8. Use of the thermopile heat flux density sensor according to any one of claims 1 to 5 as an embeddable heat flux density sensor.

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