Thermal device

Abstract

A thermal device for simulating and testing cooling solutions for integrated circuits, comprising a contact surface for mounting one heatsink, or more heatsinks, or one or more cooling device, a plurality of cells having each a thermal actuator configured to convert a current flowing through the input conductor into heat, a control circuit configured to obtain a desired power distribution in the cells simulating a power field and distribute electric current coming from a power source to the thermal actuators of the cells such that values of electric power dissipated in the cells match the desired power distribution.

Description

Thermal deviceTechnical domain

[0001] Embodiments of the present invention concerns thermal devices for testing electronic components, also known as thermal test vehicles.Related art

[0002] Thermal test vehicles are used in the art to develop and test cooling solutions for electronic components. In simple realisations, they may consist of a chip with a thermal surface (or contact surface) simulating a thermal interface in a target electronic component. The chip contains an array of power resistors that are accessible by suitable electric contacts, for example solder balls. By connecting the individual resistors to suitable power supply units and carefully choosing the voltages across the resistor, a user can simulate the distribution of dissipated power in the target component, test and validate cooling solutions, and so on.

[0003] Frequently, the thermal test vehicle includes temperature sensors as well as power sources, from which it is possible to infer the temperature distribution in the target device in a state of operation.

[0004] A limit of these thermal devices is that the desired distribution of dissipated power is achieved by hardwiring the power resistors to external power supplies. Accordingly, these solutions are not modular and are suitable to create a thermal test for a single specific component, or a special power state of a specific component. To simulate further configurations a complete rewiring of the system is necessary.

[0005] There is also a practical limit to the number of independent cells that can be used because, as the number of cells grows, the reconfiguration of the circuit becomes harder, more expensive and error prone.Short disclosure of the invention

[0006] An aim of the present invention is the provision of a device that overcomes the shortcomings and limitations of the state of the art or provides an alternative way to test cooling devices.

[0007] According to the invention, these aims are attained by the object of the attached claims, and especially by a thermal device for simulating and testing cooling solutions for integrated circuits, comprising a contact surface for mounting one heatsink, or more heatsinks, or one or more cooling device, a plurality of cells having each a thermal actuator configured to convert a current flowing through the input conductor into heat, a control circuit configured to obtain a desired power distribution in the cells simulating a power field and distribute electric current coming from a power source to the thermal actuators of the cells such that values of electric power dissipated in the cells match the desired power distribution.

[0008] Dependent claims relate with features of the invention that, while useful and important, are not essential. In this way, the thermal invention may involve a control circuit with a multichannel waveform generator that provides currents flowing in the thermal actuators of the cells. The waveform generator is controlled such that the average electric power dissipated in each cells matches the desired power distribution. Several modulation techniques can be used to this effect, including, but not limited to pulse-width modulation (PWM), pulse-frequency modulation (PFM), pulse-amplitude modulation (PAM), or a combination of the above. In general, any form of modulation in which the modulation value can be used to control the average electric power dissipated in each cell can be used in the invention.

[0009] It is advantageous, though not essential, that the modulated signal is a digital signal assuming either of two discrete values with sharp transitions therebetween. Switches capable of dealing with such signals are readily available and exhibit low power losses, even at high current levels.The currents flowing in the thermal actuators of the cells may be periodical, in the sense that they comprise trains of pulses that repeat cyclically. In this case the periodic cycle may have a frequency of at least 10 Hz and / or at most 10 kHz, preferably at least 50 Hz and / or at most 5 kHz, more preferably at least 100 Hz and / or at most 1 kHz.

[0010] The thermal actuators may be embodied by any suitable dissipative electronic element such as a power resistor, a transistor, or a subcircuit. For resistive actuators, the resistance may be at least 1 1 and / or at most 10 kfl, preferably at least 50 1 and / or at most 5 kfl, more preferably at least 100 1 and / or at most 1 kfl while the applied voltages, that is the maximum amplitude of the applied waveforms may be of at least 1 V and / or at most 250 V, preferably at least 5 V and / or at most 200 V, more preferably at least 10 V and / or at most 100 V. Cells' surfaces may be at least 0.25 mm2and / or at most 10 mm2, preferably at least 0.5 mm2and / or at most 7 mm2, more preferably at least 1 mm2and / or at most 5 mm2, while a complete thermal device may count any number of cells, preferably at least 4 and / or at most 2000, more preferably at least 10 and / or at most 1000, more preferably at least 25 and / or at most 500. The spatial resolution of the cell, which may be arranged in an orthogonal array, may be comprised at least 8 cells / mm2and / or at most 0.1 cells / mm2, preferably at least 4 cells / mm2and / or at most 0.25 cells / mm2, more preferably at least 2 cells / mm2and / or at most 0.5 cells / mm2. These figures are typical values suitable for the simulation of most integrated circuit devices, but they may be exceeded when the application requires it.

[0011] Preferably, some or all the cells include some form of temperature sensor, whereby the thermal distribution inside the array can be sampled. In some embodiments of the invention the device includes a readout circuit that acquires the temperature and reconstructs a sampled map of the temperature field in the device, which can be recorded or displayed on a screen, according to the needs. The temperature map may be refreshed periodically or whenever required, which allows to follow transient thermal phenomena in real time. The temperature sensors may be thermistors that is, resistive elements whose temperature coefficient isknown a priori or can be calibrated, or any other form of electric temperature transducer. In variants, a single device may serve double duty as thermal actuator and as temperature sensor, for example a resistive element with a known temperature coefficient will dissipate power when a known voltage is applied to its terminals and, by measuring the instantaneous current, its resistance and power dissipation can be determined. Other possible variants apply a pulsed voltage to a power resistor, which acts as a heater, and, between the pulses, use the same resistor as a thermistor.

[0012] Preferably, the control circuit can measure the instantaneous power of each cell, and / or determine the resulting instantaneous power distribution. This information can be used to control the currents flowing in the thermal actuators of the cells and in this manner correct any difference between the instantaneous power distribution and the desired power distribution. Advantageously, this feedback control stabilizes the instantaneous power to the desired level irrespective of the temperature- induced changes in the resistance of the thermal actuators. The instantaneous power can be determined in any suitable manner including, but not limited to, from any two of the following electrical quantities: instantaneous voltage, instantaneous current, instantaneous resistance.

[0013] The thermal device of the invention is designed to simulate an integrated circuit with a specific cooling solution and, advantageously, the cells may be fabricated with a process used for integrated circuit manufacturing on a semiconductor substrate. Each cell may include a thin- film power resistor as thermal actuator and a thin-film thermistor as thermal sensor or a thin-film resistor that is both a power resistor and a thermistor, as explained above. According to the needs, the device of the invention may comprise several semiconductor substrates juxtaposed, to form an array of cell having the desired dimensions and resolution, each substrate carrying a group of cells. The number of substrates may be one, two, or more, for example four or eight.

[0014] The invention is configured to distribute the current from one power source, for example a power supply, to many cells, but, in some case, may use a plurality of power sources, each delivering current to a group of cells. The number of power sources is free, and will be decided according to the application, but often will be comprised between one, or two, and ten. More power sources, for example up to one hundred or above are however possible. When there is more than one power source the control circuits may be likewise increased in number, each control circuit being configured to distribute electric current coming from one power source to the thermal actuators of a group of cells.

[0015] The thermal device of the invention may have a mechanical interface for mounting a detachable cooling device in thermal contact for testing. In this configuration, the invention could be regarded as a thermalmechanical mock-up of an integrated circuit to which diverse cooling devices can be attached and / or detached as will using the same means as it is customary in semiconductor circuits.

[0016] In embodiments, the cooling device may be liquid-cooled, aircooled, be based on heat pipes, Peltier cells, evaporative coolers, or any kind of cooling mechanism. The thermal device of the invention may have, for example, fastening holes for affixing the cooling device thereupon, fluidic connectors for circulating a thermal exchange fluid, connectors for fans or any other needed electrical apparatus, and so on. In other variants, the thermal device of the invention may comprise an integral cooling device, for example a micro-fluidic device.

[0017] The arrangement of fastening holes will be chosen in consideration of the nature of the integrated circuit and of the cooling solution that are simulated and tested. Advantageously, the thermal device of the invention will have mounting holes for placing several different cooling devices and comparing their performances. Should the provided holes not be enough, an adapter may be interposed between the thermal device and the cooling solution. The number of fastening holes may be 2, 3, 4, 6, 8 or more, and they may be spaced apart at 1 cm, 3 cm, 10 cm, 30 cm,or any value therebetween, or other suitable spacings. In other embodiments, the thermal test vehicle may include any kind of mechanical connector to affix the cooling device

[0018] The invention relates also to a method of simulating and testing cooling solutions for integrated circuits, comprising: providing a thermal test device comprising a plurality of cells in thermal exchange with a cooling device under test, obtaining a desired power distribution, distributing electric current coming from a power source to thermal actuators comprised in each of the cells such that values of electric power dissipated in the cells match the desired power distribution, preferably sampling the resulting temperature distribution. The power distribution may be a map of power values defining one power value for each cell or any suitable representation of the desired power field.

[0019] Optionally and preferably, the inventive method distributes electric current by controlling a multichannel waveform generator that provides currents flowing in the thermal actuators of the cells. The waveform generator is controlled such that an average value of electric power dissipated in each cells matches the desired power distribution.

[0020] In advantageous variants, the method includes reading a configuration of the cells comprising the number of available cells and / or a shape or a geometric arrangement of the cells and / or spatial positions of individual cells. The desired power values for each cell are chosen according to the desired power distribution and to the configuration of the cells. This may include processing the power map for example by sampling, interpolating, decimating or in any other way.

[0021] When real real-time power dissipations for each cell are available, the inventive method preferably includes a feedback control the adjusts the currents based on a difference between the -time power dissipations for each cell and the desired power distribution, for example using a proportional-integral-derivative (PID) algorithm.

[0022] Preferably, the inventive method includes a calibration of electrical sensors in the cells that are used to sample the thermal distribution. This may include reading electrical sensor signals of the cells in a stationary thermal condition, reading a reference value from a reference temperature sensor in the same stationary thermal condition, repeat the steps of reading electrical sensor signals and reference value for different values of temperature of the stationary thermal condition, and finally determine a calibration curve for each cell associating a electrical sensor signal with a value of temperature.

[0023] For safety, the inventive method and device may reduce or shut off currents if a temperature in one of the cells exceeds a predetermined threshold.

[0024] The methods disclosed herein can be performed by a computer or a programmable digital device, or else by several devices operating together and connected by a data network, local or wide area.

[0025] The methods of the invention may be embodied as software resources, for example software executable or symbolic code that could be stored in any suitable digital memory, including non-volatile memories, local storage units, removable digital storage devices and cloud-based data storage.

[0026] Thanks to the claimed structure, the thermal test devices of the invention can realize an arbitrary distribution of power in the thermal cells, and modify the power map at will, both in the spatial distribution and in the time domain, without rewiring the test system. Accordingly, the system can simulate dynamic situations in which a device for example transitions from a power state to another in a very general and powerful way.

[0027] Since the device require no rewiring, the number of independent cells can be increased, and with it the spatial resolution of the power and of the temperature map.

[0028] Additionally, some variants of the invention include special features that improve the precision of the power delivered.Short description of the drawings

[0029] Exemplar embodiments of the invention are disclosed in the description and illustrated by the drawings in which:- Figure 1 illustrates schematically a surface of a thermal test vehicle divided in four identical quadrants, each with a regular array of cells, and an idealized electrical circuit of one exemplary cell.- Figure 2 shows schematically the relationship between main functional blocks of the invention.- Figure 3 illustrates a possible structure of a cell driver, in blockfunctional fashion.- Figures 4 and 5 show a possible structure of the invention as an assembly of circuit boards in perspective, respectively side elevation. It is noted that these are simplified representation that are intended to provide a general sense of how the inventive device is assembled rather than faithful copies of complete realizations.- Figures 6 and 7 illustrate methods used in the invention as flowcharts.

[0030] Like reference symbols in the figures indicate identical, like or equivalent features. Where multiple instances appear, some reference symbols may have been omitted to avoid overcrowding.Examples of embodiments of the present invention

[0031] Figure 1 is a schematic view of a thermal test vehicle according to an embodiment of the invention, as seen from above the contact surface 45(which may also be referred to as a thermal surface 45). The apparatus comprises a plurality of cells 30 preferably arranged in a regular array. For implementing reasons, the cells are grouped in four ensembles or quadrants 40, each controlled by an individual driver circuit, as it will be shown in the following. However, this form of division is not an essential feature of the invention. The term "quadrant" is used broadly, and the number of groups is not limited to four, the invention allowing variants with one, two, three, or any number of quadrants 40.

[0032] Each cell includes, as shown in the right-hand part of the figure, a thermal actuator 35, for example an electric heating element capable of converting electrical current into heat, and a temperature sensor 37 for sensing the local temperature of the cell. The thermal actuator 35 could be embodied by any suitable dissipative electronic component such as a resistor or a transistor. In a favourable embodiment, the cells 30 are fabricated on a semiconductor substrate and the thermal actuators take the form of metal thin-film power resistors.

[0033] The temperature sensor 37 is sensitive to the core temperatures expected in the simulated device, typically from -20 °C to 120 °C for a commercial CPU, with advanced applications requiring extended temperature ranges. The temperature sensor 37 can be realised in many ways and could be implemented in a variety of ways and could be, for example, an active semiconductor circuit or a resistive transducer with a known temperature coefficient, for example. In some cases, it may be advantageous to use for the temperature sensor 37 a metal thin-film resistor fabricated with the same process as the power resistor that constitutes the thermal actuator 35, such that their temperature coefficients of resistance match.

[0034] Although the power resistor 35 and the temperature sensor 37 are represented as individual separate elements, this is not an essential feature. Embodiments where the heater 35 and the temperature sensor 37 are combined in a single element will be disclosed below.

[0035] The dimensions and number of cells may vary according to the target system. In a typical case, the total power dissipation summing up the contribution of all the cells will be between 0 W and 200 W, each cell having a maximum power dissipation of 15 W, the area of each cell 30 may be between 1 mm2and 10 mm2and the number of cells may be between 10 and 1000. These figures are typical of common implementations, for example when the invention is used to test cooling solutions for CPUs or GPUs, but may vary significantly, depending on the use case.

[0036] In a typical example, the power supply to the thermal actuators 35 has a voltage of 12 V and the resistance of the thermal actuators 35 may be of about 10 1, which results in a maximum cell power of 15 W environ, which is well suited to the requirements of this applications; however, other values are possible. Higher voltage levels, e. g. 24 V or 250 V may be desirable, and the values of the power resistors values may be increased accordingly, for example up to 10 kfl. Similarly, in other applications the voltage may be reduced to 5 V or 1 V. The temperature sensor 37 may be embodied by a thermistor with a nominal resistance of 100-1000 1, as it is typical for this application, but other values are possible.

[0037] Figure 2 shows the general architecture of an embodiment of the invention. The test vehicle comprises four quadrants 40 each having an array of 48 individual cells 30. Each quadrant is connected, via connectors 25 to a quadrant driver 50 that is configured to address the individual cells and set the desired heating power in each of them. The quadrants 40 may correspond to individual separate devices, or they may be fabricated on a common semiconductor substrate.

[0038] The system of figure 2 includes a power supply 60 that provides power to the quadrant drivers 50. The power supply 60 can preferably, provide all the power required and the quadrant drivers distribute the current to the individual cells 30 to obtain whatever power distribution is desired. In some cases, it may be desirable to have more than one power supply. The voltage level provided by the power supply 60 may be 12 V, 24 V, or any other suitable voltage, as previously mentioned. Theembodiments disclosed herein use a DC power supply and the current in the thermal actuators 35 is DC. an AC power supply could be used by, at the expense of increased complexity.

[0039] A master controller 70 interfaces with the quadrant controllers and sets the power distribution that must be reproduced. Importantly, the power distribution can be reprogrammed in software. This allows the simulation of different target systems, or of different power states of a given system, or of transient phenomena, without hardware modifications.

[0040] Figure 3 is a schematic representation of a possible implementation of a quadrant controller 50. A supply terminal 61 receives the power from the power supply 60, and output terminals 55 provide the desired supply to the cells in the corresponding quadrant. Functional block 52 is a multichannel waveform generator that can be programmed to generate digital signals that modulate the power output of each power switch 66 such that the average power dissipated in the cell 30 corresponds to the desired power distribution. The waveform generator 52 could generate PWM (pulse-width modulation) signals, PDM (pulse-density modulation) signals, or any suitable digital waveform that encodes the desired average power for each cell. Preferably, the waveform generator 52 does not need to handle the full current required by the heating elements 35 and may operate at a lower voltage. Rather, the outputs of the waveform generator are given to power switches 66 (such as power MOSFETs) for each individual cell.

[0041] In the embodiment shown in Figure 3 each power switch 66 is connected (via the output terminals 55) to the heating element 35 of a single respective cell 30. However, in another embodiment each respective power switch 66 is connected to a set of cells; in other words, each respective power switch 66 is connected (via the output terminals 55) to the heating elements of a set of cells (each set of cells comprising at least two cells); preferably each respective power switch 66 is connected in parallel to the heating elements of the cells of a set. Advantageously, providing adevice which has power switches 66 each of which are connected to a plurality of cells, allows the device to have a smaller footprint.

[0042] Pulse-Width Modulation (PWM) has been used successfully in a preferred embodiment of the invention. In this variant, the waveform generator 52 provides for each cell a digital signal with a succession of equally spaced pulses. The average power dissipated in the corresponding cell 30 is proportional to the duty cycle of the pulses, which the waveform generator can set following the instructions received from the controller 54. The PWM frequency may change between implementations and preferably will be at least 10 Hz, such that the impulsive nature of the power does not affect the temperature distribution. Preferably, the PWM frequency will be higher than 100 Hz or, better, higher than 1 kHz.

[0043] Functional block 54 is a microcontroller that interfaces with the main controller 70 via the communication interface 71. It is configured to receive the desired power levels for each of the cells of the quadrant and program the waveform generator 52 accordingly.

[0044] The quadrant driver 50 has also input terminals 57 connectable to the temperature sensors of each cell in the quadrant, and an analogue multiplexer 58 by means of which the microcontroller 54 can select and digitise each of the temperature values provided by the temperature sensors in each cell of the quadrant.

[0045] Figures 4 and 5 illustrate how the device of the invention could be realised on a printed circuit board 80 containing both the main controller 70 and the quadrant controllers 50. In this example, the power MOSFETs 66 are mounted on a separate TTV board 85 that holds also the TTV assembly. The cooling solution under test 100 is mounted on the contact surface 45 of the TTV. A stiffening element 47 (referred to hereafter as a stiffening ring 47) is provided; the stiffening element 47 is configured to surround the perimeter of the contact surface 45 of the TTV. It should be understood that the stiffening ring 47 can be any suitable shape or design (for example the stiffening ring 47 may be circular / ringshaped, or square shaped (as shown in Figure 4), or rectangular shaped, or any other suitable shape; preferably the shape of an inner perimeter of the stiffening ring 47 will match the shape of the outer perimeter of the TTV. In this example there is a gap 49 between the stiffening ring 47 and the contact surface 45 (which is defined by a surface of a chip 45a) of the TTV. The stiffening ring 47 advantageously increases the stability and minimised warping of the contact surface 45 and / or chip 45a and / or the TTV board 85. It should be understood that in the present disclosure that the contact surface 45 may be defined by a surface of a plurality of chips 45a and / or chiplets (the surface of each of the plurality of chips and / or chiplets, being located on the same plane i.e. the surfaces being located at the same height).

[0046] In the example shown in Figure 4 the stiffening ring 47 and the chip 45a whose surface defines the contact surface 45, are mounted on the same side of the TTV board 85. Specifically, in the example shown in Figure 4 the stiffening ring 47 and the chip 45a whose surface defines the contact surface 45, are mounted on a first surface 85a of the TTV board 85. However, the invention is not limited to requiring that the stiffening ring 47 and the chip 45a whose surface defines the contact surface 45, be mounted on the same side of the TTV board 85; in another embodiment the stiffening ring 47 and the chip 45a whose surface defines the contact surface 45, are mounted on the opposite sides of the TTV board 85. For example, the chip 45a whose surface defines the contact surface 45, may be mounted on the first surface 85a of the TTV board 85, and the stiffening ring 47 may be mounted on a second surface 85b of the TTV board 85; or the chip 45a whose surface defines the contact surface 45, may be mounted on the second surface 85b of the TTV board 85, and the stiffening ring 47 may be mounted on the first surface 85a of the TTV board 85.

[0047] The cooling device 100 may be an air-cooled heat sink, a liquid- cooled device, a cooling device including Peltier cells, heat pipes, evaporative or phase / change cooling units, or any other cooling device.

[0048] In embodiments, the device of the invention has fastening means, not shown in the drawings, that are configured to accommodate a specific cooling device, or several cooling devices that can be tested and compared with the thermal test vehicle of the invention. The TTV board may provide fastening holes, arranged to accept the desired cooling devices, in any number and spacing required. When necessary, an adapter, not shown, may interposed between the TTV board 85 and the cooling device 100.

[0049] The TTV board 85 and / or the host board 80 may also include fluidic connectors, not shown, to provide a cooling fluid to the cooling device 100 or electrical connectors for a cooling fan, for example, or any other necessary use.

[0050] We have already mentioned that an advantage of the system of the invention lies in the ability to simulate different power states and devices without reconfiguring the hardware. The separation between the host board 80 and the TTV board 85 further enhances the flexibility of the invention. The invention may include a range of interchangeable TTV boards to simulate devices with different configurations and packages.

[0051] Figure 6 schematically illustrates a method for simulating and testing a cooling solution for an integrated circuit implemented by the invention as computer programs running in the main processor 170 and / or in the microcontrollers of the sector drivers.

[0052] In step 120, the master controller interrogates the TTV board 85 — or the quadrant drivers 50 or their microcontrollers 54 — to determine a configuration of the available cells. This will include the spatial coordinates of each cell and the shape of the matrix. The configuration of the available cells may be obtained, for example, from a variable in a non-volatile area of the memory of the microcontrollers 54, or from any other suitable memory device. Having obtained the map, the method (step 125) also obtains a power map, prescribing a desired power distribution.

[0053] In step 130, the process checks whether the power map is compatible with the configuration of the available cells. In the event of an incompatibility, an exception management routine 135 is activated. Otherwise, the execution proceeds with step 140 with the calculation of the desired power Pset(i) for each cell. In the simplest case, the power map may already contain an explicit prescription of the Pset(i), but this is not necessary: the power map encodes the distribution as a parametric function, or as samples that are not taken at the exact positions of the available cells. In such case, the step 140 may include processing the power map to obtain the Pset(i) values, for example by sampling, interpolation, decimation, or in any other suitable processing.

[0054] Having determined the Pset(i) values, the method proceeds, in step 145 to calculate for each cell a setting of the control circuit that will provide the desired power dissipation when applied to the corresponding cell. When the control circuit is a PWM generator, this may be done by setting a duty cycle equal to the ratio between the desired power and the maximum power Pmax that the thermal actuator dissipates when it receives the full supply power. These settings are then applied to each cell by writing them to the microcontrollers of the respective sector driver, in step 150.

[0055] Once the power levels of the cells have been set, the process enters a loop 165 in which the temperatures of the cells are read from the temperature sensors (step 155) and displayed in a desired format (step 160), for example as a thermal bidimensional map.

[0056] Preferably, the invention can determine the real instantaneous values of the power dissipated in each cell and dynamically control the corresponding modulation code to obtain the desired power value. This feedback improves the stability of the power field by compensating for temperature-induced changes in the resistance of the thermal actuators. It may be implemented in a software module executed by the master controller 70, or by the microcontrollers 54 of the quadrant drivers 50.

[0057] The measure of the instantaneous cell power can be obtained, for example, by any two of the following electrical quantities: voltage drop V(i) across the thermal actuator 35, current l(i) flowing through the thermal actuator, resistance R(i) of the thermal actuator. A possible method of measuring the instantaneous power would be to measure and digitise in the microcontroller 54 the currents l(i) in each of the thermal actuators 35, and assume V(i)=Vcc, the supply voltage level, for example 12 V or 24 V, the voltage drop along the connections being negligible.

[0058] The circuit of the invention can either measure the instantaneous current during the active periods of the PWM code or an average value. This solution requires current sensing devices, for example shunt resistors, that are not shown in the figures.

[0059] Figure 7 illustrates a dynamic power control process. The main loop 165 includes steps that determine the real-time power dissipations for each cell and minimise a difference between the real-time power distribution and the desired power distribution by controlling the currents. In steps 170, the currents l(i) flowing through the thermal actuators of each cell are determined and in step 180 the process uses the formula P=VI to obtain the instantaneous power values P(i). Step 185 corrects the duty cycles using an appropriate feedback function, such as a proportional- integral-derivative (PID) algorithm, prior to the next iteration of the cycle 165.

[0060] The ability to measure the current of the thermal actuators 35 also brings the advantage that the instantaneous value of the electrical resistance of the power resistors 35 can be determined. From this and the thermal coefficient of resistance, the temperature of the cell can be determined. In this case, one resistor per cell can perform the dual function of power resistor 35 and thermistor 37.

[0061] In one variant, the instantaneous cell power is determined indirectly from the temperature value read by the separate temperature sensor 37. If the temperature coefficient of resistance of the power resistoris known, it is possible to determine the instantaneous resistance of the power resistor 35 including the thermal effects and, from that and the supply voltage, the instantaneous power. When the power resistor 35 and the thermistor 37 have matching temperature coefficients, the derivation is simpler and more reliable, since the instantaneous resistances of the power resistor and of the thermistor change with the temperature while maintaining a constant ratio, and the former can be obtained from the latter with a simple proportion.

[0062] The invention may comprise a calibration mode, preferably implemented in software, which improves the accuracy of the temperature reading. In calibration, the software senses the resistances of the temperature sensors in each cell in a stable thermal condition, acquires and stores a reference temperature from a reference temperature sensor placed in the same sensing environment as the testing device of the invention. The software associates the values of the temperature sensors in the cells with the reference temperature and stores the result. This is repeated for different values of the reference temperature that lie in the operating range of the device, until sufficient calibration points (for example three or more) are acquired.

[0063] The software is configured to generate a calibration curve for each cell in the testing device that maps resistance to temperature, thereby ensuring an accurate temperature mapping.

[0064] Advantageously, the software controlling the testing device of the invention may include safety checks and features that shut down the power if the temperature of any cell exceeds a predetermined threshold value Tmax. To reach a desired test state P(i)=Pset(i) avoiding shutdowns and hot spots, the software is preferably configured to gradually increase the power dissipated in the cells.

[0065] The parameters controlling the gradual transition may be a ramp-up time tramPand the number n of ramp-up steps. The software progressively increases the power setting for each cell in n successive steps,provided the temperatures are below the threshold value Tmax by a predetermined safety margin until the desired test state is reached. This allows the desired power state to be reached avoiding hot spots and shutdowns.

[0066] The methods disclosed herein may be embodied by a software program running on a suitable computer or programmable controller. The method may be executed totally on the master controller 70 or on a host system, not shown on the drawings, that communicates with the master controller 70 by the communication bus 71. It is also conceivable that the method may result from the execution of separate software resources in a networked system. For example, in embodiments, the master controller 70 may oversee the power feedback part of the method, while a host computer executes a program that performs other tasks of the method, such as calibration of the temperature probes, the display of the temperature distribution, or other. It is also possible that some operations belonging to the method of the invention be executed on remote computers, communicating over the internet. In an embodiment of the invention, for example, a testing facility may have several thermal test vehicles that are controllable through the internet, and a internet server through which the test vehicles can be monitored and / or controlled remotely.Reference symbols in the figures

[0067] 20 thermal test vehicle assembly25 connector30 cell35 thermal actuator, heater, power resistor37 temperature transducer, thermistorquadrant contact surface quadrant driver multichannel waveform generator, PWM generator microcontroller power lines to thermal actuators readout lines from temperature sensors multiplexer power supply connection to the power supply power switch, MOSFET master controller communication bus host board TTV board, daughterboard cooling solution under test acquire cell configuration get power map compatibility test exception management compute cells' individual desired power levels compute duty cycles set duty cycles read temperatures display temperatures loop read currents compute instantaneous power levels control duty cycles

Claims

Claims1. A thermal device for simulating and testing cooling solutions for integrated circuits, comprising:- a contact surface for mounting one or more heatsinks,- a plurality of cells having each a thermal actuator configured to convert a current flowing through the input conductor into heat,- a control circuit configured to obtain a desired power distribution in the cells simulating a power field and distribute electric current coming from a power source to the thermal actuators of the cells such that values of electric power dissipated in the cells match the desired power distribution, wherein the control circuit comprises a multichannel waveform generator providing currents flowing in the thermal actuators of the cells, and in that the control circuit is configured to control the waveform generator such that an average value of electric power dissipated in each cells matches the desired power distribution.

2. The thermal device of the preceding claim, the waveform generator being configured to generate a modulated signal using pulse-width modulation (PWM) or a pulse-amplitude modulation (PAM) or frequency modulation.

3. The thermal device of claim 1, wherein the currents flowing in the thermal actuators have a periodic cycle having a frequency of at least 10 Hz and / or at most 10 kHz.

4. The thermal device of any one of the preceding claims, wherein the thermal actuators have a resistance of at least 1 1 and / or at most 10 kfl, the waveforms have a voltage of at least 1 V and / or at most 250 V,5. The thermal device of any one of the preceding claims, wherein the cells have an area of at least 0.25 mm2and / or at most 10 mm2.

6. The thermal device of any one of the preceding claims, comprising at least 4 cells and / or at most 2000 cells, preferably wherein the resolution is at least 8 cells / mm2and / or at most 0.1 cells / mm2.

7. The thermal device of any one of the preceding claims, wherein the cells include one or more temperature sensors, the thermal device comprising a readout circuit configured to read the temperature sensors and provide a sampled map of a temperature field in the thermal device.

8. The thermal device of the preceding claim, wherein the cells have one resistor with a temperature coefficient implementing temperature sensor and thermal actuator in a single device.

9. The thermal device of any one of the preceding claims, the control circuit being configured to determine an instantaneous power distribution in the cells and correct the currents flowing in the thermal actuators of the cells in response to differences between the instantaneous power distribution and the desired power distribution.

10. The thermal device of the preceding claims, wherein the determination of the instantaneous power distribution includes measuring of the currents flowing through the thermal actuators of the cells and / or of voltage drops across the thermal actuators of the cells.

11. The thermal device of claim 1, wherein the control circuit is configured to react to temperature changes in the cells by correcting the currents flowing in the thermal actuators of the cells such that the instantaneous power distribution corresponds to the desired power distribution despite temperature-induced changes in the resistance of the thermal actuators.

12. The thermal device of any one of the preceding claims, the cells being on a semiconductor substrate, each cell comprising a thin-film power resistor as thermal actuator and a thin-film thermistor as thermal sensor or a thin-film resistor that is both a power resistor and a thermistor.

13. The thermal device of the preceding claims, comprising a plurality of semiconductor substrates, each carrying a group of cells, preferably comprising at least two semiconductor substrates and / or at most eight semiconductor substrates.

14. The thermal device of any one of the preceding claims, comprising a plurality of power sources, each delivering current to a group of cells, preferably comprising at least two power sources and / or at most 100 power sources, and / or a plurality of control circuits each configured to distribute electric current coming from one power source to the thermal actuators of a group of cells.

15. The thermal device of any one of the preceding claims, further comprising an interface for mounting a cooling device in thermal contact.

16. The thermal device of the preceding claim, the interface being configured for mounting the cooling device detachably.

17. The thermal device of any one of claims 15-16, the interface comprising one or more of: fluidic couplings for circulating a heat-transfer fluid in the cooling device, one or more mounting holes for fastening the cooling device to the interface, an electrical or mechanical connector.

18. The thermal device of any one of the preceding claims, further comprising a cooling device for cooling integrating circuits.

19. The thermal device of any one of the preceding claims, further comprising a stiffening element which is configured to increase the stability and minimised warping of the contact surface.

20. The thermal device of any one of the preceding claims further comprising, either, a plurality of power switches each of which is connected to the heating element of a single respective cell; or, a plurality of power switches each of which is connected to a plurality of heating elements of cells in a respective set of cells.

21. A method of simulating and testing cooling solutions for integrated circuits, comprising:- providing a thermal test device according to claim 1, comprising a plurality of cells in thermal exchange with a cooling device under test,- obtaining a desired power distribution,- distributing electric current coming from a power source to thermal actuators comprised in each of the cells such that values of electric power dissipated in the cells match the desired power distribution,- preferably sampling the resulting temperature distribution.

22. The method of the preceding claim, the step of distributing electric current includes programming a control circuit comprising a multichannel waveform generator providing currents flowing in the thermal actuators of the cells, and in that the control circuit is configured to control the waveform generator such that an average value of electric power dissipated in each cells matches the desired power distribution.

23. The method of any one of claims 21-22, wherein the power distribution is a map of power values defining one power value for each cell.

24. The method of any one of claims 21-23 comprising:- reading a configuration of the cells comprising a number of available cells and / or a shape or a geometric arrangement of the cells and / or spatial positions of individual cells.- choosing a power value for each cell according to the desired power distribution.

25. The method of any one of claims 21-24, comprising:- obtaining real-time power dissipations for each cell- minimise a difference between the real-time power dissipations and the desired power distribution by controlling the currents of the thermal actuators, preferably through a proportional-integral- derivative (PID) algorithm.

26. The method of any one of claims 21-25, comprising:- reading electrical sensor signals of the cells in a stationary thermal condition,- reading a reference value from a reference temperature sensor in the same stationary thermal condition,- repeat the steps of reading electrical sensor signals and reference value for different values of temperature of the stationary thermal condition,- determine a calibration curve for each cell by associating an electrical sensor signal with a value of the reference temperature.

27. The method of any one of claims 21-26, comprising reducing or shutting off currents if a temperature in one of the cells exceeds a predetermined threshold.

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