PCB and IC packaging strategy heat essentials
Introduction
Semiconductor Manufacturing Company is difficult to control the device using its system . However , mounting the IC system is critical to the overall performance of the device . For custom IC devices, the system designers often work closely together with the manufacturer to ensure that the system meets the thermal requirements of many high -power devices . This early mutual cooperation can guarantee IC to achieve electrical standards and performance standards , while ensuring normal operation within the customer's cooling system. Many large semiconductor companies to sell the device to standard parts , and there is no contact between manufacturers and end-use applications . In this case, we can use some general guidelines to help achieve a better IC and system solutions for passive cooling .
Common types of semiconductor packages or PowerPADTM exposed pad package . In these packages , the chip is mounted on the metal sheet is called a chip pad. The chip pads on the chip -to-chip processing play a supportive role , but also a good thermal path cooling devices . When exposed pad package is soldered to the PCB , the heat can be rapidly dissipated out from the package , and then into the PCB . Thereafter, the layer of the PCB to dissipate heat into the surrounding air. Exposed pad package can generally be about 80 % of the heat conduction , the heat through the bottom of the package into the PCB. The remaining 20% of the heat generated by each side of the device and the package leads to dissipate . Less than 1% of heat through the top of the package distribution. These exposed pad package , the PCB thermal design some good device performance is critical to ensure .
Fig 1: PowerPAD thermal path design showcase
A first aspect of PCB design can improve the thermal performance of the device is the PCB layout. As long as it is possible, high power components on the PCB should be spaced apart from each other. Physical separation between the components of this high-power, high power consumption enables each component to maximize the PCB area around, thus helping to achieve better thermal conductivity. Should be noted that the temperature-sensitive components on the PCB and high power components isolated. Under no circumstances may the installation location should be away from the high-power components PCB corner. PCB more middle position, you can maximize the board area around the high power components to help dissipate heat. Figure 2 shows two identical semiconductor device: Components A and B. A PCB assembly located in the corner, there is a 5% higher than the component B chip junction temperature, because the component position B by the middle of some more. Since the components surrounding area for heat dissipation plate is smaller, so the heat component A corner location is limited.
On the thermal properties of the component layout of Figure 2 . PCB assembly chip temperature is higher than the corner intermediate assembly.
The second aspect is the structure of the PCB , one aspect of its most decisive influence on the thermal performance PCB design . The general principle is : the more copper PCB , thermal performance of the system components is higher. Ideal heat dissipation chip semiconductor device is mounted on a large liquid-cooled copper. For most applications, this mounting method is impractical , so we can make some other changes to the PCB to improve thermal performance. For most applications today , the total system volume is shrinking , cooling performance adversely affected. Area larger PCB, which can be used for the greater thermal conductivity , but also have greater flexibility, leave enough space between the high -power components.
In any case possible , we must maximize the number and thickness of the PCB copper ground layer. Weight of copper ground plane generally larger , it is an excellent heat the entire PCB thermal path . For the arrangement of the wiring layers also increase the overall proportion of copper for heat conduction .
However, this electric wiring is often performed separately , thus limiting its potential role as a heat sink layer . A wiring layer on the device ground , the ground should be as many layers as in terms of power , so can help maximize heat transfer. Thermal vias situated below the semiconductor device on a PCB , the PCB to help heat into the buried layer , and transmitted to the back of the circuit board.
Is to improve the thermal performance , PCB top and bottom is " prime ." Wider use of wires away from high -power devices in place wiring , can provide a path for cooling the heat . Special thermal PCB board is a great way to heat . Usually the heat transfer plate located at the top or the back of the PCB , and is connected directly or via a copper thermal vias thermally connected to the device . The case of inline package ( only on both sides leads package), this thermal panels located on the top of the PCB can be shaped like a "dog bone" ( middle and packaging as narrow , away from where the package size compared to copper connections big middle small ends of the large ) . The four sides of the package case ( lead has four sides ) , the heat transfer plate must be located in the back or the PCB into the PCB.
Figure 3 dual in-line package "dog bone" shaped method example
Increase the thermal plate size is an excellent way to improve the thermal performance of the PowerPAD package . Different thermal plate size has a great impact on thermal performance. Product data sheets provided in tabular form will set out these general size information . However, to have an impact on the increase in copper custom PCB quantify , it is a very difficult thing . Use some online calculator , the user can select a device , then change the size of the copper pad size , they can estimate the impact on non- JEDEC PCB thermal performance . These calculation tools , PCB design highlights the degree of influence on the cooling performance . The four sides of the package , the area just under the top of the pad area of the exposed pad of the device, in this case , the buried layer is a first or a back way to achieve better cooling . For dual in-line package , we can use the " dog bone " type of style to the cooling pad .
Finally , a larger PCB can be used for cooling system . The case is connected to the screw heat thermal panels and ground layers , some of the screws for mounting PCB system can also become a leading base of the effective thermal path . Taking into account the thermal effects and costs , screw the maximum number of points to be reached diminishing returns. After connecting to the heat conducting plate , a metal reinforcing plate PCB has more cooling area . For some PCB cover has a shell application, type control welding material has a higher thermal performance than air-cooled enclosure . Solutions such as cooling fans and heat sinks , cooling system is also commonly used method , but it usually requires more space , or the need to modify the design to optimize the cooling effect.
To design a system that has a high thermal performance , just choose a good IC devices and closed solutions is not enough. IC cooling performance depends on scheduling PCB, and so the size of IC capacity for rapid cooling device cooling system . Using the passive cooling method can greatly improve the thermal performance of the system .
PCB board thermal design techniques
Heat is the main content of PCB thermal design . The purpose of cooling is to ensure reliability when the temperature exceeds the temperature of components , take appropriate countermeasures heat , the temperature is reduced to the reliability of the operating range. The main PCB heat from conduction, convection , radiation, three aspects to proceed. This paper describes the PCB circuit board thermal design techniques.
First, the components are arranged cooling
1, the elements are arranged to meet the cooling requirements dispersed staggered arrangement . When the component layout design layout should generally devices and heating components and temperature sensitive device to distinguish , around heating devices should allow sufficient cooling gas flow channel , decentralized heating element should be staggered arrangement . This uniform arrangement when the opposite is usually the layout , will help to improve heat dissipation.
When mixing the different components installed thermal performance , the best heat large components installed downwind , heat a small element mounted upwind , otherwise poor heat component in the path of the heating elements heat , which the result is a poor heat resistance of the element at higher temperature . When mixed arrangement , the basic arrangement of the heat element has the same level of order: large power consumption components , poor heat dissipation components should be installed in the upwind.
2 , the high heat of the heating device when the PCB there is always a few large heat device , the device can be added in the heat sink or heat pipe when the temperature has not come down, can be used with a radiator fan to enhance heat effect. When a large amount of heating devices , can be a large heat shield , which is at the level of the PCB and the position of the heating devices or radiators for customization in a large radiator plate cutout different components and low position . The overall buckle in the heat shield element surface, while cooling in contact with each component. However, due to the level of consistency is poor component assembly and welding , the cooling effect is not good. Usually components plus a soft surface thermal phase change thermal pad to improve heat dissipation.
Two , PCB circuit board cooling
PCB panels are clad / wood or glass cloth epoxy phenolic resin glass cloth material , as well as a small amount of paper-based materials used CCL . While these substrates have excellent electrical properties and processability , but poor heat resistance , high thermal path as is the heat from the heating element to a surface element surrounding air . But rely on a very small surface area of the surface of the component is insufficient to dissipate heat . And because a large amount of heat is generated in part passed the PCB , so the best solution is to increase the heat dissipation capability of the PCB itself in direct contact with the heating element is conducted away or dissipated via the PCB board.
1 , the selection of a good plate heat
Epoxy glass cloth sheet thermal conductivity of one of 0.2W / m ℃. General electronic circuits due to the small amount of heat , usually epoxy glass cloth substrate produced a small amount of heat generated by the general thermal design and components traces itself to dissipate . With the component miniaturization, high integration , high frequency, thermal density was significantly increased, especially with the power of the device, in order to meet this high cooling requirements later developed some new thermal conductivity sheet .
2, using reasonable alignment heat
Since the resin sheet material of poor thermal conductivity , and the holes are copper lines and a good conductor of heat , thus increasing the thermal conductivity and increasing the rate of residual copper is the primary means of cooling holes . PCB cooling capacity , need to be calculated by the thermal conductivity of the PCB with different effective thermal conductivity of the insulating substrate . The thicker foil , copper foil higher residual rate , the more layers , the equivalent thermal conductivity greater the cooling effect of the PCB as possible.
PCB thermal conductivity coefficient of thermal conductivity in the thickness direction of the surface area is much smaller . In order to improve the thermal conductivity in the thickness direction , can be thermal via . Through thermal vias are : PCB metalized holes. The effect is equivalent to a thin copper PCB thickness direction along the catheter penetrating from the surface , so that the front and back of the PCB heat conduction, heat the heating element to quickly escape or other heat conduction layer to the back of the PCB. If installed on the PCB IC bare chips in its set just below the PCB holes numerous thermal design is universal. Therefore, when the PCB traces to improve the cooling capacity should be thick line , thick copper foil , sheet, multilayer copper to a large area , plus thermal via design.
The Latest PCB Cooling Techniques
As consumer demands for “smaller” and “faster” intensify, mammoth challenges emerge when it comes to beating the heat generated by ever-denser printed-circuit boards (PCBs). As stacked-up microprocessors and logic elements reach into the gigahertz range of operation, cost-effective thermal management becomes perhaps the highest priority among engineers in the design and packaging and materials fields.
Adding to those headaches is the current trend of manufacturing 3D ICs for greater functional densities. Simulations show that a 10°C rise in temperature can double a 3D IC chip’s heat density, degrading performance by more than one-third.
Microprocessor Challenges
Projections by the International Technology Roadmap for Semiconductors (ITRS) show that within the next three years, interconnect wiring in difficult-to-cool regions of a microprocessor will consume up to 80% of the chip’s power. Thermal design power (TDP) is one measure to assess a microprocessor’s propensity to handle heat. It defines the upper point of the thermal profile as well as the associated case temperature.
The latest microprocessors from Intel and Advanced Micro Devices (AMD) feature TDPs ranging from 32 to 140 W. This number continues to rise in conjunction with increasing microprocessor operating frequencies.
Large data centers that employ hundreds of computer servers are particularly susceptible to heating problems. According to some estimates, the servers’ cooling fans—which draw up to 15% of the electrical power—actually become considerable heat sources in and of themselves. On top of that, the cost of cooling a data center can constitute about 40% to 45% of the center’s power consumption. All of these factors create a greater demand for local and remote temperature sensing and fan control.
The thermal-management challenge becomes trickier when it involves PCBs housing multicore processors. While each processor core in the array may dissipate less power (and thus less heat) than a single-core processor, the net effect on large computer servers is the addition of more heat dissipation to a data center’s computer system. Simply put, many more processor cores run for a given amount of PCB space.
Another thorny issue with IC thermal management concerns the appearance of hot spots on a chip’s package. Heat fluxes can climb as high as 1000 W/cm2, which is a condition that’s difficult to track.
PCBs play a critical role in thermal management, thus requiring a thermal design layout. Whenever possible, designers should keep power components as far away from each other as possible. Furthermore, they should be kept away from the PCB’s corners, which will help maximize the amount of PCB area around the power components to facilitate thermal dissipation.
It’s common for exposed power pads to be soldered to a PCB. Often, exposed-pad-type power pads conduct about 80% of the heat generated through the bottom of the IC package and into the PCB. The remaining heat dissipates through the package’s sides and leads.
Heat Helpers
Designers now can seek help via a number of improved heat-management products. They include heatsinks, heat pipes, and fans that allow for active and passive convection, radiation, and conduction cooling. Even the manner of the PCB-mounted chip’s interconnection helps mitigate heat problems.
For example, the common exposed-pad approach used for interconnecting an IC chip to a PCB may increase heat problems. When soldering the exposed path to a PCB, the heat travels quickly out of the package and into the board. The heat then dissipates through the board’s layers and into the surrounding air.
Thus, Texas Instruments (TI) devised a PowerPAD method that mounts the IC die to a metal plate. This die pad, which supports the die during fabrication, serves as a good thermal heat path to remove the heat away from the chip.
According to Matt Romig, analog packaging product manager at TI, its PowerStack method is the first 3D packaging technology to stack high-side vertical MOSFETs. It combines both high-side and low-side MOSFETs held in place by copper clips and uses a ground potential exposed pad to provide thermal optimization. Employing two copper clips to connect the input and output voltage pins results in a more integrated quad flat no-lead (QFN) package.
Heat management for power devices is an even greater challenge. Higher-frequency signal processing and the need to shrink package size are pushing conventional cooling techniques to the brink. Kaver Azar, president and CEO of Advanced Thermal Solutions, proposes the use of an embedded thin-film thermoelectric device that includes water-cooled microchannels.
Azar envisions one solution that minimizes spreading resistance, the largest resistance in the path of heat transfer, with a forced thermal spreader bonded directly to the microprocessor die.
This approach distributes the concentrated heat of a small microprocessor die to the larger base area of the heatsink, which transfers the heat to the ambient environment. Such a built-in forced thermal spreader combines micro and mini channels in the silicon package. The water flow rate inside the channels is approximately 0.5 to 1 liter/minute.
Simulation results showed that on a 10- by 10-mm die within a ball-grid-array (BGA) package, a 120- by 120-mm heatsink base-plate area yielded a thermal resistance of 0.055K/W. Using a heatsink material with thermal conductivity equal to or higher than diamond yielded 0.030K/W.
Paul Magill, vice president of marketing and business development for Nextreme Thermal Solutions, also suggests thermoelectric cooling, advocating that cooling should start at the chip level. The company offers localized thermal management deep inside electronic components using tiny thin-film thermoelectric (eTEC) structures known as thermal bumps. The thermally active material is embedded into flip-chip interconnects (e.g., copper pillar solder bumps) for use in electronic packaging.
Localized cooling at the chip wafer, die, and package levels delivers important economic benefits. For instance, in a data center that employs hundreds and thousands of advanced microprocessors, it’s far more efficient than removing heat with more expensive and bulkier air-conditioning systems.
In some devices like LEDs, a combination of passive and active cooling techniques can improve device performance and lifetime. For example, using a fan inside a heatsink often will reduce thermal resistance to 0.5°C/W, which is a significant improvement over the typical 10°C/W achieved with passive cooling (heatsinking) alone.
Simulate And Simulate Again
Thermal control has always been, and continues to be, one of the limiting factors to achieving greater IC performance. With space at a premium in these ever-smaller ICs and their packages, there’s little or no room to help cool them. It has forced designers to consider exotic cooling techniques and new, evolving cooling materials.
Nonetheless, the basic premise remains: Designers must pay more attention to the science of thermodynamics for optimal cooling solutions. And the entire process should start with thermal analysis software—well before a design is put into production.
That’s where simulation software tools enter the picture. Products like the Mentor Graphics’ Flotherm 3D V.9 software tool help 3D IC designers quantify thermal quantities, enabling them to address thermal problems as they arise. This computational fluid-dynamics (CFD) product provides images of bottleneck (Bn) and shortcut (Sc) fields. As a result, engineers can identify where and why heat-flow congestion occurs in their designs.
According to Erich Bürgel, general manager of Mentor Graphics’ mechanical analysis division, innovative Bn fields show where a design’s heat path is being congested as it attempts to flow from high-junction temperature points to the ambient point. The Sc fields highlight possible approaches to create a new effective heat-flow path by adding a simple element such as a gap pad or a chassis extrusion.
Flotherm 3D V.9 supports the importing of XML model and geometry data to enable the software’s integration into data flows. It also has a direct interface to Mentor Graphics’ Expedition PCB design platform. As a result, users can add, edit, or delete objects such as heatsinks, thermal vias, board cutouts, and electromagnetic cans for more accurate thermal modeling.
With thermal simulation, designers can accurately predict the thermal performance of the initial and subsequent designs without having to build and test a prototype. Design variables such as the number of heatsink fins, fin thickness, heatsink base thickness, and thermal resistance of the thermal-interface materials should be considered.
Proper thermal models are essential for future 3D ICs that plan to use stacked logic and memory devices consisting of thin die, which strongly reduces lateral heat spreading. As a die’s thickness shrinks, higher-temperature spots become more common. Hot spots on the logic die cause local temperature increases in the memory die, possibly reducing DRAM retention time.
Researchers at Belgium’s Interuniversity Micro Electronics Center (IMEC) have already proven correct thermal models for the design of next-generation 3D mixed-stack ICs. These 3D stacks, which closely resemble commercial chips of the future, consist of IMEC proprietary logic CMOS ICs stacked on top of commercially available DRAMs. Stacking is accomplished with through-silicon vias (TSVs) and micro-bumps. The research was a collaborative effort between IMEC and partners Amkor, Fujitsu, Globalfoundries, Intel, Micron, Panasonic, Qualcomm, Samsung, Sony, and TSMC.
IBM plans to use microchannel water cooling for its future 3D IC processors, such as the Power8 processor scheduled for introduction in 2013. Bruno Michel, manager of the Advanced Thermal Packaging Group for IBM’s Zurich, Switzerland research facility, says that energy-efficient, hot-water cooling technology is part of IBM’s concept of a zero-emissions data center. To cool 3D chip stacks, which generate more heat than a single processor in nearly the same space, water rather than air was used to reduce energy consumption.
Liquid cooling of CPUs is also performed in the XLR8 GTX 580 GeForce graphics card from PNY Technologies, which addresses challenging graphics-intensive gaming products. PNY and Asetek, a specialist in CPU thermal management, joined forces to produce a product for gaming enthusiasts and their GPU/CPU cooling systems.
Engineered with a closed-loop system and built with Asetek’s sealed water cooler already attached, the combination design offers consumers an out-of-the-box, ready-to-install product that costs $649.99. PNY claims the new system offers up to 30% cooler temperatures, quieter acoustics, and faster performance than the standard-reference-designed Nvidia GeForce GTX 580 graphics card.
Thermal management via water cooling also is employed in a wide variety of power devices—thyristors, MOSFETs, and silicon-controlled rectifiers (SCRs) are just a few. One example is the XW180GC34A/B developed by Westcode Semiconductors Ltd., a subsidiary of Ixys Corp. The nickel-plated heatsink has a 127-mm diameter contact plate, suiting it for press-pack devices with electrode contacts up to 125 mm in diameter.
Typical heatsink to input water thermal resistance, for flow rates of 10 L/min., is 4.3K/kW (two coolers plus one semiconductor device) and 5.6K/kW (three coolers plus two semiconductor devices). The heatsink comes with or without an integral connecting bus bar.
“Typical applications for the coolers would be mini megawatt-power-level devices and high-power rectifiers, as in heavy industrial applications, or for electric train trackside substations, as well as in applications in electricity generation and distribution,” says Frank Wakeman, Westcode’s marketing and technical support manager. “The high-efficiency cooling provided with these coolers enables customers to achieve high-power density in their systems with much reduced footprint.”
Cooling Method
The heat generated when the electronic device is working, so that the internal temperature rose rapidly, if not the heat dissipation, the device continues to heat up, the device will fail due to overheating, the reliability of electronic equipment will decline. Accordingly, heat treatment of the circuit board is very important.
A printed circuit board temperature factors
The direct cause of the temperature rise is due to the presence of PCB circuit power devices, electronic devices exist in power consumption, heat intensity with power changes in the size varying degrees.
PCB temperature rise of two kinds of phenomena:
(A) short-term or long temperature rise.
(2) local temperature rise or a large area;
In analyzing the PCB thermal power, generally to analyze the following aspects.
1. Electric power consumption
(1) Analysis of the distribution of power PCB board.
(2) Analysis of the power consumption per unit area;
2. Thermal radiation
(A) the temperature difference between the PCB and their adjacent surfaces of the absolute temperature;
(2) radiation coefficient PCB surface;
3. Heat conduction
(1) Install the heat sink;
(2) Other mounting structure of conduction.
4. Thermal convection
(1) natural convection;
(2) forced convection cooling.
5. PCB structure
(1) PCB material.
(2) PCB size;
6. PCB mounting
(1) Installation (eg mounted vertically, horizontally mounted);
(2) from the housing and the seals distance.
From the analysis of the factors mentioned above PCB is an effective way to solve the PCB temperature, often in a product and systems of these factors are interrelated and dependent on, the majority of factors that should be analyzed according to the actual situation, and only for a specific to compare the actual situation correctly calculate or estimate the parameters such as temperature and power consumption.
Second, the circuit board cooling mode
1. Heat through PCB board itself
Current sheet is widely used PCB laminates / epoxy glass cloth material or phenolic resin glass cloth material, as well as a small amount of paper-based materials used CCL. Although these substrates have excellent electrical properties and processing performance, but poor heat resistance, high thermal path as heating elements, almost can not expect a resin transfer heat from the PCB itself, but the heat from the surface element to the surrounding air. However, with the electronic member has entered the small, high-density mounting and high heat of the assembly times, if only by a very small surface area of the surface of the component is insufficient to dissipate heat. And because extensive use of QFP, BGA and other surface mount components, a large amount of heat generated by components transmitted PCB, therefore, the best solution is to increase the heat dissipation capability of the PCB itself in direct contact with the heating element through the PCB board conduction out or dissipate.
2. High plus radiator heating devices, thermal plate
When the PCB in a small number of larger devices hair when heat (less than three), you can add a radiator or heat pipes in heating devices, when the temperature can not drop down, can be used with fan heatsink to enhance heat dissipation effect. When a large amount of heating devices (more than three), can be large heat shield (board), which is based on the location and level of the PCB heating devices and custom dedicated radiator or radiator in a large flat different components on the level of the position of the cutout. The overall buckle in the heat shield element surface, while cooling in contact with each component. However, due to the level of consistency is poor component assembly and welding, the cooling effect is not good. Usually components plus a soft surface thermal phase change thermal pad to improve heat dissipation.
3. Adopt reasonable alignment designed to achieve thermal
Since the resin sheet material of poor thermal conductivity, and the holes are copper lines and a good conductor of heat, thus increasing the thermal conductivity and increasing the rate of residual copper is the primary means of cooling holes.
PCB cooling capacity evaluation, we need the thermal conductivity of the composite material composed of a variety of different PCB is calculated using one equivalent thermal conductivity of the insulating substrate (nine eq).
4. Temperature sensitive device is preferably placed in the coldest regions (such as the bottom of the device), do not put it above the heating device, multiple devices are interleaved best layout in the horizontal plane.
5. PCB cooling equipment mainly depends on the air flow, so the design to study the air flow path, the rational allocation of components or printed circuit boards. Always tend to place less resistance when the flow of air flow, so the printed circuit board configuration device, to avoid leaving a large airspace in an area. Printed circuit board machine in multi-block configuration should also pay attention the same problem.
6. Focus on the PCB to avoid hot spots, as the power is evenly distributed to the PCB, the PCB surface to maintain a uniform and consistent temperature performance. Often the design process to meet strict uniform distribution is more difficult, but be sure to avoid high power density areas, avoid hot spots affect the normal operation of the entire circuit. If the conditional, the thermal efficiency of the printed circuit analysis is necessary, such as increasing the number of professional PCB design software is now in the hot performance metrics analysis software module, can help designers to optimize circuit design.
7. The maximum power consumption and heat maximum cooling device arranged near the optimum position. Do not place a higher heating device in the corner and around the edge of the PCB, unless it is arranged in the vicinity of a cooling device. In the design of power resistors when selecting larger devices as possible, and when adjusting the PCB layout so that there is enough space for heat dissipation.
8. For a free-convection air cooling equipment, it is best integrated circuit (or other device) arranged by longitudinal, or arranged in horizontal long way.
9. Devices should be aligned as far as possible in accordance with district heat and cooling degree of the same size on a piece of PCB, heat a small or poor heat resistance of the device (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) on the cooling airflow The most upstream (inlet), heat, or high heat resistance devices (such as power transistors, LSI, etc.) on the most downstream cooling flow.
10. On the other device when the temperature in the vertical direction, the power devices are arranged as close to the top of the PCB, in order to reduce these devices work; in the horizontal direction, the power devices are arranged as close to the edge of the PCB, in order to shorten the heat transfer path .
11. Heat dissipation in the device connected to the substrate due to reduce the thermal resistance between them. In order to better meet the requirements of thermal characteristics, the underside of the chip can use some of the thermal conductivity of the material (such as smear layer thermal silica), and maintain a certain contact area for heat dissipation devices.
12. The device connected to the substrate:
(1) to shorten the length of the device leads;
(2) Select the number of pins more devices.
(3) selection of high-power devices, you should consider the thermal conductivity of lead material, if possible, try to choose a cross-section of the surface leads the largest;
13. Select the device package:
(1) should be considered to provide a good thermal conduction path between the substrate and the device package;
(2) on the heat conduction path to avoid blocking air, if this is the case can be filled with thermally conductive material.
(3) consider the thermal design should pay attention to the device's package instructions and its thermal conductivity.
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