[popular science] basic knowledge of thermal design: heat dissipation analysis of 5g optical devices

Editor's note ： This article is reproduced from Tianfu communication , Some basic knowledge of thermal design is introduced in great detail . For engineers who are concerned about high-speed signal and power integrity , This article is very worth learning .

99% Problems ignored by engineers ： Thinking heat and EMI Design considerations for damage

at present 5G It has become a hot topic of global concern , We also rub the heat , Everybody knows ,5G Compared with 4G The download rate should be increased by at least 9~10 times , stay 5G Internet Era , No matter what 5G The bearing scheme is inseparable 5G Communication devices , and 5G The requirements for optical devices are also getting higher , Small volume , High integration , High speed , Low power consumption , in the light of 5G Prequel 、 The main commonly used device rates of intermediate transmission and return transmission are 25G、50G、100G、200G as well as 400G Optical devices , among 25G and 100G Optical devices are the most widely used 5G Communication devices .

   The rate is getting higher and higher , Smaller and smaller , This is the inevitable trend of the development of optical devices , At the same time, it also brings higher requirements to the internal thermal management of optical devices , How to heat quickly and effectively is a problem that must be taken seriously .

   One 、 Dissipate heat

   Why consider thermal design ？

   as everyone knows , Our photoelectric chip is working , It will not inject current 100% Convert to output photoelectrons , Part of it will be used as energy loss in the form of heat , If a lot of heat continues to accumulate , It cannot be eliminated in time , It will have many adverse effects on the performance of components , generally speaking , As the temperature rises, the resistance value decreases , Reduce the service life of devices , Poor performance , Material aging , Components are damaged ; In addition, high temperature will also produce stress and deformation on the material , Reduced reliability , Device malfunction, etc .

   I have seen a company QSFP-DD 200G modular , When coupling and packaging devices , The module is too hot to touch , The temperature is at least 80℃, Only one side can be coupled , While using the cooling fan , To stabilize the device power , So when considering the device packaging structure , Thermal design is one of the most important considerations .

   Let's first popularize the three basic ways of heat transfer ： heat conduction 、 Heat convection 、 Thermal radiation

   heat conduction : When there is no relative displacement between the parts of the object , Rely on molecules 、 Atoms and free electrons The heat generated by the thermal movement of microscopic examples is called heat conduction . such as , The chip dissipates heat through the heat sink underneath , The optical device contacts the shell for heat dissipation through heat dissipation silicone grease , All belong to heat conduction .

   Two 、 Basic knowledge of thermal design

   The heat transferred during heat conduction is in accordance with Fourier Calculation of heat conduction law ：Q=λA(Th-Tc)/δ

   among ：A Is the area perpendicular to the direction of heat transfer , Unit is m2;Th And Tc They are the temperature of high temperature and low temperature surfaces respectively ;δ Is the distance between two faces , Unit is m;λ Is the thermal conductivity of the material , Unit is W/(m*℃)

   It can be seen from the formula that , Heat conduction process and heat dissipation area 、 Thickness of material 、 thermal conductivity , There is also a relationship between the temperature difference between the contact surface and the heat dissipation surface , Larger area , The thinner the material 、 The greater the thermal conductivity , The more heat is transferred by heat conduction .

   In general , The thermal conductivity of a solid is greater than that of a liquid , The of liquid is greater than that of gas . For example, the thermal conductivity of pure copper at room temperature is as high as 400 W/(m*℃), The thermal conductivity of pure aluminum is 210W/(m*℃), The thermal conductivity of water is 0.6 W/(m*℃), And air only 0.025W/(m*℃) about . Aluminum has high thermal conductivity and low density , Therefore, radiators are basically made of aluminum alloy , But in some high-power chip heat dissipation , In order to improve the heat dissipation performance , Aluminum radiator inlaid with copper block or copper radiator is often used .

   Give a few examples of heat conduction in life ：

  ① Pan fried vegetables , The iron pot conducts heat and quickly fry the vegetables ;

  ② When I was a child , Sell popsicles at the door and wrap them in a quilt , Popsicles will not melt for a long time , The quilt has poor heat conduction ;

   The following figure summarizes the performance comparison of some commonly used materials as heat sinks ：

   Our selection rules for heat sink materials ：

  (1) The thermal conductivity should be high ;

  (2) Match the thermal expansion coefficient of the chip ;

   It can be seen from the above table , High thermal conductivity , The coefficient of thermal expansion matches the chip material ： Tungsten copper alloy 、 Diamond 、 Beryllium oxide 、 Aluminum nitride , Considering the economic cost, the most widely used ： copper 、 Tungsten copper 、 Aluminum nitride, etc .

   Convective heat transfer ： It refers to when a moving fluid flows through a solid surface with a different temperature , The process of heat exchange with the solid surface , This is the most widely used heat exchange method in the heat dissipation of communication equipment .

   Convection heat transfer is mainly divided into natural convection heat transfer and forced convection heat transfer ：

   Natural convection ： It mainly uses the buoyancy force caused by the density difference of high and low temperature fluid to exchange heat , It is a passive way of heat dissipation , Applicable to the environment with low calorific value . And on mobile phones 、 End products such as optical modules are mainly natural convection heat transfer .

   Forced convection heat transfer ： Through the pump 、 Fan and other external power sources to accelerate the speed of fluid heat transfer caused by an efficient way of heat dissipation , Need additional economic input , Suitable for high calorific value 、 Poor cooling environment ; The fan cooling and heat dissipation of optical modules working in cabinets or switches is a typical forced convection heat transfer .

   Examples in life ：

  1、 When the electric teapot is boiling , When opening the lid , Convection of hot and cold water can be seen ;

  2、 Open the tea just made with hot water , You can see air convection .

   Thermal radiation ： Refers to the process of transmitting energy through electromagnetic waves , Thermal radiation is the process of emitting electromagnetic waves when the temperature of an object is higher than absolute zero , The transfer of heat between two objects through thermal radiation is called radiative heat transfer . The formula for calculating the radiation force of the object is ：

E=5.67e-8εT4

   The calculation of thermal radiation between object surfaces is extremely complex , The simplest two have the same area and

   The formula for calculating the radiant heat transfer between the opposite surfaces is ：

Q=A*5.67e-8/(1/εh +1/εc -1)*(Th4-Tc4)

   In the formula ：T It refers to the absolute temperature of the object = Celsius temperature value +273.15;

  ε Is the blackness or emissivity of the surface .

   Emissivity depends on the kind of substance , Surface temperature and surface condition , It has nothing to do with external conditions , It has nothing to do with color . Apply green oil on the surface of the printed circuit board , Its surface blackness can reach 0.8, This is conducive to radiation and heat dissipation . For metal enclosures , Some surface treatment can be carried out to improve the blackness , Enhance heat dissipation . But here's the thing , Blackening the shell does not necessarily enhance thermal radiation , Because the temperature of the object is lower than 1800℃ when , The wavelength of thermal radiation is mainly concentrated in 0.76~20μm In the infrared band , The proportion of thermal radiation energy in the visible light band is not large . Therefore, blackening the shell or interior of the module can only enhance the absorption of visible light , It has nothing to do with the infrared radiation that brings heat .

   Examples in life ：

  1、 When you are by the fire , There will be a burning sensation ;

  2、 The radiation of the sun produces heat .

   3、 ... and 、 Thermal analysis of optical devices

   The overall heat dissipation path of the device ：

   The thermal environment of optical devices during operation is shown in the figure below . After the pluggable optical transceiver module is inserted into the panel , A small part of the heat generated inside is dissipated by the natural convection of the surrounding air , Most of them dissipate heat through conduction , Heat is always transferred from the high temperature end to the low temperature end , The module heat is transferred upward to the package shell , Pass it down to the motherboard . The following figure shows the overall schematic diagram of the packaging structure of the optical module , Analyze the main heat dissipation path of the module .

   Internal heat dissipation path of optical device ：

   The main internal heating components include TOSA Launch components 、ROSA Receiving components 、PCB On board devices and IC Control chip . The heat generated by the chip mainly passes through the top ① And the bottom ③ And the side ② Dissipate heat , The heat transmitted from both sides to the outside through the lead frame ②, In fact, because of ①、② Too small to be ignored , To improve the overall heat dissipation efficiency of the module , It needs to be improved as much as possible ③ Heat dissipation capacity , Reduce the size of thermal resistance in each path and improve its thermal conductivity .

Chip heat dissipation path

   Important factors affecting the heat dissipation of optical devices ：

   Through the internal and external analysis of optical devices , It can be seen that the important factors affecting the heat dissipation of optical devices are as follows ：

  (1) The heat of power devices is exported in time ： For devices with high heat flux , Such as under the chip and laser PCB The plate is treated with through-hole copper plug or copper inlaid block , Improve the thermal conductivity of the heat sink .

  (2) Thermal conductivity of shell ： Under the same heat dissipation conditions , Improving the thermal conductivity of the shell is conducive to reducing the shell temperature of the device , At the same time, it is conducive to reducing the temperature difference between the module shell and the radiator

  (3) Device layout ： Shorten the distance between the base plate of the heat sink and the heating component , It is conducive to reducing the device shell temperature and the temperature difference between the device shell and the radiator .

  (4) Contact thermal resistance ： The contact thermal resistance between the device shell and the radiator is an important factor affecting the heat dissipation of the device . Reducing the contact thermal resistance is conducive to improving the heat dissipation performance of the device , Thus, the device shell temperature and the temperature difference between the device shell and the radiator are reduced .

  (5) The contact area between the radiator and the device shell ： By increasing the length of the radiator contact surface , The device shell temperature and the temperature difference between the device shell and the radiator can be reduced by about 1-2 ℃.

   Four 、 Thermal simulation example

   With TOSA For example , Through difference Receptacle The structure design of the can see the curve of temperature changing with time , As shown in the figure below , Through thermal simulation, it is known that the temperature difference between the two structures reaches 5℃ about .