Thermal Analysis of 5G Optical Devices

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Currently, 5G has become a hot topic of global interest. Everyone knows that compared to 4G, 5G's download speed has increased by at least 9 to 10 times. In the 5G network era, regardless of the type of 5G bearer scheme, it cannot be separated from 5G communication devices. The requirements for optical devices in 5G are also becoming increasingly demanding, with smaller sizes, higher integration, higher speeds, and lower power consumption. The main commonly used device speeds for 5G fronthaul, midhaul, and backhaul are 25G, 50G, 100G, 200G, and 400G optical devices, with 25G and 100G optical devices being the most widely used 5G communication devices. As speeds increase and sizes decrease, this is an inevitable trend in the development of optical devices, which also places higher requirements on internal thermal management. How to quickly and effectively dissipate heat is a serious issue that must be addressed.

Part I: Thermal Dissipation

Why Consider Thermal Design?

As we all know, when photoelectric chips are operating, they do not convert 100% of the injected current into output photons. A portion of this energy is lost as heat. If this heat accumulates and cannot be dissipated in time, it can have numerous adverse effects on the performance of components. Generally speaking, as temperature rises, resistance decreases, reducing the lifespan of the device, deteriorating performance, aging materials, and potentially damaging components. Additionally, high temperatures can cause stress deformation in materials, reducing reliability and potentially causing malfunctions.

Let's discuss the three basic modes of heat transfer: conduction, convection, and radiation.

Thermal Conduction: This occurs when there is no relative displacement between different parts of an object, and heat is transferred through microscopic particles such as molecules, atoms, and free electrons. For example, chips dissipate heat through a heat sink at the bottom, and optical devices use thermal grease to transfer heat to the outer casing. Both of these processes are examples of thermal conduction.

Part II: Basic Knowledge of Thermal Design

The amount of heat transferred during thermal conduction is calculated using Fourier's Law of Heat Conduction: Q = λA(Th-Tc)/δ, where A is the area perpendicular to the direction of heat transfer (measured in m2), Th and Tc are the temperatures of the hot and cold surfaces, respectively, δ is the distance between the two surfaces (measured in meters), and λ is the thermal conductivity of the material (measured in W/(m*℃)).

From this formula, we can see that thermal conduction is affected by factors such as the surface area, material thickness, thermal conductivity, and the temperature difference between the contacting surfaces. The larger the area, the thinner the material, and the higher the thermal conductivity, the stronger the heat transfer will be.

Generally speaking, solid materials have higher thermal conductivity than liquids, and liquids have higher thermal conductivity than gases. For example, the thermal conductivity of pure copper at room temperature is as high as 400 W/(m℃), while pure aluminum has a thermal conductivity of 210 W/(m℃). Water has a thermal conductivity of 0.6 W/(m℃), and air only has a thermal conductivity of around 0.025 W/(m℃). Aluminum is commonly used in heat sinks due to its high thermal conductivity and low density. However, in some high-power chip applications, aluminum heat sinks with embedded copper blocks or copper heat sinks are used to enhance heat dissipation.

When selecting materials for heat sinks, we consider two main criteria:

1. High thermal conductivity.

2. Compatibility with the thermal expansion coefficient of the chip.

Materials with high thermal conductivity and thermal expansion coefficients compatible with chip materials include tungsten-copper alloys, diamond, beryllium oxide, and aluminum nitride. Among these, copper, tungsten-copper, and aluminum nitride are widely used due to economic considerations.

Convective Heat Transfer: This occurs when a fluid in motion passes over a solid surface with a different temperature, and heat is exchanged between the two. This is the most widely used heat transfer method in communication equipment cooling.

Convective heat transfer can be divided into two categories: natural convection and forced convection.

Natural Convection: This mainly utilizes buoyancy forces caused by differences in fluid density at different temperatures. It is a passive cooling method suitable for low heat generation environments. Natural convection is primarily used in mobile phones, optical modules, and other terminal products.

Forced Convection: This is an efficient heat dissipation method achieved by using external power sources such as pumps or fans to increase fluid heat transfer speed. It requires additional economic investment and is suitable for high heat generation and poor heat dissipation environments. Fan cooling commonly used in optical modules operating in cabinets or switches is a typical example of forced convection.

Thermal Radiation: This refers to the process of transferring energy through electromagnetic waves. Thermal radiation occurs when an object's temperature is above absolute zero, emitting electromagnetic waves. Heat transfer between two objects through thermal radiation is called radiant heat transfer. The radiative power of an object is calculated using the formula: E = 5.67e-8εT4, where T is the absolute temperature of the object (measured in Kelvin) and ε is the emissivity or surface emissivity.

Calculating thermal radiation between object surfaces is complex, but for two parallel surfaces with the same area and emissivity, the formula simplifies to: Q = A * 5.67e-8 / (1/εh + 1/εc - 1) * (Th4 - Tc4), where A is the surface area, εh and εc are the emissivities of the hot and cold surfaces, respectively,

Emissivity depends on the type of material, surface temperature and surface condition, and is unrelated to external conditions and color. When green oil is applied on the surface of printed circuit board, its blackness can reach 0.8, which is favorable for radiative heat dissipation. For metal shells, some surface treatment can be applied to improve blackness and strengthen heat dissipation. However, it should be noted that painting the shell black can not necessarily enhance thermal radiation, because when the object temperature is below 1800℃, the thermal radiation wavelength is mainly concentrated in the infrared waveband range of 0.76-20μm, and the proportion of thermal radiation energy in the visible light waveband is not large. So painting the module shell or interior black can only enhance the absorption of visible light radiation, and is unrelated to the infrared radiation that brings heat.