Heat Dissipation Measures Using Thermal Fluid Analysis
When taking measures to dissipate heat from box housing products, the majority of such measures involve transferring heat from the housing wall to the outside air.
This page briefly explains heat dissipation measures that use the housing. But first, here are the three elements of heat transport.Three elements of heat transport
- Convection heat transfer
▷ Heat transfer due to the movement of heated molecules- Radiation
▷ Heat transfer by electromagnetic waves- Heat conduction
▷ Heat transfer due to lattice vibrations and free electrons in matter
Table of contents
- Model specification used in descriptions
- Heat dissipation effect via convective heat transfer
- Summary of heat dissipation effect via convective heat transfer
- Heat dissipation effect via radiation
- Heat dissipation effect via heat conduction
- Heat conduction blind spot
- Are you spending a fortune on heat dissipation?
- How to make an exact model of a heating component
- Heat measurement technology that analyzes heating components with high precision and high accuracy
Model specification used in descriptions
Element Material Thermal conductivity (W / mK) Emissivity Heat value (W) Housing Aluminum die-cast 121 0.03 – Fins Aluminum die-cast 121 0.03 – IC placement table Aluminum die-cast 121 0.03 – IC Resin 1 0.9 20
Heat dissipation effect via convective heat transfer
When designing a plate fin, you may be wondering how many fins should be set.
★ There is an appropriate number of fins (optimum solution).
★ It’s best to design heat dissipation designs that make the most of the convection heat transfer effect.
Fin wing count:
- If there are too few fins, then the heat transfer area from the fin surface to the outside air will decrease, and the effect will be reduced.
- And, if there are too many, there won’t be enough wind between fins, and the effect will be reduced.
In other words, a fin is a heat dissipation member whose count needs to be optimized for maximum heat dissipation.
Below are the verification results for this model.
This is the result of analyzing 8 to 30 fins at intervals of 2 sheets.
Summary of heat dissipation effect via convective heat transfer
8 fins 18 fins 30 fins Low surface area of fins Optimal range Range where difficult for wind to flow between fins Wind speed distribution map Consideration Although the wind speed is sufficient, the surface area of the fin that comes into contact with the outside air is small and the heat radiation effect is reduced. Since the wind speed and the surface area of the fin are both appropriate sizes, the convective heat transfer effect is maximized. Although the surface area of fin is sufficient, the required wind speed is not obtained, thus the convective heat transfer effect is reduced.
When it comes to minor changes in product development, such as product outlines, materials, and positions of heat-generating members, you do not necessarily have to refine the optimal fin configuration.
If you change the way you look at the graph…
…you can find the optimal fin pitch
Heat dissipation effect via radiation
Radiation is a heat transport that is often overlooked.
Radiation heat dissipation is an effective means of heat dissipation for metal enclosures and sealed products.
Aluminum die-cast
The emissivity of aluminum die-cast differs depending on the surface condition as shown below.
In addition, the amount of radiation increases with the fourth power of the surface temperature of the object, as shown in the following equation.
Thermal radiation amount = constant × emissivity × object surface temperature 4
For Your Reference
What is heat radiation?
In midsummer, sandy beaches exceed 70 degrees Celsius. However, the atmospheric temperature is only about 30 degrees Celsius. The high temperature of the beach is caused by the heat radiated from the sun to the earth (ground).
Heat radiation is heat transported by electromagnetic waves.
The results of a thermal analysis of the aluminum die cast by the surface treatment described above are as follows.
<Emissivity ε = 0.03> <Emissivity ε = 0.95 (paint)>
Heat dissipation effect via heat conduction
Because of the heat dissipation effect due to heat conduction (shown on the right), the selection of the thermal conductivity and thickness of the material is the key to heat dissipation.
● When an IC is installed on housing walls, the temperature of IC_Tj varies depending on the selection of the TIM (* 1) material.
(* 1) TIM: Thermal Interface Material
Consideration:
A steep temperature change is seen at a thermal conductivity lower than 0.6 W / mK. 0.6W / mK or more should be selected.Consideration:
Since the change is linear, a material that is as thin as possible should be selected.
Heat conduction blind spot
Reducing the thickness in order to increase heat transport by heat conduction will not necessarily increase the heat dissipation effect.
To increase heat transfer by heat conduction, reduce fin base thickness
Although the thermal resistance between Tj and Tfin should decrease, the results show the opposite trend.
This is a phenomenon that occurs because the heat spread due to heat diffusion (45°diffusion) does not match the vertical size of the fin.How can this phenomenon be avoided?
Example countermeasures
Spread heat before transferring it to fin.
Improves the thermal conductivity of the IC stage and spreads the heat near the heat source:
Employing Cu (thermal conductivity 385W / mK), etc…
Heat is diffused by a material having high thermal conductivity:
Graphite sheet (thermal conductivity 1500W / mK (in plane)), Cu sheet (thermal conductivity 385W / mK), etc…
Are you spending a fortune on heat dissipation?
Why does the cost of heat-dissipating components matter?
Heat dissipating components do not improve product functions per se.
(Products can still function even if there are no heat dissipation components.)↓
Reduce the cost of heat dissipation components = product prices decrease
(Materials with high heat dissipation have higher costs, as shown in the graph.)↓
Correctly grasping the temperature helps to suppress the thermal margin to the limit
In product thermal design, thermal fluid analysis tools (Sim) are often utilized for heat calculation. (The accuracy of thermo-fluid analysis is improving year by year.)
However, the thermal design of the product has a large temperature margin with respect to the temperature solved by Sim, and still guarantees the quality of the product.Unnecessarily expensive heat radiation parts are often used in products, leading to increased product costs. This is because product designers leave much more thermal margin than simulated, resulting in employment of too many heat dissipation measures. The reason for excessive thermal margin is the lack of simulation accuracy.
High-precision heat estimates of heating components are the key to cost reduction.
How to make an exact model of a heating component
As shown in the flowchart below, three “knows” and two “know-hows” are required
The following section explains the differences between existing technologies and our proprietary WTI technologies.Chip analysis by opening mold Investigation of structure size via cross section polishing PKG simulation model Chip surface temperature distribution during local heating
Heat measurement technology that analyzes heating components with high precision and high accuracy
How WTI technology is different from thermal resistance analysis manufacturers’
Method used by most thermal resistance analysis providers Our patented technology
Problems
The following problems occur when the power supply (Vcc) and GND are connected in reverse, and measurement is performed using parasitic Di in the semiconductor.
- Entire chip surface does not reach uniform temperature
- Unable to identify measurement location → Can’t even determine if it’s measuring correctly… sigh. (-_-;)
- Joule heat generated in bonding wires affects measurement accuracy
↓
Technology which solves these problems
Features:
For high-voltage blocks in semiconductors,
the specific block is clamped by forward current.
The temperature of only the specified part is then
measured by a technique that generates heat
uniformly.
→
After heat
generation
- Model specification used in descriptions
- Heat dissipation effect via convective heat transfer
- Summary of heat dissipation effect via convective heat transfer
- Heat dissipation effect via radiation
- Heat dissipation effect via heat conduction
- Heat conduction blind spot
- Are you spending a fortune on heat dissipation?
- How to make an exact model of a heating component
- Heat measurement technology that analyzes heating components with high precision and high accuracy
Visit the WTI blog for more information:
- No more heat margin for the product? How a knowledge of semiconductors is key to making accurate predictions
- Thermal stress generated by temperature change is greater than you’d imagine
- Mounting reliability evaluations for semiconductor packages (Daisy chain samples and identifying breakage points)
- Mechanism design: the feedback destination of simulation results
- Avoid those heat problems with preventive diagnosis!
