The parameters of heat sink fins are the core factors determining their performance, size, cost, and reliability. These parameters are interrelated and mutually restrictive. Excellent thermal design involves finding the optimal balance among them. The following is a detailed introduction to various fin parameters, including their definitions, impacts, and design considerations.
I. Core Geometric Parameters
These parameters directly define the physical shape and arrangement of the fins, as shown in Figure 1 below.
Figure 1: Fin Geometric Parameters
1. Fin Pitch (P)
Definition: The center-to-center distance between two adjacent fins. Sometimes it also refers to the clear distance between fins.
Impact:
1.Small Pitch: More fins per unit volume, larger heat dissipation area, but increased air resistance and susceptibility to dust accumulation.
2.Large Pitch: Lower air resistance, less prone to dust accumulation, but reduced total heat dissipation area.
Design Consideration: It is necessary to find the optimal balance between heat dissipation area and airflow resistance. This is one of the most critical trade-off parameters.
2. Fin Height (H)
Definition: The vertical height of the fin from its root to its tip.
Impact:
1.Increased Height: Significantly increases the heat dissipation area, thereby enhancing cooling capacity.
2.Negative Impacts: As fins become taller, it becomes more difficult for heat at the tip to be conducted away through the baseplate, leading to decreased fin efficiency (larger temperature difference between tip and root). Mechanical strength also decreases, making them prone to vibration.
Design Consideration: Height must be designed in coordination with pitch and thickness. Tall fins typically require a larger pitch to ensure unobstructed airflow.
3. Fin Thickness (δ)
Definition: The material thickness of the fin itself.
Impact:
1.Increased Thickness: Facilitates heat conduction from the root to the tip (reduces the fin's own conductive thermal resistance), improving fin efficiency. Also provides better mechanical strength.
2.Decreased Thickness: Under the same pitch, allows for more fins (i.e., smaller clear distance), thereby increasing the heat dissipation area, but also increases air resistance.
Design Consideration: Thinner fins are used when pursuing lightweight and miniaturization, but process feasibility and structural strength must be ensured.
4. Fin Inner Radius (R)
The fin's inner radius (R) is ensured by the mold. Generally, the design should not specify too small an R angle. An excessively small R can lead to defects such as fin cracking and poor flatness after stamping.
5. Perpendicularity Deviation Angle (a)
The deviation angle (a) of each fin wave's axis relative to the perpendicular of the base surface. This is typically determined by the mold and machining/alignment processes. This angle is generally controlled within ±3°. Exceeding this tolerance leads to poor fin formation.
6. Fin Shape
Definition: The macro-geometric shape of the fin.
Overview: Fins come in various structural types such as serrated, plain (straight), and perforated. The appropriate fin type can be selected based on different operating conditions. The fin's extended surface area and its ability to disturb the fluid flow determine the heat exchange capability.
Plain Fin Characteristics: Feature long rectangular fins with smooth walls. Their heat transfer and flow characteristics are similar to fluid flow in long circular pipes. See Figure 2.
Figure 2: Plain Fin
Serrated Fin Characteristics: The fluid channels are stamped to be uneven, increasing fluid turbulence and enhancing the heat transfer process. They are known as "high-efficiency fins." See Figure 3.
Figure 3: Serrated Fin
Wavy Fin Characteristics: Involves pressing certain corrugations/waves into a plain fin to promote fluid turbulence. The denser the waves and the larger the amplitude, the better the heat transfer performance. See Figure 4.
Figure 4: Wavy Fin
Perforated Fin Characteristics: Created by punching many holes into a plain fin. They are often placed in inlet/outlet distribution sections and where fluid phase change occurs. See Figure 5 below.
Figure 5: Perforated Fin
II. Fin Performance and Physical Parameters
These parameters describe the performance characteristics and physical properties of the fins.
1. Fin Efficiency
Definition: The ratio of the fin's actual heat dissipation to its ideal heat dissipation (assuming the entire fin is at the root temperature).
Impact:
1.The taller, thinner, or made from poorer thermally conductive material the fin is, the lower its efficiency (lower tip temperature).
2.Higher efficiency indicates better utilization of the fin material.
Design Consideration: The goal is to make fin efficiency as high as possible (typically desired >70-80%). This means selecting materials with good thermal conductivity and rationally designing the height-to-thickness ratio.
2. Hydraulic Diameter
Definition: A comprehensive parameter used to describe the characteristics of the flow channels between fins.
Impact: Determines the flow characteristics and Reynolds number (Re) within the channel, thereby affecting the convective heat transfer coefficient and flow resistance.
Design Consideration: Engineers use it for fluid dynamics and heat transfer calculations.
3. Surface Area to Volume Ratio
Definition: The heat dissipation surface area provided per unit volume of the heat sink.
Impact: A higher ratio indicates better compactness of the heat sink, allowing greater cooling capacity within a limited space.
Design Consideration: In space-constrained applications (e.g., laptops, mobile phones), achieving a high surface area to volume ratio is critical. This typically implies smaller, denser fins.
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