Analysis of electromagnetic radiation impact of CPU radiator

This paper studies in detail the influence of the aspect ratio of the bottom surface size of the heat sink, the orientation and height of the fins on the first resonant frequency (the first resonant frequency analyzed in this paper, hereinafter referred to as the resonant frequency), and the electric field gain and radiation direction at the resonant frequency point. Through the study, the general rules are found to provide a basis for the design and selection of the heat sink.

1 Numerical model establishment

In the research of EMC standard issues, the CPU heat sink problem is one of the main issues of electromagnetic compatibility. For the modeling of traditional CPU heat sinks, the heat sink is usually decomposed into three parts: ground plane, excitation source and heat sink. From the electromagnetic characteristics of actual integrated circuits, the electromagnetic characteristics of the CPU core can be simulated as a conductor patch. Brench believes that the heat sink can be simulated as a solid block to simplify the calculation. Das and Roy concluded from experimental results that the excitation source can be simulated with a monopole antenna.

Compared with traditional processors, the structure and packaging of the P4 processor are different: a heat sink is integrated on the top of the integrated chip and is insulated from the chip package. Therefore, the numerical model of the heat sink of the P4 processor is different from that of the traditional processor. In the literature, the two models are compared, and a simple multi-layer structure numerical model has been proposed. Based on the simple multi-layer numerical model of P4, this paper establishes a more realistic fin-shaped heat sink, as shown in Figures 1 and 2. In Figure 2, from bottom to top, they are ground plate (Ground), patch (Patch), medium (Substrate), integrated heat sink (IHS), and heat sink (HS). Based on this model, the following two points are analyzed in detail: (1) The effect of the change in the aspect ratio of the bottom surface of the heat sink on the resonant frequency, the electric field gain at the resonant frequency, and the radiation direction; (2) The effect of the orientation and height change of the fin on the resonant frequency, the electric field gain at the resonant frequency, and the radiation direction. The materials of each part in Figures 1 and 2 are shown in Table 1.

2 Simulation Analysis

First, the radiator is regarded as a solid block, with a standard size of 88.9 mm × 63.5 mm × 38.1 mm, and a model is established. The frequency is swept from 1 to 6 GHz to obtain the reflection coefficient, as shown in Figure 3. Comparing the results of Figure 3 with those of Figure 4 in the literature, it can be seen that they are basically consistent at the first resonance point, and the simulation results are more accurate at low frequencies.

2.1 Effect of the aspect ratio of the heat sink bottom on the resonant frequency, electric field gain and radiation direction

The long side and height of the bottom surface of the radiator are fixed, and the wide side is changed to observe the changes in the resonant frequency and the changes in the electric field gain and radiation direction at the resonant frequency. The long side of the radiator is set to 88.9 mm, the height is 38.1 mm, and the wide side is changed from 40 mm to 95 mm. The calculation is performed every time the 5 mm increase is made, that is, the bottom surface aspect ratio changes from 0.93 to 2.22, and the electric field gain and resonant frequency are obtained with the bottom surface aspect ratio, as shown in Figure 4.

As can be seen from Figure 4, (1) when the aspect ratio is greater than 1.25, the resonant frequency does not change significantly and remains at 2.6 to 2.65 GHz. When the aspect ratio decreases, the resonance point will shift significantly to low frequency. For example, when the aspect ratio is about 0.93, the resonance point has dropped to 2.4 GHz. This is because when the wide side is less than the long side, the long side is the dominant dimension, which determines the resonant frequency of the radiator. When the long side is 88.9 mm, its resonance point is around 2.65 GHz. When the wide side is greater than 88.9 mm, the wide side becomes the long side and becomes the dominant dimension. The change in the dominant dimension mainly affects the change in the resonance point. In addition, when the aspect ratio is greater than 1.65, the resonant frequency shifts slightly downward; (2) when the aspect ratio is greater than 1.3, the electric field gain remains above 8 dB, which is greater than the antenna gain of portable devices in most wireless communication systems, and the radiator exhibits an antenna effect. When the aspect ratio is about 1, the electric field gain drops by more than 1 dB. In addition, the change trends of the electric field gain and the resonant frequency are basically consistent.

Figure 5 shows the two-dimensional radiation diagram of the electric field gain of the CPU heat sink when the width size is 3 different (3 = 0). It can be seen that the radiation direction is around θ = 30. Figures 6 (a) to 6 (c) show the changes in the three-dimensional radiation diagram of the CPU heat sink as the width size increases. Figure 6 (a) is the radiation diagram when the width is 40 mm; Figure 6 (b) is the radiation diagram when the width is 60 mm; Figure 6 (c) is the radiation diagram when the width is equal to the long side of 88.9 mm. From the changes in Figure 6 (a) to Figure 6 (c), it can be seen that as the width size increases, the radiation direction gradually changes from two radiation directions on the xz plane to four radiation directions on the xz plane and the yz plane. This is because the long side is the dominant size for the resonance point, which mainly affects the radiation direction characteristics of the heat sink at the resonance point. When the wide side is smaller than the long side, the long side determines the radiation characteristics, and there are two radiation directions, as shown in Figure 6(a) and Figure 6(b); when the wide side is close to the long side, the wide side and the long side together determine the radiation characteristics, and there are four radiation directions, as shown in Figure 6(c).

2.2 Effect of fin orientation and height of heat sink on resonant frequency, electric field gain and radiation direction

Here, the bottom of the heat sink is 88.9 mm × 63.5 mm in length and width, the fin thickness is 2 mm, the fin spacing is 2 mm, and the height of the bottom of the heat sink is 5 mm.

2.2.1 Effect of lateral fins

The transverse fins, that is, the fins run along the x-axis, and there are 8 on each side symmetrically about the z-axis. When the fin height is between 0 and 60 mm, the interval is 5 mm, and simulation analysis is performed to obtain the curve of the resonant frequency and the gain of the electric field at this frequency changing with the fin height, as shown in Figure 7. As can be seen from Figure 7, when the fin height changes from 0 to 60 mm, the resonant frequency is between 2.5 and 2.65 GHz. As the fin height increases, the electric field gain increases. When the fin height exceeds 20 mm, the electric field radiation gain basically remains at around 8 dB.

When the fin height is 0 mm, 35 mm and 55 mm respectively, it can be seen from the 2D diagram of the electric field gain of the heat sink that with the increase of the lateral fin height, obvious radiation is generated at the bottom of the heat sink, and its radiation direction also changes with the increase of the fin height, as shown in Figure 8, but it has little effect on its two main radiation directions.

2.2.2 Effect of longitudinal fins

The longitudinal fins, that is, the fins run along the y-axis, with 11 on each side symmetrically along the z-axis, and the fin height varies from 0 to 50 mm, with an interval of 5 mm for simulation analysis, and the first resonant frequency and the electric field gain at this frequency are obtained as the curve of the change with the fin height, as shown in Figure 9. As can be seen from Figure 9. The change of the longitudinal fin has a large and complex impact on the resonant frequency, especially when the fin height is <20 mm. As the fin height increases, the first resonant frequency drifts by 350 MHz. When the fin height is 20 mm, multiple different resonant points appear. When the fin height is >20 mm, the resonant frequency basically remains at 2.65 to 2.7 GHz. At the same time, it is also observed that the change of the longitudinal fin height has little effect on the electric field gain, which remains at 8.0 dB with a deviation of about 0.4 dB. 20 mm is a special point. At this time, there are 3 close resonant points in the simulation. Only the strongest resonance of 2.7 GHz is observed, so a smaller electric field gain is obtained.

When the fin height is 0 mm, 33.1 mm, and 50 mm, respectively, it can be seen from the 2D diagram of the electric field gain of the radiator that with the increase of the longitudinal fin height, obvious radiation is generated at the bottom of the radiator, and its radiation direction also changes with the increase of the fin height, but it has little effect on its two main radiation directions, as shown in Figure 10.

From the simulation analysis of the longitudinal fins and transverse fins, it can be seen that the longitudinal fins and transverse fins show almost the same effect in general, that is, the orientation of the fins has little effect on the radiation direction of the radiator. However, the change in the height of the longitudinal fins still has a significant impact on the resonant frequency, especially when the height of the fins changes below 20 mm, the resonant frequency drifts greatly.

3 Conclusion

This paper focuses on the influence of two factors on the resonant frequency, electric field gain at the resonant frequency and radiation direction of the radiator, namely the aspect ratio of the bottom surface of the radiator, the fin orientation and the fin height change. Through the study, it can be seen that the change of the aspect ratio of the bottom surface of the radiator has a significant effect on the resonant frequency; the orientation and height of the fin also have a certain influence on the resonant frequency. With the change of the fin, the resonant frequency has a drift of about 100 MHz, especially for the longitudinal fin, when its height is <20 mm, the influence is more obvious; these three factors also have an impact on the electric field gain, but the overall impact is not significant, basically maintained at about 8.0 dB. However, the electric field gain is already greater than the antenna gain of portable devices in most wireless communication systems, making the radiator show a significant antenna effect; in addition, it can be seen that the electric field radiation of the radiator has obvious directionality, but it is also affected by the bottom surface size of the radiator and the fin height. Therefore, when designing or selecting a radiator, it is necessary to comprehensively consider these factors to minimize the electromagnetic radiation and interference of the radiator.