散热器辐射分析研究

0-7803-5015-4/98/$10.00 0 1998 EEE
EMI engineer to quite a disadvantage, since the heatsink is usually placed as close as possible to the processor, which carries the maximum amount of high frequency energy in the entire system. The heatsinks are made of metal and usually consists of fin type structures. Depending upon the processor harmonics the heatsink dimension can become electrically long enough to start radiating. Due to its close proximity the energy from a processor gets coupled to the heatsink sitting on top of it. The radiated emissions from the heatsinks could couple to the intemal cables or nearby slots. Quite often the processor noise coupled through these paths i.e. intemal cables, vent panels, etc., could leak outside the enclosure. The radiated emission from 3 heatsinks is being studied by Li et al [1 where the authors used FDTD method to study resonances and different modes supported by the heatsink structure. In this study the radiated emission pattern for a circular heatsink has been investigated. The radiation pattern depends on the geometry of the heatsink and excitation. The heatsinks have been modeled by using finite element method (FEM) in frequency domain. The radiation pattern of an actual circular heatsink is also measured in a semi-anechoic chamber. The circular heatsink is compared to a monopole. Depending on the harmonic frequencies, the heatsink dimension will change from electrically small to quite significant fraction of the wavelength. This not only changes the radiating efficiency of the heatsink but also changes the pattern. Moreover the secondary coupling elements are usually located in the near-field of the heatsink The objective of this study is to characterize a n circular heatsink a d to investigate its radiating performance.
BACKGROUND A typical board is composed of a printed circuit board (PCB), asics, passive components like decoupling capacitors, ferrite beads, fdters, terminating resisters, etc. Quite often there are sockets for plug-in daughter cards. The PCBs usually contain a stack of alternating conducting layers and insulating dielectric layers. The signals, power and ground planes are routed through these
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conducting layers. Most of the components are placed on the top or bottom of the FCB. These components are either surface mounted on the PCB or mounted via through Heatsink holes. Heatsinks are usually placed directly on top of the asics. The basic principle of cooling through heatsink is I I t to place the heatsink as close as possible to the dye of the t 1 asic and provide a path for good thermal conduction I EPOXY \ , I ' between the dye and the heatsink. One way of achieving Dye this, is by putting a thermally conductive epoxy on top of the dye and then placing a metal lid over the epoxy layer. If the heatsink is not very heavy then its quite common to attach the heatsink to this metal plate by means of thermally conductive glue. Sinceglue is not strong enough to stand rigorous shock and vibration tests, bigger Figure 1. The General placement of a heatsink and a chip on a PCB is shown he=. Some of the details like U0 heatsinks are either bolted on to the metal lid or screwed connectors are ignored. The epoxy layer between the in through plastic fixtures sitting on top of the asic. ra dye and metal lid is shown by the hatched a e Even though heatsinks have evolved to provide only thermal solution, it produces unwanted EMI. The authors represented by electric and magnetic dipoles [11. Since the attempted to investigatethe contribution of these thermal ground inside the ask is electrically tied to the top metal paths in creating an electrical coupling path between the lid, it is assumed that the heatsink and the top metal lid is heatsink and the asic or rest of the PCB. The authors basically driven with respect to the PCB ground. The studied several types of heatsinks and their various simplified model of the ask and the heatsink are shown in mounting strategies. For a typical scenario, where a figure 1. Different types of heatsinks were studied initially but heatsink is placed on an asic on a PCB, there are lot of EMI sources around the heatsink and energy can get coupled by finally circular heatsinks were chosen for more in-depth various methods. Along with the main asic (which has the study. Smcturally, this particular heatsink has similarity heatsink on top) adjoining active components e.g. with a monopole. In this paper, the performance of this oscillators could capacitively couple to the heatsink [2]. type of heatsink over the frequency range and its radiation The traces routed on the top layer (adjacentto the heatsink) p a " were investigated. The authors have tried to can also drive the heatsink. However, there is a good compare the heatsink's radiation performance with a chance that the most probable driving source will be the monopole over a finite ground plane. A second model of h main asic itself. In the main asic even though the silicon the heatsink over an asic could be created by taking tetop dye is not a conductor but there are lot of metal traces and metal lid into account. The metal lid acts a small patch in vias inside the dye which carry high speed signals. between the heatsink and the finite ground plane. The Moreover the epoxy layer on top of the heatsink is performance of the actual heatsink was also compared electrically conductive. The top metal lid is usually tied with a monopole on a patch over a finite ground plane. with the ask ground which is again c o ~ e ~ t to d main e the ground in the PCB through ground pins. There is of course EXPERIMENTALSETUP certain amount of package inductance existing between The simplified model shown in figure 1 is constructed the two grounds which could put the two grounds at two for both experimentalpurpose and theoretical simulation. different potentials. The ask ground is furtherlinked with n the top metal cover by means of vias a d epoxy which In order to represent a ground plane a rectangularpiece of pushes the lid to a slightly different potential than the asic metal (54cm x 42 cm) was taken. In order to create the ground. The heatsink is placed on top of the metal lid with excitation source, an N-type connector was punched through the center of the rectangdar metal plate and a either thermally adhesive glue or bolted lugs. subminiature type-A connector was hooked up to the In this investigation, the primary source of excitation N-type connector through an adapter. The subminiature is assumed to be the main asic itself. The possible sources connector was c o ~ e c t e d a tracking generator through to are assumed to be the vias and traces which can be a 50 Ohm coaxial cable. The metal plate was placed on a