Abstract:
A method and apparatus for determining EMI compliance provides a conductor that is mutually impedance coupled to an integrated circuit on a semiconductor package. The conductor is attached to a lid covering the integrated circuit and RF noise energy on the lid is mutually impedance coupled to the conductor. By measuring the voltage at the conductor, an indirect measurement of the EMI generated by the integrated circuit is made.

Description:
FIELD OF THE INVENTION 
     The present invention relates to electromagnetic interference measurements (EMI) of digital systems and, more particularly, to determining electromagnetic compatibility (EMC) of integrated circuits (ICs). 
     BACKGROUND OF THE INVENTION 
     Electromagnetic radiation emitted from a digital system is mainly at the fundamental frequency of its swiching operation and that frequency&#39;s harmonics; for systems with multiple clocks, multiple fundamental frequencies and harmonics will be present. At other frequencies, system radiation levels are typically undetectable. 
     International as well as national regulatory standards exist which specify allowable levels of EMI emissions from unintentional radiators. The main purpose of these standards is to protect the radio frequency spectrum for equipment licensed to operate at appropriate frequencies. Because the regulated frequencies reach into the gigahertz range and the allowable signal levels are typically in the microvolt range, the test set-up when measuring EMI often influences the test results. Therefore, in addition to allowable signal levels, these regulations also specify measuring methods in an attempt to standardize the compliance tests and improve their repeatability. These factors increase both the length and cost of typical EMI tests. 
     Typically, EMI regulations directly apply only to full systems, such as a personal computer, and not directly to system components, such as video cards or microprocessors. An accepted industry practice, however, to test the EMC of system components is to use the “substitution method”. In this method, a compliant system is used, and an original component is replaced with a new component. If the system is still compliant, then the new component is determined to be electromagnetically compatible. 
     Because the microprocesor is one of the major contributors to system level EMI, EMC assessment of microprocessors is a concern of CPU manufacturers. This assessment encompasses two facets: verification of compliance to EMC standards and component characterization. When performing the substitution method to verify EMC compliance, multiple tests of multiple components on multiple systems are needed to provide any degree of confidence in the test results. Component characterization, the other facet of EMC assessment, identifies the effects that design choices have on EMI emissions. For purposes of characterization, a high number of package design options (e.g., package layer stack-up, chip-cap configuration, etc.) are evaluated and compared to arrive at valid manufacturing decisions. 
     One common feature of both facets is the large number of tests which need to be performed in order to properly assess a microprocessor&#39;s EMC. The far-field tests typically used for radiated emission measurements are both time consuming and expensive and the component industry needs an alternative method to perform EMC assessment. The Society of Automotive Engineers in developing their standard SAE J1752-1, entitled “EMC Measurement Procedures for Integrated Circuits”, has investigated using near field EMI measurement methods for ICs; but these methods typically involve specially manufactured test boards attached to a modified Transverse Electromagnetic Mode (TEM) cell and introduce measurement errors when operated above one gigahertz. Presently, an alternative EMC assessment method and arrangement is needed, which produces EMI data corresponding to a microprocessor or other IC with minimum influence from other components such as cables, power supplies, and which allows testing of multiple parts in a timely and economic manner. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the need for alternative EMI measurement methods and arrangements for digital electronic systems, especially alternatives to far-field measurement requirements as described in regulatory standards on EMI. 
     This, as well as other needs, are met by the present invention which uses the RF noise voltage present on an IC package lid to determine the likely far-field EMI behavior of that IC during system testing. In particular, the voltage resulting from mutual impedence coupling of a conductor with the package lid is used to assess the EMC of the IC. The present invention produces stable and repeatable measurement data in the frequency ranges of typical electronic system emissions. Further, it produces data which, while representative of the EMI contribution of the IC, also correlates well with the far-field system test behavior of the IC. 
     The needs are also met by embodiments of the invention which provide a determination of EMI compliance of a system by first operating a replaceable IC component at an operating speed, measuring the noise voltage level on the lid of the IC and then calculating a system level EMI valve based on the measured noise level. 
     The needs are further met by embodiments of the invention which provide a test apparatus for measuring the voltage potential on a lid of an IC comprising the IC attached to the IC so that it is mutually impedance coupled with the lid and a meter connected to the conductor to measure the voltage potential. 
     The foregoing features, as well as other aspects and advantages, of the present invention will become more apparent from the following detailed description, claims and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a typical IC package for which the present invention performs EMI measurements 
     FIGS. 2 a  through  2   c  illustrate different test system set-ups in accordance with embodiments of the present invention. 
     FIG. 3 illustrates a schematic depiction of the test set-up depicted in FIG. 2 a.    
     FIG. 4 illustrates a flowchart depicting a method for determining EMI performance of an IC device in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the present invention is presented below specifically in terms of a CPU or microprocessor within a computer system. However, the scope of the present invention also includes similar ICs and systems other than this specific embodiment. 
     A typical CPU device  106  as depicted in FIG. 1 includes pins  110 , or other connectors, for electrically coupling the device  106  to a motherboard  108 , or other circuit board. The device  106  also includes package substrate  112 , die  114 , and solder bumps, pads or pins  116  for connecting die  114  and conducting traces (not shown) on substrate  112 . The lid  120  seals and protects die  114  and its connections  116  to substrate  112  and provides a mounting surface for heat sink  122 . The region between a lid  120  and substrate  112  may be filled with a resin or similar material for both its protective and thermal properties. FIG. 1 is only an exemplary IC; the present invention is not limited to the particular flip-chip arrangement depicted therein, other IC package types as known in the art are also contemplated within its scope. 
     Substrate  112  and connectors  110  are sometimes referred to as an IC package. For EMI measurement purposes, an IC package is basically a passive structure and the EM radiation depends on the harmonic contents of the currents that are flowing in it. The “package plus die” combination of a microprocessor contributes to the EMI performance of a computer system in the following two ways: direct radiation from the die and package through the heat sink assembly; and noise introduction into the supply and ground planes and the resulting radiation of this noise from the motherboard PCB traces and power cable. 
     At frequencies other than the core and I/O clocks and their harmonics, the influence of the microprocessor on the overall system EMI performance is negligible. 
     Far-field measurements of system EMI emissions which are used to compare CPU designs or perform EMC verifications are both costly and time consuming because of the testing equipment, set-up and procedures needed to acquire reproducible measurements. These disadvantages are only compounded when many measurements are required to obtain meaningful data. 
     One aspect of the present invention provides a novel method to measure the EMI emitted by the IC device. The RF noise voltage available on the lid  120  covering the CPU device  106 , measured with respect to the system&#39;s ground, is used as a measure of the CPU&#39;s emitted EMI. There are a number of justifications for using the lid  120  in this manner. First, the lid  120  of the CPU device  106  acts as a closely and precisely positioned sensor which capacitively couples the radio frequency (RF) energy from the die  114  and package, while remaining insulated from the die  114  and package. Also, because the energy at the lid  120  is what is coupled to the heat sink  122  and ultimately radiated, the voltage difference between the lid  120  and ground is a good indicator of the “die and package” radiation for a given motherboard, while considerably isolating the contribution of other sub-systems like the power supply and VGA cards. Third, the precise positioning of the lid  120  on each CPU device  106 , as compared to a near-field probe introduced during testing, allows more stable and reproducible data. Also, the manufacturing tolerances of dies, and thus the transmission lines within a package, are very precise. Therefore, any change in the RF potential signature on the lids  120  for CPU devices  106  with various package design options, are mainly attributable to changes in package design. 
     One method of measuring the lid&#39;s RF potential with respect to ground is a direct measurment in which the center conductor of one end of a coaxial cable is soldered to the lid and the outer conductor is soldered to the nearest ground. The other end of the cable is then connected to a spectrum analyzer. Although feasible, this method presents the difficulties of soldering to aluminum lids (which are common on many CPUs), choosing a common lid location to solder to, keeping the open-end ground length as short as possible, and soldering and desoldering the cable to each device under test. 
     FIG. 2 a  illustrates an exemplary embodiment in accordance with the present invention for indirectly measuring the RF voltage available on an IC&#39;s lid. Instead of directly measuring the lid potential, the set-up depicted in FIG. 2 a  allows coupling, through mutual impedance coupling, of the RF noise voltage from a lid  210  to a connected conductor  212 . In this arrangement the lid  210  and conductor  212  are both inductively and capacitively coupled (i.e. mutually impedance coupled) and the coupled energy indicates the noise that would have been coupled to the heat sink in a similar manner. 
     In the mutual impedance coupling (MIC) measurement set-up of FIG. 2 a , a motherboard  220  is supported by spacers  222  and has its ‘0 volts’ connected through metal reference plane  224  to ground. This arrangement simulates the presence of a typical PC chassis. Socket  202  accepts a CPU or other IC  204  which has a lid  210  physically adjacent to a conductor  212 . There are many known methods of securing heat sinks to microprocessors including various clips, clamps and adhesives. The present invention uses similar methods to secure the lid  210  and conductor  212  as well as to secure conductor  212 , heat sink  214  and fan  216 . Alternative connection methods include adding a weight  232  on top of heat sink  214  to operatively, but temporarily, connect the lid  210 , conductor  212  and heat sink  214 . In this alternative, a brass grille  230  is interposed to help isolate weight  232  from the fan  216 . 
     In certain embodiments, conductor  212  is constructed of copper and has substantially the same dimensions as CPU  204 . As for the thickness and other dimensions such as the side lengths, conductor  212  is preferably smaller compared to the wavelength of the highest frequency being measured; otherwise, the coupled noise voltage may vary depending on the location of the measurement of the conductor  212 . In certain embodiments of the present invention, the conductor  212  is shaped to fit the shape of lid  210  to better facilitate coupling of the RF potential and to assist with heat dissipation. 
     A coaxial connector  240  provides connection between a spectrum analyzer  248  and the conductor  212 . The outer conductor  242  of the connector  240  is electrically connected to the metal reference plane  224 . The inner conductor  244  of connector  240  is connected through a DC blocking capacitor  246  to conductor  212 . The connections between conductor  212 , capacitor  246  and connector  240  are accomplished using conventional soldering methods. 
     FIG. 2 b  illustrates another test set-up embodiment of the present invention. In this arrangement, coaxial cable  250  is used to connect conductor  212  to spectrum analyzer  248 . Specifically, the outer conductor  252  of cable  250  is electrically connected to ‘0 volts’ of the motherboard  220  and the inner conductor  254  is connected through the blocking capacitor  246  to conductor  212 . 
     FIG. 2 c  illustrates another test set-up embodiment which positions conductor  212  differently than the previous two figures. In this drawing, the conductor  212  is inserted between the weight  232  and brass grille  230  instead of being physically adjacent to lid  210 . The spectrum analyzer  248  still measures the voltage potential mutually impedance coupled to conductor  212  which, even in this arrangement, remains an indication of the RF noise energy on the lid  210 . 
     In testing operation of the present invention, the RF energy at lid  210  is coupled to conductor  212 , through mutual impedance coupling, and measured by a spectrum analyzer or other meter connected through connector  240  or coaxial cable  250 . 
     FIG. 3 depicts an equivalent circuit of the test set-up of FIG. 2 a . In this figure, V n    310  is the noise voltage available on the package lid (FIG. 2 a ,  210 ). Z m    312  is the mutual impedance between the conductor (FIG. 2 a ,  212 ) and the lid  210 . Z SA    318  is the input impedance of the spectrum analyzer and V m    320 . is the measured voltage at the spectrum analyzer. C hs    314  and R hs    316  are, respectively, the capacitance between the conductor  212  and the heat sink (FIG. 2 a ,  214 ) and the frequency-sensitive radiation resistance of the heat sink  214 . 
     In theory, the radiated energy from a device is obtained by adding (in logarithmic terms, in the frequency domain) the frequency spectrum of its excitation current with its coupling transfer function. A CPU and heat sink are considered to radiate like a monopole over a ground plane. Similarly, a mutual impedance coupling, or cross-talk, model has its own coupling transfer function. The consequence is that by subtracting the cross-talk coupling transfer function from the signal measured at the coupled conductor, the RF noise signal on a package lid can be determined. To calculate the EMI data, the radiation coupling transfer function is then added to the previously determined RF noise signal. In practice, however, the transfer functions of the devices and structures are not as simple as the theoretical models. Therefore, an empirical relationship between the two methods is used to predict the radiated emission data from the cross-talk data. As long as the insertion of the conductor between the heat sink and package lid does not change the current distribution, amplitude and waveform on the package and the conductor&#39;s thickness is small compared to the highest measured frequency&#39;s wavelength, then the radiated emission data can be predicted from the measured, cross-talk (or coupled) data. In particular, the mutual impedance coupling method correlates well with the standard far-field EMI measurement method if positive deltas in one method remain positive in the other method and vice versa, and the order of performance remains the same for a variety of packages at all spot frequencies of interest. In general, experiments undertaken by the present inventor, show an empirical correlation between the mutual impedance coupling (MIC) method and the far-field method; this correlation justifies the use of data acquired using the MIC method for qualitative comparison of the EMI performances of various IC packages and systems. 
     FIG. 4 depicts a flowchart which describes one method of utilizing the measured MIC data, described earlier, to determine far-field EMI performance of a device. In step  410 , far-field measurement of EMI data is first collected for a variety of devices to provide a basis by which to analyze MIC measured data. Next, for the same devices, MIC data is collected, in step  412 , and then compared to the far-field data to determine, in step  414 , the correlation between the two. After this correlation is determined, the expected EMI performance of an untested device can be determined using the novel mutual impedance coupling (MIC) method of the present invention instead of the traditional far-field testing techniques currently used. 
     When an EMC assessment of a new device, identified in step  416 , needs to be performed, or different design options need to be compared, each of the devices being assessed are tested, in step  418 , according to the MIC method of the present invention. During this MIC data collection, the test set-up, as depicted in FIGS. 2 a - 2   c , are operated so that RF noise energy is created on the IC lid and consequently coupled to the conductor. The spectrum analyzer is used to measure the resulting voltage levels on the conductor at a variety of frequencies. The voltage potentials observed at different frequencies are then used as indicators to determine, in step  420 , the EMI behavior of the devices. This determination is possible because of the correlation between MIC and far-field data discovered in step  414 . Thus, MIC testing allows, in step  422 , either EMI performance comparisons between different package designs or verification of compliance with EMC standards for a particular package without the need to perform far-field testing. 
     The present invention addresses the need for EMI measurement methods and arrangements other than the far-field measurement requirements described in EMI regulatory standards. This invention arranges a conductor and IC package such that RF noise energy on the IC&#39;s lid is mutually impedance coupled with the conductor. A test set-up is also presented for measuring the voltage levels induced in the conductor. Finally, these voltage levels are then used as indicator of the device&#39;s EMI performance. Thus, an assessment of a device&#39;s EMC can be accomplished without far-field test measurements being performed.