Abstract:
A new method is introduced to characterize EMC at the IC/ASIC package level. It is designed to excite the modes of radiations found in IC/ASIC packages, and permits the characterization of packages in a way that represents their real application. In the new method, a small antenna designed on a die is mounted inside the package under test. The package is mounted on a carrier PCB that is used to feed a signal into the antenna on the die. Radiated emissions from this set-up are measured. The antenna on the die is measured in free space to obtain a reference level which represents its capability to radiate without shielding. The antenna is then measured inside a package to obtain its capability to radiate with package shielding. The difference between the two measurements will represent the shielding effectiveness or the EMI reduction the package offers to the die. All measurements may conveniently be performed inside a TEM (Transverse Electromagnetic Mode) cell. The antennas designed on the die will permit the measurement of shielding effectiveness, ground bounce and immunity properties of the packages.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method and apparatus for testing integrated circuit packages for their effectiveness in shielding electromagnetic interference (EMI) emissions. 
     BACKGROUND OF THE INVENTION 
     EMI radiation from electronics mounted within a package or enclosure must be minimized in order to reduce or eliminate its potentially interfering effects on surrounding equipment such as communications receivers etc. Standards exist which stipulate the maximum EMI radiation which is permitted. These are referred to as EMC (electromagnetic compatibility) standards. Historically, the focus on reducing EMI radiation has been on enclosures or boxes which surround the electronic equipment mounted within the box. However, with new ICs (integrated circuits) and ASICs (application specific integrated circuits) being designed to operate at faster and faster speeds, individual chips are becoming significant contributors to the EMI problem and it has become important to design IC packages which are effective in reducing EMI, thereby reducing the burden of EMI reduction placed on the enclosures in which the IC packages are to be installed. 
     Existing methods for the EMC characterization of silicon devices were developed to measure radiation from printed circuit boards in operation inside electronic devices. These methods are not appropriate or capable of characterizing ASIC packages on their own without electronics. From a mechanical package point of view, various methods exist for measuring shielding effectiveness of metal enclosures for electronic equipment. However, these methods are not appropriate for small devices such as an ASIC package. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to obviate or mitigate one or more of the above identified disadvantages. 
     A new method is introduced to characterize EMI at the IC/ASIC package level. It is designed to excite the modes of radiation found in these packages in a way representative of the electronics which would normally exist inside them. In the new method, a small antenna designed on a test die is mounted inside the package under test. The package is mounted on a carrier PCB (printed circuit board) that is used to feed a test signal into the antenna on the die. Radiation is then measured from this set-up. 
     Preferably, the antenna is first measured in free space to obtain measurements which represent its capability to radiate without shielding. The antenna is then measured inside a package to obtain measurements representing its reduced capability to radiate with package shielding. The difference between the two measurements represents the shielding effectiveness or the EMI reduction the package offers to the die. Preferably, all measurements will be performed inside a wideband TEM (Transverse Electromagnetic Mode) cell. 
     According to a first broad aspect, the invention provides a method of characterizing the EMI (electromagnetic interference) emissions of a die package comprising the steps of: installing a test die in the package in place of where a normal die would be located, the test die having a first antenna thereon; injecting a test signal to the first antenna; measuring the EMI emissions produced by the first antenna at a point outside the package. 
     According to a second broad aspect, the invention provides a method of characterizing the EMI (electromagnetic interference) immunity of a die package comprising the steps of: installing a test die in the package in place of where a normal die would be located, the test die having a first antenna thereon; injecting a test signal to a second antenna located outside the package; measuring a resulting signal received by the first antenna. 
     According to a third broad aspect, the invention provides a test die for mounting within a package for performing EMI characterization of the package, the die comprising: a die with at least one antenna etched thereon; and connecting pads for connecting the antenna to connecting elements on the package. 
     According to a fourth broad aspect, the invention provides a test package for performing EMI characterization of a sample package, the test package comprising: a sample package with a test die installed in place of its normal die, the test die having at least one antenna etched thereon; 
     the test die having connecting pads connecting the antenna to connecting elements on the package. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described with reference to the attached drawings in which: 
     FIGS. 1 a,    1   b,    1   c  are side sectional views of a BGA (ball grid array) package, a SBGA (super ball grid array) package and a FSBGA (Faraday super ball grid array) package, respectively; 
     FIG. 2 is a plan view of a test die according to an aspect of the invention; 
     FIG. 3 is a bottom view of the die of FIG. 2 mounted inside an SBGA package; 
     FIG. 4 a  is a top view of a carrier PCB (printed circuit board); 
     FIG. 4 b  is a perspective view of the carrier PCB of FIG. 4 a  with the test die of FIG. 2 mounted thereon; 
     FIGS. 5 a  and  5   b  are schematic illustrations of a EMC shielding effectiveness measurement test setup; 
     FIG. 5 c  is a graph containing test EMI readings for each of the packages of FIGS. 1 a,    1   b  and  1   c;    
     FIGS. 6 a  and  6   b  are side sectional views of two SBGA packages with each having a slightly different heat sink and mounted on a carrier PCB; 
     FIG. 7 is a schematic illustration of an immunity characterization measurement test setup; and 
     FIG. 8 is a top view of test die according to another aspect of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the invention will be described as it may be applied to testing the shielding effectiveness of various enhancements to a basic BGA (ball grid array) package, but of course it is to be understood that the invention may be used with many other packaging types. 
     For the purposes of this example, three different test EGA packages will be considered these being the basic plastic BGA, the SBGA (super ball grid array), and the FSBGA (Faraday super ball grid array). 
     Referring to FIG. 1 a,  a basic plastic BGA package generally indicated by  10  has a test die  11  mounted within a plastic package  12 . The package  12  has a set of connecting elements which consists of an array of balls  14  located on the bottom side of the plastic package  12 . Although not evident from the sectional view of FIG. 1 a,  the array of balls consists of concentric rectangular rings of balls around the test die, and include an outer ring  14   o  and inner ring  14   i.  Connections  16  from various balls  14  to the test die  11  may be made. The package  10  is mounted to a package carrier such as a carrier PCB (printed circuit board)  18  which has a complementary footprint (not shown) by placing the array of balls  14  onto the footprint on the board  18 . The regular plastic BGA package  12  has no emissions reductions features. 
     Referring now to FIG. 1 b,  an SBGA (super ball grid array) package is shown which is similar to the BGA package of FIG. 1 a  except that the test die  10  is mounted beneath a metal lid  20  rather than within a plastic package. A plastic layer  22  separates the metal lid  20  from the array of balls  14 . The metal lid  20  in this case was introduced primarily to dissipate heat out of the die. It also provides a level of shielding effectiveness and should result in a decrease in measurable EMI when compared with the basic plastic BGA package of FIG. 1 a,  but there are still areas of the package through which EMI may leak. 
     Referring now to FIG. 1 c,  a FSBGA (Faraday super ball grid array) package is shown which is similar to the SBGA package of FIG. 1 b  except that in this case the metal lid  20  is connected to the outer balls  14   o  of the array of balls  14  through a metallic multilayer interconnect  24 . The package is shown mounted to a carrier PCB  18  which has a ground plane  26  connected with the outer balls  14   o  with the result that the lid  20  is grounded through the package to obtain a Faraday cage around the test die  11 . This should result in a further level of improvement in EMI reductions when compared with the SBGA package of FIG. 1 b.    
     Referring now to FIG. 2, a test die according to an aspect of the invention is shown and is generally indicated by  11 . The test die  11  has the same size as a normal die which would be mounted in the packages under test, in this example, the various BGA packages. There are three different antenna patterns  30 ,  32 ,  34  on the die. Two of the patterns  30 ,  32  each consist of a pair of straight lines across the die  11 . The third pattern  34  consists of a plate area. To minimize cost and increase design flexibility, the die  11  may be fabricated on the fibreglass material used for PCBs using conventional print and etch techniques for PCBs. It is preferably finished with soft gold plating over nickel to permit wire bonding inside the package. 
     In FIG. 3, the antenna die  11  of FIG. 2 is shown mounted inside a test SBGA package  36 . The first pair of straight lines  30  are connected to two pairs of outside balls  38  in the outside row of balls  14   o  of the ball array  14 , so as to permit the testing of EMI due to signals carried through the outside balls. The second pair of straight lines  32  are connected to two pairs of inside balls  40  in the inside row of balls  14   i  of the ball array  14 , so as to permit the testing of EMI due to signals carried through the inside balls. Having separate connections to inside balls and outside balls will permit the measurement of the difference in radiation due to signals on the inside balls and the outside balls. These pairs of lines  30 ,  32  will form loop antennas with the rest of the package inter-connection and carrier PCB, as discussed further below. 
     The plate area  34  is designed to test noise distribution in ground balls (ground bounce) as well as some RF emission modes of the package. Each package has a number of ground balls  42  (shown in shaded grey) which are to be connected to ground. The plate area  34  consists of a plane of ground which is tied to the ground balls  42  of the package. As many ground balls  42  as possible should be bonded to this plate area  34 . However, the quantity of balls used should be consistent with the actual quantity of ground balls in each of the packages being tested. One signal pin  44  is bonded to the middle of the plate  34  to permit the injection of a test signal into the ground plate  34 . This will permit the measurement of the effectiveness of each package ground structure at returning current. 
     Referring now to FIG. 4 a,  a multilayer carrier PCB generally indicated by  18  with a BGA footprint  52  is shown which is designed to mount the three types of packages described previously. The carrier PCB  18  also has three terminals for injecting test signals into a BGA mounted to the carrier PCB. Referring now to FIGS. 3 and 4 a,  a first terminal  54  is for injecting a test signal into one of the outside balls  38  connected to the first pair of lines  30 , a second terminal  56  is for injecting a test signal into one of the inside balls  40  connected to the second pair of lines  32 , and a third terminal  58  is for injecting a test signal into the ball  44  connected to the plate area  34 . The carrier PCB  18  also has connection traces  60 ,  62  between where two of the outside balls  38  and two of the inside balls  40  are to be connected. When a test die  11  is installed on the PCB  18 , these traces  60 ,  62  make each pair of lines  30 ,  32  into a loop antenna, with one end of each loop connected to one of the terminals, and the other end of the loop connected to ground or a terminating resistor load. The carrier PCB has ground planes on its outside layers to shield the signal traces and prevent radiation. This will permit the measurement of emissions originating from the die and package structure, and not from the carrier PCB. The carrier PCB  18  also preferably has heat sink grounding straps  63  to test their effectiveness at reducing heat sink re-radiation. A perspective view of the PCB of FIG. 4 a  is shown in FIG. 4 b  with the test die  11  installed without its package for clarity. 
     Referring now to FIGS. 5 a  and  5   b,  during test, the carrier PCB  18  with the package  36  under test will be mounted on a special bulkhead  70  that together with the PCB is installed over an opening  72  in a TEM (transverse electromagnetic mode) cell  74 . Only the package  36  under test will be inside the TEM cell  74 . This setup will permit the measurement of emissions from the package  36  only. Any leakage from any external equipment will be shielded out of the measurements. 
     A signal generator  76  is provided for injecting a signal into one of the terminals on the carrier PCB  18 . An antenna  78  in the form of a septum or plate as is commonly used in TEM cell devices (see FIG. 5 b ) is mounted inside the TEM cell  74  and has a connection to an externally located spectrum analyzer  80 . 
     Testing is performed for each of the various types of BGA packaging. The measurement may start with the plastic BGA, thereby providing a reference point. Measuring the BGA package not offering any shielding is equivalent to measuring the antenna in free space. Alternatively, a shielded package can be measured with its shielding lid removed. 
     The signal generator  76  will source a test signal into one of the antenna patterns ( 30 ,  32 , or  34 ) on the die  11  through a SMA (subminiature series A) coax connector to one of the terminals ( 54 ,  56  or  58 ) on the carrier PCB  18 . The spectrum analyzer  80  attached to the TEM antenna  78  will measure an output signal produced by the antenna  78  representing the emissions of the package resulting from the test signal. Preferably, the signal generator will sweep the frequency range from 30 MHZ to 5 GHz with the test signal to characterize the package over the frequency range of interest. The test will be repeated for each antenna pattern with the same input signal. In each case the emissions detected by the antenna  78  in the TEM cell  74  will be measured over the same frequency range. 
     The test will be repeated on each type of package using the same test signal sweep. By comparing the spectrum analyzer output for the different package types, a determination of the effectiveness of each shielded BGA package compared to the basic BGA package can be made. 
     FIG. 5 c  is a graph containing sample plots of the EMI emissions of each of the three BGA package types. Two graphs are shown for the FSBGA package type, FSBGA OUT being the performance measured for outer balls, and FSBGA IN being the performance measured for inner balls. By preparing graphs such as this, testing personnel can determine the relative performance of each package type over the entire frequency range of interest. In the example of FIG. 5 c,  it is evident that the FSPGA package offers a substantial performance improvement particularly at frequencies below 3 GHz. 
     In the illustrated embodiment, in order to connect to the different antennas different connections to the die must be made. Alternatively a switching mechanism may be provided on the test die itself which allows an operator to switch between antennas on the die without requiring any change to the connections to the test die. This might be done with a manual switch, for example a screwdriver adjust through a non-radiating aperture. An electronic switch might alternatively be provided for switching between antennas. 
     In the illustrated embodiment, the noise is piped into the die from an external source. Alternatively, a noise source may be implemented on the test die itself. For example, a power transistor on the test die connected to an internal or external clock drive may be used to implement a noise source. For a design having an internal clock drive, the only connections required during testing would be to a power supply and to ground. 
     According to another aspect of the invention, methods are provided to determine the effect of a heat sink installed over a package from an EMC point of view. In the context of the above example, the testing procedures are repeated again with a heat sink attached to each of the basic plastic BGA, the SBGA and the FSBGA. The test may be performed with various grounding topologies for the heat sink, for example with one, two and four ground straps, and with different locations of the ground post, for example centre or corner. In FIG. 6 a,  an example SBGA test package is shown with an ungrounded heatsink  90  mounted on its surface, while in FIG. 6 b  the package has a grounded heatsink  92 . 
     According to another aspect of the invention, methods are provided for characterizing packages for immunity from EMI. This will determine the effect of the packages on the IC/ASIC&#39;s immunity from external sources of EMI. The immunity characterization test method is similar to the shielding effectiveness test method except that the signals on the antennas are reversed. An example test setup for immunity characterization is shown in FIG.  7 . In this case, an antenna on the die  11  is used as a receiving antenna and has its output connected to the spectrum analyzer  80 . The TEM cell antenna  78  is used as the test signal injecting antenna, and has its input connected to the signal generator  76 . The quantity of noise measured inside the ASIC package determines the immunity characteristics of the package. It may again be used to characterize package immunity over the frequency range 30 MHZ to 5 GHz, for example. 
     Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practised otherwise than as specifically described herein. 
     While a test die has been described which has a pair of lines and a ground plate area, other test die configurations are possible. For example, a configuration such as the one illustrated in FIG. 8 may be used in which two loop antennas are etched directly on the test die. 
     The guiding principle in these cases is that the antenna configuration is adjusted to imitate the flow pattern of high-frequency currents expected in the final integrated circuit device; this may be different for specific integrated circuit designs. 
     While the illustrated example has used connecting elements which are balls, other types of connecting elements may be used, such as pins for example. 
     In the illustrated example, signals are injected in single ended mode. However, differential signals could also be used to drive a pair of antenna lines for example. In this case, the differential signal would be applied across one end of the pair with the remaining end (both lines) being connected to ground.