EMC Performance of IC Packages

J. J. Rollin and G. Arcari None1 Networks

PO Box 35 11 Station C Ottawa, Ontario, Canada, KIY 4H7

Abstract: There arc obvious benefits to controlling Electromagnetic Interference (EMI) directly at the chip level through appropriate Integrated Circuit (IC) packaging. This paper presents radiated emission levels of Plastic Ball Grid Array (PBGA) and Super Ball Grid Array (SBGA) packages up to 10 GHz, and examines the features that can lead to improved performance. It is concluded that the Electromagnetic Compatibility (EMC) characteristics of SBGA packages are substantially better than those of PBGA packages. Any further EMC improvements are highly dependant on the SBGA grounding configuration employed.

INTRODUCTION

With the continuing trend towards higher operating frequencies, there is a need to assess EMC directly at the Application Specific Integrated Circuit (ASIC) level so that unwanted radiation may be controlled before propagating to the board and module levels [l], [2]. New IC packages are being developed to improve shielding and heat sinking capabilities, but there has been no systematic comparison of the EMC performance of different IC packages. This paper presents radiated emission levels of PBGA and SBGA packages up to 10 GHz, and examines the features that can lead to improved perfomnnce. Nate1 has recently tiled a patent application with the United States Patent Office regarding the technique employed in this paper for measuring IC package shielding effectiveness.

TEST PROCEDURE

The technique employed for determining the shielding effectiveness of IC packages is based on the radiated power obtained from an integrated circuit antenna in a GigaHertz Transverse Electromagnetic (GTEM) cell, as described in [3]. The die containing the IC antenna is shown in Fig. l(a). It consists of printed metal strips on a 6mm x 6mm silicon substrate, which can be configured to radiate as a dipole or a loop antenna depending on the feed connections. The die is then mounted inside an IC package, as shown in Fig. l(b).

The connections shown in Fig. l(b) correspond to loop antenna operation. It is well known that this mode of operation, resulting from currents along thin conductor tracks printed on a planar dielectric substrate, is the primary cause of high-speed IC Electromagnetic Interference (EMI) [4].

L. Roy University of Ottawa

School of Information Technology and Engineering Ottawa, Ontario, Canada, KIN 6N5

(a)

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Fig. 1. An antenna designed on a substrate (6mm x 6mm), (a), is mounted inside the IC package, (b).

In order to perform the electromagnetic radiation tests, the setup illustrated in Fig. 2 is employed. The package of Fig. l(b) is now mounted on a Printed Circuit Board (PCB) carrier with SMA connectors. The PCB carrier is gasketcd directly to the outside metal wall of the GTEM cell, with the IC package sitting inside the cell through an opening. The dense surface ground layer directly under the IC package allows for a continuous metal surface on the inside of the GTEM cell.

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Fig. 2. The test setnp for measuring the shielding elleeliveness of an IC package in a GTEM cell.

The following procedure allows shielding effectiveness to he determined for various packages.

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The IC antenna-is placed in a reference package and excited with a sweep generator over the frequency range of interest. The radiated power is collected at the opposite end of the GTEM cell and measured on a spectrom analyzer. The same IC antenna is mounted on the new package under test and measured under the same conditions. The difference between the two recorded traces represents the shielding effectiveness of the new package with respect to the reference package.

A PBGA package (Fig. 3(a)) was used as the reference for determining the shielding effectiveness improvement of the SBGA package (Fig. 3(b)). The additional heat spreader on the SBGA design was expected to reduce radiated emissions.

Fig. 3. The struchme of a PBGA package, (a), and of an SBGA packw, m.

Fig. 4 shows the measured results up to 10 GHz for the inside bell configuration (Loop B in Fig. l(b)). The absolute levels measured for each package are given to show individual characteristics such as resonances and level variations. The shielding effectiveness curve indicates an improvement of up to 38 dB with the SBGA configuration.

Having established that the SBGA package is superior to the PBGA package, it is appropriate to investigate the EMC characteristics of different configurations of the SBGA design. Fig. 5 compares the radiated emission levels of the SBGA fed by the outside balls with the SBGA fed by the inside balls (Loop A versus Loop B in Fig. l(b)). It is clear that signals fed through the inside balls result in lower radiated emissions, as would be expected from the shielding nature of the outside balls.

A typical implementation of the SBGA package is with the addition of a heatsink on top of the device. The effect of possible choices of heatsink grounding is assessed in Fig. 6. Generally speaking, below 4 GHz an increased number of grounding points results in a reduction of unintentional heatsink radiation. However, in the test setup implemented here, a 1 GHz resonance occurred for the 4 point grounding case, which involved grounding all 4 corners of the heatsink to the PCB. Although this phenomenon must be researched through testing and theoretical analysis, the most probable cause of this resonance is the physical geometry of the IC package.

It is interesting to determine the degradation of the EMC performance based on the use of a heatsink. Fig. 7 compares tbe radiated emission levels of the best configuration of heatsink grounding (4 point) with those of an SBGA without a heatsink. On average, the use of a heatsink entails an 8 dB penalty on radiated emissions. This could be explained by hypothesizing that the addition of a metal heatsink, even a grounded one, would tend to reradiate the emissions created by the IC itself. A more detailed analysis would need to be performed in order to prove, or disprove, this hypothesis.

As a final comparison, Fig 8 shows the improvement in EMC performance of an SBGA with an optimally grounded heatsink, over a PBGA package employed in a similar configuration. Typically, below 4 GHz, the SBGA package outperforms the PBGA package by 10 dB, with the exception of the 1 GHz resonance.

CONCZUSIONS A simple method has been employed for measuring the EMC performance of IC packages up to 10 GHz. A comparison of the PBGA and SBGA packages revealed substantially lower radiated emission levels for the SBGA. Several configurations of the 2 packages have been studied, showing the radiated emission level tradeoffs associated with different feed implementations and the use of a heatsink.

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REFERENCES

[l] Paul, C. R., Introduction to Electromagnetic Compatibility, Wiley Interscience, New York, 1992.

[2] R. De Smedt, S. Criel, F. Bonjean, and G. Spildooren, “Checking radiation effects of components with EMC test chips,” CEMCOMPO 99 (IERSET) Proceedings, Toulouse, pp. 28-33, January 1999.

[3] J. J. Rollin, L. Roy, and G. Arcari, “A novel technique for measuring IC package shielding effectiveness,” 16” IEEE Instrumentation and Measurement Technology Conference, Venice, 1999.

[4] K. Naishadham, J. B. Berry, and A. N. Hejase, “Full- wave analysis of radiated emission from arbitrarily shaped printed circuit traces,” IEEE Transactions on Electromagnetic Compatibility, vol. 35, no. 3, pp. 366- 377, August 1993. -

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-FSGA absolute level - hskle bat& (dBuV) s -SBGA absolute level - 4 heatsink grounds (dBuV) ------.SBGA absolute level- lnsMe bans (dBuV) .-----.SBGA absolute level _ No heatsink(dBuV) -knpravement in !%C performance of SBGA over F0GA (dB) -hcrease h radiated emissions of SBGA with heatsink relative to SAGA without heatsink (dB)

Fig. 4. The absolute measured levels for the PBGA package, the SBGA package, and the improvement in shielding effectiveness of

Fig. 7. The absolute measured levels for the SBGA with a

the SBGA package over the PBGA package. heatsink, the SBGA with no heatsink, and the increase in radiated emissions of using a heatsink over no heatsink.

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-SBGA absolute level-Outside balls (dBuV)

- - - - . - . SBGA absolute level - hskte bals (dBuV)

- hcraasa in radiated Mssions of SBGA fed by outside bals r&&e to i&de ball feed (dB)

- PBGA absolute level - 4 hsatsink grounds (dBuV)

-_ _- --.SBGA absolute level - 4 heatsinkgrounds (dBuV)

-!qmvemni in EN performance of SBGA w iih heatsink over PBGA with h~atshk (dB)

Fig. 5. The absolute measured levels for the SBGA fed by outside balls, the SBGA fed by inside balls, and the increase in radiated

Fig. 8. The absolute measured levels for the PBGA with a

emissions of using outside balls over inside balls. heatsink, the SBGA with a heatsink, and the improvement in

shielding effectiveness of the SBGA over the PBGA.

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Fig. 6. The absolute measured levels for the SBGA package using different grounding configurations for the heatsink.

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