Choosing Conducting Material Interfaces for Seams and Joints

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

An MD&DI January 1999 Column

EMI FIELD NOTES

Selecting proper materials minimizes corrosion EMC problems.

Increasingly, the medical electronics industry—along with the rest of the electronics industry—is being forced to use shielding to meet electromagnetic compatibility (EMC) needs. Effective shielding requires enclosure seams and joints that mate conductively along the joining boundaries. This can cause considerable difficulty for mechanical designers, because it requires the surfaces to mate at frequent intervals. Designers must either provide frequent positive contact points or interpose a conductive resilient gasket.

Even after taking such a step, however, designers face one more hurdle: corrosion. When corrosion occurs at the mating interface, conductive contact is lost and, thus, the shield's effectiveness is degraded. Worse yet, the corrosion may not occur during development but rather surface only after the product has been in the field for some time. So, it is wise to consider the issue of materials compatibility during the design phase, when more options are available and sensible design decisions can be made.

Even with effective shielding, it is still important to consider materials compatibility. Photo courtesy of Tecknit (Cranford, NJ).

The purpose of choosing the proper conducting material interfaces for seams and joints in electronic products is to minimize the penetration of the electromagnetic wave either into or out of the product and to minimize the impedance at the seam interface. The failure mode that produces electrical discontinuity at the otherwise conducting material interfaces is galvanically induced corrosion, which results in nonconductive film growth between the conducting interfaces. Between 10 and 20% of EMC design problems are the result of failures caused by corrosion. The potential effects of corrosion on EMC can include one or more of the following:

This article discusses four methods for choosing appropriate conducting material interfaces for seams or joints.

THE NATURE OF CORROSION

Galvanic corrosion can occur when two metals are joined in the presence of an electrolyte or humidity. When corrosion occurs, the impedance of the mating surfaces increases. In extreme cases, one of the metals will corrode (e.g., aluminum corrodes when mated to steel, a fact discovered by anyone who places an aluminum camper onto the steel bed of a pickup truck).

To combat such corrosion, some industries (notably the military-supply and automotive industries) have established test methods to determine materials compatibility and have published lists of compatible materials. These methods are summarized below.

Method 1. Military Standard 889B presents a limited choice of metals and alloys that are evaluated as compatible in marine and industrial environments. The difficulties with this standard are that no impedance values are given, the choice of metals or alloys is limited, and no surface treatments are recommended.

Method 2. Military Standard 1250A lists a number of metals and alloys, but recommends no surface preparations. Metals and alloys are considered compatible when the galvanic potential is equal to or less than 0.1 V (a few combinations at 0.25 V are also listed). No impedance values are given, and no environments are cited.

Method 3. The Society of Automotive Engineers (SAE) APR 1481 standard provides a more complete listing of metals, alloys, and surface preparations than MIL-STD 889B or 1250A. It states pressures and impedances but gives no environmental conditions.

Method 4. The present article is based on a fourth method, which provides more definitive data. Three studies have been performed using this method and their results have been reported.^1–3 Although all three used the same general method, differences in test fixturing preclude direct comparison. Accordingly, results from all three sources are provided.

Table I. Range of transfer impedance, both initial and final, after exposure to environment. Pressure and area were not stated, but the same type of fixtures were used throughout these experiments. Exposure to hydrogen sulfide (10 ppb), nitric oxide (200 ppb), and chlorine (10 ppb) give a 5–8-year lifetime for a commercial computer product.¹

Table II. Transfer impedance, both initial and final, after exposure to environment. The environment was 40°C at 95% relative humidity for 1000 hours.²

Table III. Transfer impedance, both initial and final, after exposure to the environment. The first two samples were exposed to a Battelle Class 3 flowing mixed gas, whereas the last two samples were exposed to 40°C/90–95% relative humidity for 240 hours.³

Basically, each study addressed the transfer impedance of the mating surfaces before and after the test. The test itself exposes the surfaces to various atmospheric conditions designed to encourage corrosion. Impedances of some mating surfaces increased significantly after the test. Note that some corrosion occurred even when joining similar metals, an indication that galvanic corrosion is not the only factor to consider.

The data from the studies are provided in Tables I–III. These data include the interfacial pairings, initial impedance, percentage change in impedance with aging, and final impedance after aging. These data—based on changes within each study's findings—are consistent for impedance per unit area and for pressure. However, they are not easily translated from author to author because of the differences in transfer impedance fixturing.

As presented here, the base material is treated as independent of the conductive interface. This is not always the case, since the base can sometimes become a part of the galvanic corrosion reaction. Base materials are usually selected separately according to factors such as cost as well as structural, mechanical, and jointing properties (e.g., welding). However, under certain conditions, the base may diffuse to the conductive interfaces, causing electrical discontinuity. When considering porous films such as chromates, the base material is especially important, because the electrolyte can penetrate the film and corrode the base material.

All three investigators measured the transfer impedance before and after exposure to corrosive environments. The measurement technique is discussed in IEEE P1302 and by Kunkel.²

CONCLUSION

Although the three tables address different materials and test conditions, they all indicate that compatibility of conductive mating surfaces must be considered during the design phase to prevent initially satisfactory mating from degrading and becoming ineffective. Once the bond of a seam or joint becomes ineffective, the device will no longer perform properly and may require expensive redesign to address EMC issues.

REFERENCES

1. B Archambeault and R Thibeau, "Effects of Corrosion on the Electrical Properties of Conducted Finishes for EMI Shielding" in Proceedings of the IEEE EMC Symposium (Piscataway, NJ: IEEE, 1989), 46–51.

2. G Kunkel, "Gasket and Gasketed Joint Considerations for EMI Shielding," ITEM (1994): 38–48.

3. WD Peregrim, "Comparison of RF Joint Corrosion in Different Environments" in Proceedings of the Third International Symposium on Corrosion and Reliability of Electronic Materials and Devices (Pennington, NJ: The Electrochemical Society 1994), 134–141.

Richard Haynes is the principal of Richard Haynes Consultants (Princeton, NJ). In addition to consulting on EMI issues, he conducts several seminars focusing on corrosion and reliability issues.

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