RoHS Compatabilities

No matter where you look today, you'll find the increasing familiar "complies with the RoHS" directive symbol in many advertisements, data sheets and catalogs. RoHS, the Removal of Hazardous Substances directive (Directive 2002/95/EC of the European Parliament) came into full force this year even though it was released Jan. 27, 2003. In the rush to RoHS compliance, there are two other compatibility issues to consider.

The first is electromotive force (emf) compatibility, otherwise known as the "Electrochemical Series." When metals are brought in contact, an exchange of electrons occurs between the surfaces. If the voltage potential between the two metals is too great, corrosion will occur. The corrosion will impact the integrity of the metal and seriously degrade conductive contact.

The RoHS move toward silver immersion plating on circuit boards can degrade the emf compatibility if the characteristics of the chassis surface are not considered simultaneously. When circuit boards are "plated" with tin-lead alloy solders, the surface of the chassis such as a zinc plate is usually emf compatible. Silver immersion plating causes a very different result. Determining emf compatibility and preventing corrosion and a serious degradation of electrical conductivity over time, requires a review of the electromotive force series.

There are two representations of the emf series: 1) as an index approximation of values compared to gold, where gold is the index reference value of "zero" or, 2) as a sequence of the actual electrode potentials that would be listed in well-known references, such as the Handbook of Chemistry and Physics. Of these two methods, the general approximations of the index version offer a very functional listing though the values of reference are often grouped into classes of metals. This index approach, even though broadly utilized in industry, requires that the potentials are typically grouped into approximate classes of metals and rounded off in a summary form resulting in less precision. Additionally, since the index version is a listing referenced to gold, conversions back to the electromotive series of the elements are required to determine the standard electrode potentials. Dichotomies will be found in these conversions due to the imprecision of the "index" approach.

Index Approximation vs. Electrode Potential

The "index approximation method," lists groups of metals in a table with gold as the reference. This table has been popular since the 1960s and is often used as a reference for various EMI shielding materials.

From the table, you can recognize that tin/lead solders exhibit an "index" voltage value of -0.65V. On the other hand, zinc exhibits a value of -1.25V. The emf potential differential value between these commonly used materials would be approximately 0.6V.

Now, look at the value of silver. It displays an emf potential of -0.15V, much further away from zinc. The emf potential difference between zinc and silver, as in a silver immersion plated circuit board, would be a whopping 1.1V! Corrosion would be guaranteed from the combination of these metals.

A more accurate method would be to use the electrode potential series rather than the index approximation. The real electrochemical series version of the electromotive force series defines the electrode potential in volts of the elements compared to a hydrogen electrode as a reference. The hydrogen electrode is known as a "hydrogen half cell." The hydrogen electrode, utilized as a base reference to determine electrode potentials of the elements, is comprised of a platinum foil immersed in a 1.0 molar solution of hydrogen ions at 1 atmosphere pressure. Hydrogen gas is bubbled over the foil in this immersion. The measurements are taken as electrode potentials at 25C. You may hear electrode potentials referred to as "reduction potentials."

Comparing tin or lead with the approximate value of -0.130V to zinc, with a value of -0.763V, yields a differential of 0.633V, which is very close to the comparison of the "index" method. Comparing silver with a potential of +0.799V to zinc at -0.763V, yields a differential value of an even larger 1.562V! This value is probably more representative of what would happen in real products.

Paired Performance

The goal is to pair metal surface combinations that are understood to be compatible in order to circumvent corrosion that would significantly impair or deteriorate the surface-to-surface contact conductivity over time. Considering the vital interactions that "skin effects" (and the related skin depth and skin effect resistance) impart into high frequency shielding performance, and given that compressible conductive contact surfaces are vital in the design implementation of shielding, the compatibility of the metallic junctions must be emphasized as imperative for metal selections during design.

The actual acceptable differential value of emf potentials that may be utilized with implied reliability depends upon several factors. These factors include the nature of closely adjoined, or faying surfaces, for example static-quiescent or motional and abrasive, and the expected operating environment. In high salient environments (such as overlooking ocean surf motions) the required electrode potentials can be very low (marine or Naval environments would certainly qualify for rigorous use) for acceptability, often in the range of only 0.1 to 0.25V maximum differential. In more typical commercial environments with quiescent surfaces, empirical results indicate that differentials in the approximate maximum range of 0.35 to 0.40V may be acceptable. It is suggested that emf differentials approaching 0.50V in commercial environments may be acceptable for surfaces that are subject to mutual abrasion, though avoided in quiescent applications. Differential values above 0.60V should be avoided in any situation.

Another RoHS Compatibility Factor

Electromagnetic Compatibility (EMC) issues are the second RoHS consideration. EMC has two factors: surface conductivity (resistance) and high frequency "skin effect." Surface resistance is an issue with pressure required to "access" the conductive surfaces. By knowing the surface "strike area" (in square inches) and the pressure applied to two adjoining surfaces, it is possible to measure surface resistance in "Ohms per square."

For most high frequency performance, the goal is values for surface resistance of equal to or less than 50 mO/square, but in various instances, values as high as 100 mO/square may suffice. Before you ask "per square what," it is important to recognize that in the resistive lattice model of square surfaces, the series and parallel resistive equivalent elements always balance out. As long as it is a square, it does not matter whether it is a square inch, centimeter, or furlong. My recommendation is that you know what the performance is of the material you're replacing for RoHS compliance, and equal or better it under the RoHS requirements.

A final EMC shielding compatibility issue involves the skin effect. Change the conductivity, thickness, and relative permeability of your metal surfaces, and almost certainly you will alter the shielding. If your surface plating conductivity is the same or better than what you are replacing and the relative permeability is the same, your RoHS changed product is probably okay for EMC compliance. If it is not the same or if you don't know, you have to evaluate the structure.

Shields that are multiple skin depths thick are essentially comprised of two plated surface boundaries and one core. To consider the functions of and within a shield structure fabricated from sheet metal, it is appropriate to view the shield as it would be engaged through impingement by the electromagnetic wave. From this electromagnetic wave view, there are three components embedded within the metal:

• Two opposing surfaces, which could be termed "surface skins," and,

• A core material.

The first effect is the performance of the "surface skins" to any electromagnetic wave impedance. A change of the surface skin such as the plating may require recertification of your compliance to EMC standards. Calculating the skin depth may provide some insight into the amount of change that has occurred.

According to the International Telephone and Telegraph Corporation "Reference Data for Radio Engineers," the value of skin depth, d, is calculated by:

d = v (2/µrsr?) (in meters since the values of µr and sr are expressed with relationship to meters). For the complete discussion, go to http://rbi.ims.ca/4928-577.

RoHS is Not the Only Compatibility Issue

Before concluding that a RoHS-compatible design is complete, make sure it is also compatible with the electromotive force series and the surface compression required for low resistance, and that the skin effect values do not alter your EMC performance. Otherwise, it is just a matter of time before these older design compatibility issues become apparent.

CREEPING EFFECTS OF INCOMPATIBILITY

Emf incompatibilities and the related effects can progress in stealthy ways.

Initially, the effects may be essentially non-visible. Conductive contact performance will start degrading to impact RF skin effect resistance. This first effect will show up particularly at higher (greater than 200 MHz) frequencies.