Humidity and pollution effects on Pb-free assemblies: a study of how metallization and operating voltage influence electrochemical migration.

A leading automotive industry survey expects an electronics part growth of 30% to 40% by 2008. The most common failure mechanisms are described in this recent scientific synopsis, namely electrochemical migration and leakage currents induced by corrosion. (1) As briefly described in a recent contribution to Kester's Lead-Free Connection newsletter, the presence of silver in common Pb-free alloys has been identified as critical to the assemblies' long-term reliability. The two major sources of climatic-induced failures are described here in detail to better understand their cause and minimize potential failures.

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To cause electrochemical migration, humidity is necessary, where a humidity film of a few monolayers is fully sufficient. A certain number of layers sufficient to start corrosion processes will be formed at a critical humidity level of about 60-70% RH at room temperature, depending on the substrate polarity and its surface energy. (2), (3) Hygroscopic pollution lowers the critical humidity in extreme cases to 40% RH. So, even with a regular office climate, computers, for example, are endangered. (4)

Given constant climatic conditions, the humidity adsorption for substrates (i.e., by epoxy resin) with SnPb metallization is concentrated mainly at the polymer surface (i.e., regular epoxy resin). This is in strict contrast to thermal cycles, in which the dewation occurs at the warmest, indolent places: the metallic areas. Dusts, salt particles and adsorbed S[O.sub.2], part of industrial atmosphere, work as seeds for condensation and support dewation. In addition to a critical humidity film, a metallization with sensitivity to migration behavior is required. In pure, condensed water, the migration tendency can be established through pH-value potential diagrams, so-called "Pourbaix" diagrams. (5) Interestingly, in the presence of chlorides and other complex ligands (i.e., pollution-induced) even stable elements/metals like gold, indium, palladium and platinum can migrate.

Visual phenomena. Bridges with tree-like, band-like and fibrous structures growing from the cathode to the anode can be observed independent of the concentration gradient at the cathode (Figure 1).

[FIGURE 1 OMITTED]

If the solubility of the corrosion complex created at the anode is very low, so-called "staining"--bridges growing from the anode to the cathode--can also be observed. (6) The composition of such bridges varies between metallic at the cathode and anodic dendrites, to metal salt complexes at the "staining" bridges.

Basic Electrochemical Migration Mechanisms

Anodic metal dissolution. Anodic solubilized metallic ions transfer to the cathode, where they are deposited via microscopically small peaks. The driving force of the dissolution is the electrolysis of the humid film by the operating voltage, normally between 2 and 5V. The free corrosion potential of a SnPb metallization shifts from about -279 SHE to less than -500 SHE, given electrolytic pH values higher than 12.

Metal dissolution kinetics occur directly at the crystal lattice level; they are initiated at the corner positions of the crystal surface, mainly caused by dislocations and crystal structure irregularities, and through adsorbed ad-atoms via continuous hydration. In addition, printed circuit board contamination supports bridge formation and growth. Harmful gases, such as nitrogen compounds [H.sub.2]S, S[O.sub.2] and C[O.sub.2], are dissolved in the humidity films and have similar effects. The dissolution and solvation behavior influences bridge formation. For silver migration (part of SAC305-based alloys) the water soluble Ag(OH), which is easily formed in aqueous solutions, is sufficient. In contrast, gold dissolves only if a tetra-chloro-gold complex can be formed. This example shows that, in fact, some elements do not form bridges under pure aqueous conditions. Naturally, migration processes increase significantly in the presence of complexes forming/binding contamination.

Metallic ions' diffusion. The electrical field determines the dissolved metallization ions' diffusion. The electrolyte conductivity depends primarily on the nature of metal salt ions, not on the degree of contamination. If the potential gradient is small relative to the concentration gradient of the migrated ions, the ions diffuse along the concentration gradient. If the concentration gradient is sufficiently large, a diffusion of the ions against the electrical field is actually possible, and staining bridges are formed. Operation voltage and conductor distance determine the potential gradient. The concentration gradient is determined by the current density, the active anodic area size and the solvated metallization ions' diffusion velocity.

Metallic ions' deposition. The deposition is dominant in areas with high field strength. If the solution conductivity is small, due to a small concentration of ions for example, the field lines are concentrated at peaks and edges. The increase of lattice dislocations on surface leads to a micro roughness, and the pyramidal growth to dendritic structure formation. In cases of high ion concentration, the growth of smoother structures is typically preferred.

Corrosion-Induced Leakage Currents

Apart from electrochemical migration, corrosion-induced leakage currents are electronics assemblies' second main failure mechanism. In sulfurous industrial gas atmospheres and humidity levels of about 60% RH, especially at copper contacts and metallic surfaces (depending on the Pb-free alloy), corrosion-induced leakage currents can be detected. This effect is induced by defects--for instance, because of pores and cracks in solder masks (Figure 2) or at glass of TO 5 transistor packages. (7) In contrast to electrochemical migration, few bridges were formed. The copper sulfate solution increases humidity film conductivity, until a shortcut occurs. The advanced corrosion processes probably cause conformal coating delamination, as a result of the presence of alkaline surfactants/saponification. (8) Therefore, corrosion processes and bridge growth will be accelerated. Bridge expansion takes place through diffusion processes. In the humidity film, gases such as S[O.sub.2] are solvated. NOx-combinations support the oxidation of S[O.sub.2] to sulfuric acid (9) and, therefore, to the industrial gas's corrosive effects.

[FIGURE 2 OMITTED]

In addition to corrosion-induced leakage currents, corrosion-induced cutoffs can occur (Figure 3). As a result, the glass conductor lines of hybrids are endangered. In this particular case, it is the contamination that supports the corrosion-induced defect, especially sulfur compounds.

[FIGURE 3 OMITTED]

Electrochemical migration and leakage currents cause most electronics components failures in humid atmospheres. Conductor spacing reduction enhances risks of shortcuts, especially sensors and control circuits in standby mode.

This study described the various steps of electrochemical migration, consisting of metal solubilization, metal diffusion and metal deposition. The influence by the type of metallization and operating voltage was stated. Further, it is noteworthy that silver, a component in most common Pb-free alloys, was found to be a main contributor to electrochemical migration. The type and concentration of contaminants mostly caused by flux residues determines the chemical composition of the bridges.

Leakage currents arise with air pollution, especially of sulfur dioxide. Solder mask imperfections, such as cracks and delamination, form crevices and induce corrosion. With SEM, it is now possible to prove the leakage path, even in cases where only extremely thin moisture films remain. Furthermore, the authors found that it is possible to analyze the chemical composition of the released salt bridges. Additional studies are underway to further correlate globally available Pb-free alloy compositions to the aforementioned failure mechanisms.

References

(1.) Study conducted at Zestron Analytical Centers in North America and Europe, 2005.

(2.) S. Lee and R.W. Staehle, "Adsorption Studies of Water on Copper, Nickel, and Iron Using the Quartz-Crystal Microbalance Technique: Assessment of BET and F Models of Adsorption," Materials and Corrosion, no. 48, Wiley-Interscience, 1997.

(3.) Helmut Schweigart, "Functional Safety of Conformally Coated Assemblies When Exposed to Moisture/Climatic Stressors," Hieronymus, 1997.

(4.) PC World, 1993.

(5.) Umut Tosun, Is Climatic Reliability Endangered by Lead-Free Assemblies? SMTA Boston chapter meeting, November 2005.

(6.) Simeon J. Krumbein, "Metallic Electromigration Phenomena," Chapter. 5, Christou: Electromigration and Electronic Device Degradation, Wiley-Interscience, 1994.

(7.) W. Ernst, "The Impact of Air Pollution on Components in the Electronics Industry, Electronic Industry, 7/8, 1986.

(8.) Helmut Schweigart, "Impact of Environmental Conditions on Conformal Coatings," Society for Environmental Simulation, Pfinztal, Germany, 1998.

(9.) D. Hajas, "Electrical Resistance on Solder Masks," Color & Coating, 94, 1988.

Helmut Schweigart, Ph.D. and Harald Wack, Ph.D.

Helmut Schweigart, Ph.D., is head of application technology, Zestron Europe (zestron.com); [e-mail]. Harald Wack, Ph.D., is executive vice president and CEO, Zestron America; [e-mail].