Environmental Testing of Electronics

Electronics, Corrosion and Environments

Electrical contacts, microelectronic circuits, and connectors found in common electronics are extremely sensitive to corrosive atmospheric exposure and surface contamination associated with corrosion attack. The presence of less than 1 microgram of corrosion product on the surface of a connector is in some cases a sufficient amount to interrupt the flow of electrons between mating contacts, thus rendering the component useless. Surface contaminants that affect the functioning of electronics are virtually everywhere; plant environments, areas with high levels of air pollution, storage areas, industrial settings, business offices and humid environments.

Electronics are exposed to a wide range of outdoor and indoor environments generally considered as atmospheric exposure. The corrosion behavior is determined by the actual environment which can be as benign as a simple low humidity, purified atmosphere, indoor location, to the aggressive environment existing at a pulp and paper mill or on an automobile which is subjected to road salt splash and spray. The electronics design as well as the nature of the environment are important because failures in printed circuit boards, integrated circuits, and other components have been known to occur even in extremely low levels of moisture and contaminants. Electronics components are mostly indoor or sheltered from direct exposure to liquid splash, spray, rain, snow, etc., and therefore the environment is considered atmospheric exposure.

Materials used in electronics are susceptible to corrosion in a wide range of environments. For example, sulfidation of silver in H₂S occurs in dry as well as humid air. However, moisture in the form of humidity is generally required for atmospheric corrosion to occur. When the humidity is increased, a moisture film can form on the metallic surface and can yield an increased rate of corrosion. The duration of time at which this occurs is referred to as time of wetness (TOW). However, the humidity at which wetness occurs is dependent on a number of factors including the nature of the material, surface roughness and composition, temperature, and surface contamination including atmospheric pollutants. Since the nature and significance of the moisture film is dependent on very complex and synergistic interactions among a large number of variables, this subject has received a great amount of attention and has resulted in conflicting points of view. This behavior is significant because corrosion tests must be designed properly and include the effects of complex and synergistic reactions. Otherwise, the test environments will not represent those to which electronics are exposed.

In some cases, a critical humidity exists above which significant corrosion occurs. This behavior has been described for steel in SO₂. However, many of the materials used in electronics do not exhibit this behavior. Copper in SO₂, for example, corrodes at a steadily increasing rate with increasing humidity, while in the absence of pollutants, its corrosion rate is very low even at 100% relative humidity. The presence of certain atmospheric pollutants therefore enhances corrosion by reaction on the surface in the presence of moisture to form corrosive species or corrosion products, or both. Their properties are different for each metal and therefore the corrosion behavior will vary. There are also a wide range of submicron atmospheric particles including various compounds of sulfate, chloride, nitrate, sodium, ammonium, potassium, magnesium, and calcium. These particles deposit on surfaces and react with moisture to form corrosive electrolytes. Sulfate and ammonium ions are the most common ones found in particulates in outdoor and indoor environments. Another source of contamination is chemicals from out-gassing of organic materials.

Short circuit in microelectronics due to corrosion and dendritic growth.

The damage to circuit board components resulting from corrosion attack is irreparable and compromising to the reliability of electronic devices. That’s why environmental testing of electronics and the development of corrosion prevention and control programs is fast becoming a necessary precaution in product development. To evaluate and safeguard your electronics investment from surface contamination and corrosion attack by atmospheric pollutants, an established testing procedure referred to as the Mixed Flowing Gas (MFG) Test (ASTM B-845), can be performed.

Another phenomenon in electronics degradation is whisker growth, defined as filimentary growth on metallic materials. Whiskers can be metallic or inorganic compounds. Although this behavior is still under investigation, it is reported to be a form of induced recrystallization related to metallurgical imperfections and occurs under the influence of stress. Whiskers grow on tin, zinc, cadmium, and silver and can grow long enough to short out circuitry.

An unexpected form of degradation of printed circuit boards (PCBs) is the growth of whiskers of copper, zinc, tin or even silver under the influence of tiny flows of galvanic currents in adsorbed moisture films on connectors. The one in the micrograph to the left formed on a large tin-coated circuit breaker contact. These whiskers commonly grow like “rosettes” at the interface between metal connectors and insulating bodies of transistors or integrated circuits. The center image above shows such rosettes along each of the connectors on an integrated circuit. The roses had originated at individual whiskers. Sulfur concentrations in the local atmosphere react with the whiskers to form ‘black roses” of sulfide crystals. Eventually these will short out the components. The solution to such a problem on PCBs is proper control of atmospheric pollutants or “potting” of the PCBs.

Corrosion in the electronics industry has become a significant factor in recent years because of the extremely complex systems that have been developed and the increasing demand on their reliability. Technological advances have resulted in the development of sophisticated components with closer spacing so that extremely low levels of corrosive contaminants can cause failure. Testing for this type of behavior is difficult and costly. Further advances in electronics can only be made where corrosion issues are addressed and reliability is maintained.

Materials-Electronic materials include a broad range of metals and alloys depending on the specific system, equipment, and components. The system as a whole can include structural materials such as steels, copper, nickel and their alloys, aluminum, and titanium. The system can also include structural materials used for cabinets as well as those used for electronic components. The broadest range of materials used in electronics is in components including printed circuit boards, contacts, connectors, switches and relays, grounding contacts, thermal contacts, and integrated circuits. Table 1 lists various electrical components and includes important materials of construction.

Another group of contaminants causing failure in electronics is residual chemicals. These are generally introduced during manufacturing and include fluxes, cleaning compounds, plating solutions, and metal processing fluids. Also included here are chemicals from fingerprints and saliva. Many of these include chlorides, and when these contaminants, such as residual chloride flux, are not removed, corrosive electrolytes are formed.

Classification of Environments

Corrosivity depends on a wide range of factors, including material and physical and chemical properties of the environment, a classification system should be based on these factors. However, the complex interaction of these factors requires that some simplification be made. In general, two systems of classification are used. One is based on corrosion of copper measured in the environment. The other is based on the levels of relative humidity and pollutants. Classification according to copper corrosion is used because copper provides a good representation of how metals behave in environments and is used widely in electronics. Classification according to levels of relative humidity and pollutants has gained wide acceptance, especially in combination with copper coupons. In some cases, the systems of classification are not adequate and can be related to specific factors. For example, behavior of silver contacts in sulfide environments can only be correlated with silver test material. In general, where local conditions vary significantly, correlation may be poor.

Corrosion Mechanisms-Corrosion mechanisms in electronic components have been the subject of intense study. Since electronics are largely found indoors and/or within packages or cabinets, the mechanisms leading to corrosion problems are not easily defined. Problems are compounded by the fact that these systems are fabricated by a number of complex processes and consist of a variety of dissimilar materials. Miniaturization and the requirement for high component density has resulted in smaller components, closer spacing, and thinner metallic paths. Thus, the effect of bias potentials and small defects is magnified.

Uniform and localized corrosion mechanisms are all important in electronic systems. However, some extremely important mechanisms of degradation are unique to this field of interest. Pore corrosion is one of these and is frequently associated with noble metal coatings such as gold on copper/nickel. Defects in the noble metal coatings give rise to galvanically accelerated pitting of the underlying base metal. When that process occurs, the corrosion products (for example from copper) can migrate over the surface of the metal coating (such as gold).

Several mechanisms of migration are known to cause problems in electronics and involve metal migration and ion migration. Metal migration occurring electrolytically involves: (1) electrodissolution, (2) ion transport, and (3) electrodeposition. The metallic material is oxidized producing ions that are transported through an electrolyte by electrical migration, diffusion, or convection, and then cathodic reduction of the metal ions at dendritic nucleation sites. Failure results due to the resulting conductive path formed across the dielectric between biased electrodes. This behavior has been reported for silver, gold, copper, and tin on devices such as integrated circuits and circuit boards.

Ion migration involves movement of metal ions over a surface away from the source metal. The phenomenon is commonly referred to as corrosion product creep. The mechanism is not well understood, but it is known to occur in humid or dry environments depending on the materials. Tarnish films that form on copper in environments containing H₂S and high humidity can creep over adjoining gold surfaces causing increased contact resistance of the gold.

One of the most widely investigated effects is the atmospheric sulfidation of silver used in electronics. The mechanisms of silver corrosion in polluted dry and humid atmospheres have been studied. The presence of NO₂ with H₂S greatly enhances silver sulfidation and when high relative humidity is added. The rate of sulfidation is extremely rapid and is limited by gas-phase diffusion even at high flow rates. Therefore, the solid-state transport of silver atoms from the bulk is faster than the interfacial reaction and does not affect the rate.

An illustrated formal report documents the results of the investigation. If necessary,  MATCO personnel can usually be on site almost immediately when requested during emergency situations.

Corrosion Testing and Analysis Team

Our team is comprised of certified and experienced personnel from a variety of technical disciplines including: metallurgical engineers, mechanical engineering, corrosion engineers, electronic specialists, chemistry, metallurgy, and materials science.  We also have a vast array of laboratory and in field testing equipment. All of MATCO's testing equipment is calibrated on a routine basis in accordance with both national and international standards, and is ready to be put to use at a moment's notice.

Our team includes NACE Certified Materials Selection/ Design / Corrosion/Coating Specialists (*) includes:

Dr. M. Zee(*)                                             Mr. Geoff Rhodes (*)

Dr. George Bayer                                      Mr. Marty Latona

Mr. Ed Larkin                                            Dr. Huping Xu

Ms. Heather Groll (*)                                  Mr. Sam Scheinman (*)