Chapter 70 - Electronics
Robert Baboian,
Electrochemical and Corrosion Laboratory, Texas Instruments
Incorporated, Attleboro, MA 02703
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.
| Printed Circuit Board | Metallic conductor separated by insulating materials | Copper, copper alloys, copper clad materials, epoxy, resins, ceramics, woven glass fiber, electroplate, solder, tin, lead, conformal coatings |
| Contacts | Electrical contact maintained by mechanical force | Base Metal - copper alloys, steels (clad) Contact Surface - gold, palladium, silver-palladium, silver, tin, tin-lead, copper |
| Connectors | Electrical connections between systems or boards | Spring Material - beryllium copper, stainless steels Contact Surface - gold, palladium, silver, silver-palladium, tin, tin-lead |
| Switches and Relays | Cyclic electrical connection | Copper alloys, steels, stainless steels, electroplate and contact surface materials |
| Grounding contacts | For shielding | Copper, steels, aluminum, nickel, tin, tin-lead and contact surface materials Copper, aluminum |
| Thermal contacts Integrated circuits | Heat sinks Small dimension complex systems | Gold, silver, aluminum, Kovar, solder, glass, ceramic, silicon, silicon dioxide, silicon nitride, tungsten |
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.
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.
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.