Page 206 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

184 CORROSION CAUSES

TABLE 3.15 Instruments System Automation Society Classes
Class Description Expected Time-to-Failure
G1 (mild) Corrosion not a factor No corrosion-related failure
G2 (modern) Corrosion <1000 Å per month Failure in 3–4 years
G3 (harsh) High corrosion <2000 Å per Failure in 1–2 years
month
G4 (severe) Considerable corrosion <3000 Å Failure in 1 year
per month

The various forms of corrosion that may be encountered in the field of electronics
are as described below.

1. Anodic Corrosion. The spacing between components of the ICs is small,
and when a voltage is applied to a device, voltaic gradients of the order
6
5
of 10 –10 V/cm can exist across surfaces, accelerating electrochemical
corrosion reactions and ionic migration. In ICs, positively biased aluminum
metallizations are susceptible to corrosion. A combination of electric fields,
atmospheric moisture, and halide contamination leads to corrosion of
aluminum. Gold and copper metallization are also subject to corrosion under
these conditions.
2. Negatively biased aluminum metallizations can corrode in the presence of
moisture because of the high pH (basic) produced by the cathodic reaction
of water reduction. The high pH can dissolve the passive oxide on aluminum
along with the corresponding increase in conductor resistance possibly up to
open-circuit value.
3. Electrolytic Metal Migration. Detected early on in electromechanical
switches, this problem occurs in silver-containing compounds. In the pres-
ence of moisture and an electric field, silver ions migrate to the negatively
charged cathodic surface and plate out in the form of dendrites. The dendrites
grow and eventually bridge the gap between the contacts causing an electric
short and an arc. Large dendrites may be formed even from small volumes
of the metal. Under certain humidity and voltage gradient conditions, a
30-day exposure becomes equivalent to 4 years of service in a typical office
environment (42). Other metals susceptible to metal migration are gold, tin,
palladium, and copper.
4. Pore-Creep in Electrical Contacts and Metallic Joints. To prevent tarnishing
of connectors and contacts, a noble metal such as gold is plated on the contact
surface. The coverage of noble metal such as gold may not be perfect, and
corrosion can occur at the site of imperfections. If the substrate is copper or
silver and exposed to either chloride or sulfur-bearing environment, corrosion
products can creep out from the pores and cover the gold plating with a layer
of high contact resistance.