Page 327 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION FORMS IN THE ELECTRONICS INDUSTRY 305

water reduction reaction. High pH leads to dissolution of surface passive oxide and
aluminum substrate with corresponding increase in conductor resistance.
Electrolytic metal migration occurs in silver-containing compounds. In the pres-
ence of moisture and an electric field, silver ions migrate to negatively charged surface
and plate out, forming dendrites. The dendrites grow and bridge the gap between the
contacts, causing an electric short and an arc. Even very small amounts of dissolved
metal can form a large dendrite. Under certain humidity and voltage gradients, a
month-long exposure equals 4 years of service in an office environment. Other metals
susceptible to metal migration are gold, tin, lead, palladium, and copper.

4.32.2 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. Substrate may corrode at imperfections of plating. When the
substrate is copper or silver and is exposed to sulfur or chloride containing environ-
ment, corrosion products can creep out of the pores and over gold plating, forming a
layer of high-contact resistance.

4.32.3 Fretting Corrosion of Separate Connectors with Tin Finishes
Fretting corrosion in electronic components is manifested as continuous formation
and flaking of tin oxide from the mated surface on tin-containing contacts. As the tin
is consumed, the problem becomes more severe. The best available solution to this
problem is to replace the part.

4.32.4 Galvanic Corrosion
Galvanic corrosion occurs when two dissimilar metals such as aluminum and gold are
coupled together as in package-integrated circuits. The polymers used for packaging
are porous, and the gaskets around hermetic covers such as ceramic or metal might
leak. In humid environments, moisture can permeate to the IC bond pad, creating
favorable conditions for galvanic corrosion. Processing-related integrated circuits are
exposed to aggressive media used in reactive ion etching (RIE) or wet etching for pat-
terning of aluminum lines, which can lead to corrosive residues. Reactive ion etching
of aluminum metallizations utilizes aggressive chlorine-containing gases. If removed
untreated from the etcher, patterned structures are covered with aluminum chloride,
which on hydrolysis forms hydrochloric acid in the presence of moisture.

4.32.5 Micropitting on Aluminum
Aluminum metallizations, alloyed with copper, form intermetallic compounds such
as Al Cu along the grain boundaries, which act as cathodic sites relative to aluminum
2
adjacent to the grain boundaries. This leads to dissolution of aluminum matrix in the
form of micropitting during the rinsing step after chemical etching.