IC circuits are exposed to a number of aggressive media used in reactive ion etching (RIE) or wet etching for patterning of aluminum lines, which can lead to corrosive residues. RIE of aluminum metallizations utilizes a combination of aggressive chlorine-containing gases. If removed untreated from the etcher, patterned structures are covered with aluminum chloride residue, which is hydroscopic and forms hydrochloric acid in the presence of moisture. (reference)
Ionic contamination can arise from fluxes used during soldering processes, from handling and other materials such as dust and dirt. Standard test procedures have been developed to measure the level of contamination which essentially involve washing the whole PCB in a solvent and measurement of the ionic conductivity. The severity of the washing procedures vary from simply removing surface soluble ions to extracting materials that has been adsorbed within the PCB laminate. The ionic contamination level is calculated from the ionic conductivity of the wash solution and the total board area. The result is an average value across the whole surface.
Aluminum metallizations, alloyed with copper, can form intermetallic A12Cu compounds along the grain boundaries, which act as cathodic sites relative to the aluminum adjacent to the grain boundaries. This leads to dissolution of an aluminum matrix in the form of micropitting during the rinsing step after chemical etching.
Both liquid and vapor-phase halogenated solvents used for production of ICs and PCs readily corrode aluminum-containing components. Water contamination of the solvents increases the time-to-corrosion on the one hand; however, on the other hand, it increases the subsequent corrosion rate. Dilution of the stabilized solvents with aromatic or alcohol solvents leads to the breakdown of the halogenated solvent and the formation of chloride ions, which corrode aluminum and aluminum-copper alloys.
The corrosion resistance of lead-tin solder in aqueous and gaseous environments is a function of the alloy composition. It improves significantly when the tin content increases above two weight percent. Lead forms unstable oxides, which easily react with chlorides, borates, and sulfates. (reference)
Frankel GS. Corrosion of Microelectronic and Magnetic Storage Devices. In: Marcus P, Oudar J, eds. Corrosion Mechanisms in Theory and Practice. New York, NY: Marcel Dekker, 1995.