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Failure due to humidity

SAQ

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The two main mechanisms which lead to failure in a humid environment are corrosion and electromigration:

• In corrosion, the surfaces are subjected to a chemical attack, which ‘eats away’ the material being corroded. One typical corrosion site is the metallisation of a semiconductor chip
• In electromigration, dendrites of conductor material grow across a surface under the influence of applied voltage, given a continuous (but very thin) film of water, leading to short-circuits between tracks. Board surfaces are typical electromigration sites.

Both mechanisms need a source of ions as well as water penetration to the surface, and are accelerated by increased temperature and applied voltage.

Materials and material compatibility are the key to minimising corrosion failure. The moulding compound should have as low as possible an ionic content, and minimal shrinkage; the chip passivation must be both inert and crack-free; all the materials must have compatible CTEs and form strongly adhered interfaces; the lead-frame design must maximise both strengths and the path length for moisture; processing must minimise contamination and mechanical stress during assembly.

The production defence against PCB electromigration aims to minimise ionic contaminants by appropriate processing and handling, and reduce water transmission by coating with solder resist. However, a primary protection is to use materials or platings which are less susceptible to migration.