Chemical Degradation
An oxidation process destroys the metallic connections between a silicon die and the package lead frame when exposed to moisture and ionic contaminants. This wire bond corrosion increases the electrical resistance of the joint and can eventually lead to an open circuit. The reaction is often accelerated by the presence of halogen compounds in the molding material or surrounding environment.
Reliability testing is performed to evaluate the resistance of bonds to this degradation.
Intermetallic Formation
High temperatures promote the growth of intermetallic compounds at the junction between gold and aluminum wires. These compounds are more susceptible to corrosive attacks when moisture penetrates the package. The rate of this chemical reaction increases exponentially with temperature.
Reliability Testing
Testing involves exposing packaged devices to high humidity and elevated temperatures under electrical bias to accelerate the degradation. Post-test analysis uses scanning electron microscopy or acoustic imaging to detect cracks or voids in the bond region. The wire pull strength is also measured to evaluate the mechanical integrity of the joint after exposure.
These tests confirm the long term durability of the connections.
Material Selection
Using halogen-free mold compounds and corrosion-resistant alloys minimizes the risk of joint degradation. Solder alloys and wire compositions are selected for their compatibility to prevent galvanic reactions. Proper hermetic sealing also protects the internal wire bonds.
