Intermetallic Compound
Chemical reaction products forming during soldering operations on electroless nickel immersion gold surfaces govern mechanical joint reliability under mechanical shock. Characterization of electroless nickel coatings identifies ni3p as a phosphorus-rich interfacial layer generated when tin reacts with nickel. Electroless nickel deposits contain six to ten percent dissolved phosphorus by weight within the metallic matrix.
As tin-based solder wets the nickel layer, tin reacts preferentially to form nickel tin intermetallics, displacing phosphorus into adjacent unreacted plating. The resulting phosphorus enrichment forms a continuous crystalline compound layer adjacent to the solder interface.
Phase Enrichment
Excessive phosphorus concentration at the wetting boundary creates micro-voiding and structural embrittlement during thermal aging. When tin consumes underlying nickel, phosphorus concentrates in a thin band that converts to crystalline phase structures. High-phosphorus chemical baths suppress nickel corrosion during gold immersion but form thicker phosphorus-rich interfacial layers during assembly reflow.
Solder joint shear strength declines rapidly when this intermetallic layer exceeds critical thickness thresholds. Thermal shock cycling drives Kirkendall void growth along the intermetallic margin, leading to catastrophic brittle solder joint detachment under shock loading.
Fracture Boundary
Mechanical stress concentrates at the interface between crystalline nickel phosphide and adjacent nickel-tin intermetallic structures. Mechanical shock testing exposes flat, featureless fractures along this brittle plane. Control of electroless nickel bath chemistry maintains phosphorus content within tight limits to inhibit excessive phase growth.
