Segregation Mechanism
Chemical partitioning of dissolved elements away from advancing intermetallic reaction fronts occurs during soldering on electroless nickel deposits. Microstructural analysis measures phosphorus rejection as tin selectively consumes nickel to form binary nickel-tin intermetallic phases. Electroless nickel plating holds amorphous phosphorus in solid solution within the nickel matrix.
Molten tin reacts with nickel at four hundred degrees Celsius, forming intermetallics that cannot accommodate phosphorus atoms in their crystal lattice. Expelled phosphorus moves into remaining unreacted nickel metal immediately below the solder interface.
Interface Accumulation
Concentration of expelled phosphorus creates a hyper-eutectic layer containing up to twelve percent phosphorus by weight. Elevated phosphorus concentration promotes phase transformation into crystalline nickel phosphide compounds. High reflow temperatures and extended liquidous times increase the total mass of nickel consumed, forcing greater phosphorus accumulation.
Liquid acid drag-out during bath operation creates localized phosphorus concentration spikes across plating panels. Void formation along the segregated boundary reduces mechanical shear energy absorption during drop testing.
Deposition Constraint
Plating bath chemistry limits determine initial phosphorus content in electroless nickel layers to balance corrosion resistance against assembly solderability. Mid-phosphorus formulations holding seven to nine percent phosphorus balance corrosion protection against excessive interfacial segregation. Controlling solder reflow peak temperature minimizes elemental expulsion during joint formation.
