Solder Interface
Solid-state intermetallic growth at the boundary of a copper land and tin-based solder alloy yields a dual-phase structure that evolves during high-temperature storage. An engineered process for Cu3Sn layer planarization suppresses the uneven, scalloped growth of the copper-rich intermetallic phase. It ensures a uniform sub-micron thickness across the pad area.
Uneven growth leads to voids and joint weakness.
Chemical Treatment
Active chemical additives introduced into the copper plating bath or the finishing chemistries alter the grain structure to inhibit localized, preferential diffusion along copper grain boundaries. This controlled surface modification promotes a uniform Cu3Sn layer planarization by regulating the atomic diffusion pathways during the initial reflow and subsequent thermal aging cycles. Additives reduce the migration rate of copper atoms.
Thin, flat layers result from this treatment, preventing the coalescence of micro-voids at the interface and securing the solder connection.
Structural Growth
Progressive thickening of the intermetallic compounds under thermal stress causes planar interfaces to degrade when thermal aging is uncontrolled. The presence of Cu3Sn layer planarization maintains a stable intermetallic barrier that prevents the formation of Kirkendall voids. These microscopic cavities are the primary cause of solder joint separation during drop testing.
Smooth layers ensure long-term mechanical strength.
