Interfacial Intermetallic Compound Kinetics in Lead Free Solder Joints
Control reflow peak dwell and solid-state thermal exposure to limit interfacial intermetallic growth below four micrometers, preventing brittle joint fracture.
Cu6Sn5 Scallop Growth
Meaning
Metallurgical intermetallic compound formation measured against reflow duration and thermal aging cycles describes the kinetic development of eta-phase copper-tin structures at the solder-substrate interface. Microstructural cu6sn5 scallop growth occurs during liquid-state soldering when molten tin reacts with copper pads to form hemispherical crystalline grains. The thickness and morphology of this intermetallic layer determine initial mechanical bonding and long-term joint reliability in printed circuit board assemblies.
Excessive growth depletes copper from the pad and creates stress concentration troughs between adjacent scallops.
Kinetic Mechanism
Liquid tin dissolves copper rapidly during the liquidus phase of surface mount reflow soldering. During this liquid-solid reaction, cu6sn5 scallop growth proceeds through grain boundary diffusion and mass transport across the liquid solder interface. The individual scallops coarsen over successive reflow passes, converting their initial rounded morphology into a continuous planar layer.
Adding trace elements like nickel or bismuth to the solder alloy alters diffusion rates and modifies scallop dimensions.
Structural Risk
Intermetallic compounds possess high hardness but low fracture toughness compared to bulk solder alloys. As cu6sn5 scallop growth progresses, deep channels form between adjacent scallop bases where tensile stresses concentrate during drop testing or thermal cycling. Microcracks propagate preferentially along these intermetallic interfaces under mechanical shock loading.
Thermal Aging
Solid-state diffusion during operating temperature exposure drives secondary transformation into epsilon-phase Cu3Sn intermetallic layers.