Joint Fracturing
Structural solder failure occurs when mechanical or thermal stress causes a clean separation along the brittle boundary layer between different metals in a joint. Under excessive mechanical shock, intermetallic cleavage presents as a sudden fracture that separates the bulk solder alloy from the copper pad. The separation develops because the boundary layer has lower fracture toughness than the surrounding metal matrix.
Material Failure
The growth of copper-tin or nickel-tin compounds during reflow generates a thin interface that is susceptible to mechanical cracking. During thermal cycling, intermetallic cleavage develops along these brittle intermetallic compound lines because of mismatches in the coefficient of thermal expansion. The resulting fracture exhibits a smooth surface finish when examined under high magnification.
This characteristic helps failure analysis engineers distinguish interfacial failures from ductile solder tears. The transition from a ductile failure to a brittle fracture often occurs during rapid mechanical loading, such as drop testing.
Inspection Assessment
Microscopic analysis identifies these specific fracture modes by examining the chemistry of the exposed pad after joint separation. Handheld pull testers and automated shear stations measure the force required to induce intermetallic cleavage in test coupons. These mechanical tests establish the strength boundary of the solder joint.
