Managing Kirkendall Microvoiding and Fracture Risks in High-Density Reflow
Manage Kirkendall voiding by capping plating bath sulfur impurities under 10 ppm and clamping reflow time above liquidus strictly between 45 and 60 seconds.
Cu6Sn5 Layer
Meaning
An intermetallic compound phase forms at the boundary between copper and tin-based solders during the soldering process. This cu6sn5 layer provides the physical bond between the board pad and the solder, though its excess thickness can lead to embrittlement. It grows rapidly during the reflow cycle when molten tin reacts with the solid copper substrate.
The resulting compound has a hexagonal crystal structure that is mechanically hard but brittle.
Intermetallic Growth
Solid-state diffusion continues to drive the expansion of the boundary phase even after the solder has solidified. This cu6sn5 layer thickens over the operating life of the electronic device, particularly when the assembly is exposed to high temperatures. If the reaction continues unhindered, it can consume the available copper and compromise the electrical pad.
Solder Joint
Reliability of the electrical connection depends on keeping this boundary region thin. A thin cu6sn5 layer ensures a strong mechanical connection, while a thick layer causes stress concentration.
Thermal Ageing
Long-term exposure to high operating temperatures alters the chemical composition of the board-level interface. Over time, the cu6sn5 layer can transform into a more copper-rich phase like Cu3Sn if the copper supply is plentiful. This transformation often creates microscopic voids along the interface, which severely reduces the shock resistance of the solder joint.