Surface Finish Layer Thickness Limits for High Density Circuit Boards
High density circuit board surface finishes require tight thickness boundaries to prevent nickel hyper-corrosion, black pad, and solder joint embrittlement.
AuSn4 Embrittlement
Chemical Transition
Solder joint degradation occurs when excessive gold on a printed circuit board pad dissolves into tin-lead or lead-free solder alloys to form weak intermetallic compounds. This chemical reaction leads to ausn4 embrittlement during the liquid phase of the reflow process or through solid-state diffusion over extended thermal aging. The resulting structure loses its ability to absorb mechanical shock and fails under low strain levels.
Phase Growth
Gold dissolves rapidly into molten solder at typical reflow temperatures of two hundred and fifty degrees Celsius. When the concentration of gold in the liquid solder exceeds three percent by weight, the alloy forms brittle platelet structures during cooling. These platelets distribute throughout the bulk solder joint instead of remaining at the boundary interface.
This dispersion creates numerous internal stress concentrations that weaken the solder matrix. As the solder solidifies, these crystal needles grow in size and form a network of crack-propagation pathways. This microscopic disruption reduces the overall shear strength of the electrical interconnection.
Fracture Susceptibility
High-stress environments like drop tests or thermal shock reveal the loss of ductility in the joint. The component suffers sudden interfacial fracture during shock events. Mechanical failure occurs along the planes where the intermetallic platelets concentrate.
This risk demands strict control of the gold plating thickness on copper pads.