Grain Structure
Metal deposition processes create a crystalline morphology where the grains exhibit approximately equal dimensions in all directions. Equiaxed copper plating differs from columnar structures by offering isotropic mechanical properties throughout the deposited layer. This crystal orientation provides ductility and thermal shock resistance during the assembly of multilayer boards.
Chemical additives in the plating bath, known as grain refiners, interrupt the vertical growth of crystals to force this uniform multidirectional distribution.
Reliability Outcome
Isotropic grain boundaries resist the propagation of cracks during the rapid temperature changes of reflow soldering or thermal cycling. When a printed circuit board undergoes expansion along the vertical axis, equiaxed copper plating absorbs the resulting strain more effectively than rigid columnar copper. This flexibility reduces the likelihood of barrel cracking or corner cracks in plated through holes.
Boards used in harsh environments or high reliability applications specify this plating type to ensure the structural integrity of the vertical interconnects. Failure to achieve the correct grain structure often leads to latent defects that only appear after the board has been in service for several months. Microsectioning and electron backscatter diffraction provide the means to verify the presence of these uniform grains within the vias.
Plating Chemistry
Bath composition and current density determine the final arrangement of the metal atoms. Organic suppressors and levelers manage the rate of ion arrival at the cathode surface to prevent the formation of large or elongated crystals. Maintaining a specific ratio of these additives ensures the copper grows in the desired non-directional pattern.
