Electrodeposited Copper Recrystallization Mechanics in High Density Circuit Stackups
Thermal cycles in high density stackups drive copper recrystallization, causing grain growth and vacancy coalescence that trigger microvia interface failures.
Abnormal Grain Growth
Metallurgical Transformation
Metallurgical transformation involving the rapid expansion of specific crystal grains at the expense of their neighbors. Secondary recrystallization, or abnormal grain growth, occurs during the annealing of electrodeposited copper on a circuit board when certain grains possess a lower surface energy or higher strain energy than the surrounding matrix. This process results in a bimodal grain size distribution.
The phenomenon remains active until the large grains consume the smaller ones or reach a physical boundary such as the laminate interface or the trace wall.
Structural Evolution
Electroplating additives influence the rate at which grains expand during thermal excursions. During the initial deposition, the presence of suppressors and accelerators creates a highly strained lattice with a high density of defects. When the board enters a reflow oven, these stored energies drive the movement of grain boundaries to reduce the total free energy of the system.
In cases where the additive concentration is uneven, some grains grow at an accelerated rate and reach sizes several orders of magnitude larger than the initial deposit. This evolution changes the physical properties of the copper from a hard and brittle state to a softer and more ductile one. Stress concentrates at the interface between the coarse grains and the fine grained regions.
Microstructural Inspection
Microscopy analysis through cross sectional imaging allows for the direct observation of grain size variations. An inspector identifies abnormal grain growth by looking for crystals that have reached the full height of the plating layer.