Quantifying Microvia Plating Fatigue and Target Pad Separation Rates under Rapid Thermal Cycling
Continuous high-speed Kelvin monitoring during rapid thermal cycling isolates latent target pad separations that re-nest and pass static ambient tests.
Plating Additive Inclusion
Material Contamination
Material contamination occurs when organic molecules used to control the rate of metal deposition become trapped within the growing copper grain structure. Plating additive inclusion alters the mechanical properties of the copper, making it more brittle and prone to cracking under thermal stress. This contamination happens when the concentration of brighteners or levelers in the plating bath is too high.
Pure Metal
Pure copper is ductile and can withstand the expansion of the dielectric material during the reflow process. However, plating additive inclusion reduces this ductility and can lead to corner cracks in the via barrel or knee. These internal defects are difficult to catch with electrical testing because they often remain conductive until the board undergoes further thermal cycling.
Monitoring Method
Monitoring the levels of organic additives using cyclic voltammetric stripping is the standard method for maintaining the health of the plating bath. If plating additive inclusion is detected, the bath may need to be treated with carbon or diluted to restore the proper balance. High-reliability boards require high-purity copper to ensure that the material can handle the mechanical loads of a multilayer stackup.
Precise regulation of the current density and the temperature of the bath also helps in minimizing the amount of organic material that is incorporated into the metal.