To understand the effects of the decomposition of a suppressor on the filling performance for microvias on a printed circuit board, the characteristics of copper (Cu) electroplating solutions were investigated using linear sweep voltammetry (LSV) and galvanostatic measurements (GMs). As a suppressor, polyethylene glycol (PEG) was utilized. With conducting Cu electroplating, the filling performance was lowered and the PEG molecular weight (MW) was decreased as a result of the decomposition. When fresh PEG was added to the aged solution, the cathodic potential at a given current density recovered to the initial value but Delta eta measured at two different rotating speeds was negligibly affected. When PEG with small MW (MW <= 600) was added to a plating solution containing PEG with large MW (MW >= 2000), the filling performance was lowered, exhibiting an unaffected cathodic potential and a reduced Delta eta value. The more negative cathodic potential could be associated with the higher filling. However, when the filling performance is higher than 82% or the cathodic potential shifted negatively below a certain level (-0.18 V, in this work), the filling performance was found to be more closely related to Delta eta; a larger value of Delta eta brings better filling performance. Based on these results, an electrochemical method that more accurately predict the filling performance of the solution during continuous Cu electroplating is suggested.