The influence of Cu electroplating solution on porous silica low-k films was investigated. The leakage current of the porous silica film with hydrophobic treatment increased significantly after dipping it in Cu electroplating solution. It is found that the Cu electroplating solution permeates the porous silica film owing to its suppressor, which is one of the additives in the Cu electroplating solution, resulting in the increase of leakage current. The permeation characteristics of the electroplating solution were examined using polyethylene glycol (PEG) as a suppressor. It is found that the leakage current in the porous silica film depends on the molecular weight (Mw) of PEG. The leakage current increased when the mean Mw of PEG exceeded 1000. The contact angle measurement of the Cu plating solution to the porous silica film shows that the mean Mw of PEG determined the wettability of the porous silica film to the solution. The increase of leakage current in the porous silica low-k film can be suppressed by controlling the mean Mw of PEG less than 1000.