A study of plated through-hole reliability of formaldehyde-based electroless copper depositions

With the growth of electroless depositions, the need to understand the formaldehyde-based electroless plating process in multi-layer board (MLB) applications is becoming increasingly important. The purpose of this work is to understand the conditions that insure reliable, crack-free electroless copper in plated through-holes (PTHs). This is accomplished by correlating multiple processing and plating variables to PTH cracking in actual MLBs. A design of experiment (DOE) and statistical analysis was employed to understand the most important processing and plating conditions on PTH cracking. It was found that cracking is not only affected by plating thickness and PTH diameter, but also by plating bath variables such as formaldehyde (HCHO) concentration. These results indicate that a strict adherence to plating bath concentrations had the largest effect on insuring crack-free PTHs. In addition, the microstructure of the electroless copper foils in this experiment was analyzed and correlated to cracking. It was found that the copper from the PTHs with more cracking had voids in the microstructure, believed to originate from hydrogen evolution, increased dimpling and less plastic deformation in the region of the fracture surface from elevated temperature (250/spl deg/C) tensile testing.