Hydrogen Embrittlement and Nano Void Classification within Electroless Copper Deposits

Abstract

Within the chemical reactions occurring during the deposition of electroless Copper, a low level of Hydrogen gas is evolved, and unless immediately removed, which can be problematic, is prone to accumulation on the freshly formed Cu surface. As further Cu deposition continues, the likelihood of Hydrogen encapsulation is high, and this can lead to a nonporous, low ductility Copper structure with an internal stress characteristic which leads to a high tendency for blistering. All of which are undesirable! This process is typically referred to as “Hydrogen embrittlement” and was often identified during the 1980/1990s as the probable cause for preferential failure of plated through holes in PCBs. With the development of modern electroless Copper baths, the reports of Hydrogen embrittlement related issues have been virtually eradicated, yet it is only recently that published data has offered insight into why this has occurred. As part of a larger ongoing investigation into the formation and presence of nano scaled voids within blind micro via designs, this paper reports on the impact of electrolyte stabilizers and Ni co-deposition on the occurrence and nature of Hydrogen induced voiding. Utilizing a combination of SEM, FIB and TEM techniques, it has been determined that a Ni content in excess of 30ppm in the electroless Copper solution itself, is sufficient to virtually eliminate “Hydrogen induced voids” within the resulting Copper deposit. For the single component stabilizer additives investigated, much as expected, all were found to have a positive effect in terms of grain refinement, yet, when compared to the effect arising from Ni codeposition, they were all unable to fully suppress the reduction of Hydrogen induced voids. With this further understanding, the paper concludes by identifying, classifying and offering route cause formation mechanisms for a range of nano void types that have been observed as part of these investigations as well as routine analysis on failed micro via assembles, all of which is believed to be vital in determining the best solution for void reduction across BMV interfaces and so, maximizing joint reliability.