Anette Alsted Rasmussen

Abstract: Immersion Ni/Au has been the overall dominant surface finish on PWB's for the last 10 years. During the last 5 years Mobile Phones have been extremely popular and spread all over the world in different climate zones in very high volumes. At the same time the Mobile Phone terminal for many people has become a necessity that is brought with them in any activity they practice. These changes in user behaviour have heavily changed the impact from moisture, sweat, corrosive atmospheres and mechanical drop. As a result of this the requirement to solder joint reliability, corrosion stability and wear resistance are heavily increasing to keep a high reliability of the terminal. A paradigm shift to avoid use of Immersion (Imm.) Ni/Au is ongoing nowadays because the thin and porous Imm. Ni/Au can't address these challenges in a satisfactory way. Already some years ago OSP has replaced Immersion Ni/Au on solder pads in terminals from Nokia with positive impact on solder joint reliability. Theoretical electrochemical studies, experimental work and full scale high volume production has now demonstrated that Carbon surface finish for Key- and spring contact-pads, combined with the right concept design will make use of Imm. Ni/Au unnecessary in the near future. The end result is higher reliability with less expensive and simpler processes. This paper will discuss the various considerations for choice of surface finish for rigid PWB's and results from the extensive feasibility studies performed by Nokia Mobile Phones, supplemented by theoretical electro-chemical studies performed by IPU/IPL/DTU.