Autocatalytic Tin: How to Overcome Process Limitations to Introduce a New Solution for Thick Tin Plating

The autocatalytic deposition of tin is gaining high interest as it offers several benefits compared to the immersion deposition. Despite the fact that in contrast to the immersion plating process there is no technical limitation of the thickness of the tin deposit, it also allows the deposition of tin on different materials other than copper or aluminum. With this, it can open up new application fields for plating deposits, which are not considered for electroless plating yet. Even though there is a great potential for this technology, there are limitations that have prevented the successful introduction of this process so far. One limitation of possible electrolyte formulations is for example, that the plating rate of the process is instable in so far as it tends to drop already after few minutes of dwell time. Target of the investigations presented in this paper was to understand the root cause of this deposition rate drop. In order to clarify the reaction mechanisms different electrochemical approaches such as Electrochemical Impedance Spectroscopy (EIS) or Quarz-Microbalance (QMB) were used to determine the physical and chemical background. It could be proven, that pyrophosphate which is used as complexing agent in most autocatalytic tin solutions adsorbs on the electrode surface and that due to that adsorption, the plating reaction of tin is inhibited. Based on this finding a bath formulation was created, which is able to overcome this inhibiting effect being able to provide stable plating performance over bathlife. In this paper exemplary tin deposits are presented which were plated with an autocatalytic electrolyte comparing their layer properties to those of other conventional final finishes like immersion tin or electroless nickel/immersion gold. To judge the performance of the final finish, solder wetting behavior and solder joint reliability were rated and compared. It is also shown, that the electrolyte solution can be tailored in composition and plating parameters to fulfill requirements such as defined layer roughness or the capability for plating of fine patterns. The presented results will allow to consider the autocatalytic tin deposition as a potential solution for specific applications. Thicker tin layers would improve the solderability. In addition, higher tin thickness (>3 µm) is also of interest in IC substrates. Here, mechanical solder paste printing is currently limited to areas with a diameter of ~50 µm. Commonly used structures