Electrolytic Solder Deposition for IC-Substrates: From Deposition Uniformity to Solder Ball Geometry

Abstract

The current screen printing process for solder deposition on IC substrates is coming to a technology boundary due to the continuous down sizing of the solder structures. This will require soon new methods of solder transfer to the substrate. This paper describes an alternative process based on electrolytic deposition of Tin and Tin alloys for sub 100 μm solder balls. A suitable process sequence is evaluated and described in detail. Uniformity of the electroplated deposits both in terms of solder thickness and chemical composition is of vital importance for the process. Thickness determines the solder ball diameter after reflow and elemental composition influences the reflow performance and solder joint reliability. We will present in this paper results from an investigation program to identify main parameters for the distribution of the plated solder. Main emphasis is here on difference between direct plated (DC) and reverse pulse plated (RPP)deposits. We want to clarify if reverse pulse plated layers exhibit a better distribution than dc plated deposits as known from other applications. As a 2nd very important parameter electrolyte flow is identified. We will show how variations in these parameters itself are translated first in variations of the plated layer and then – after reflow- into different solder ball geometries. Understanding these influences will help introducing this technology into future production scenarios.