Electrochemical Express Analysis of Organic Additives in Lead-Free Wafer Level Packaging Plating Baths
Authors: Pavlov Michael , Lin Danni , Shalyt Eugene , Tsimberg Isaak , Liu Zhi 
Published: May 2020

Semiconductor packaging migrated from wire bond to flip chip level interconnect to meet size, weight and electrical performance requirements. Lead-free alloys for flip chip attachment has become a common material in electronics. Alloys containing Sn and Ag as well as pure Sn are the most common ones. The mechanical strength, wetting, and fatigue resistance of these alloys and pure Sn is either comparable or surpass lead contained alloys. Electroplating with lead-free alloys is the most desirable technique due to its low cost and usefulness in building fine pitch solder bumps [1].

The new generation of lead free plating systems targeting faster deposition rates, better uniformity with-in die and longer bath life. The co-deposition of elements may be complicated by the large difference in the electrode potentials of tin and alloying metals (e.g. silver, bismuth). To produce uniform and dense deposits, these plating baths include organic additives that are typically separated for two groups with different functions: polarizers that significantly affect the plating rate and control the uniformity of deposit; and grain refiners [2]. For desired properties of deposit, the bath components must be kept within specific concentration ranges. In some cases, the analytical results for bath components should be available more frequently in order to provide feedback to dosing system [3]. To develop most accurate and repeatable analytical methods, the interactions between lead free plating bath components were previously investigated and understood [2]. That allowed to develop efficient analytical techniques for analysis of lead free plating bath components. However, faster analytical methods are required for control of high speed plating baths. The effect of changes in the concentration of organic components on the structure of deposit were studied more recently and will be reported in this work.

When the speed of analyses increases, it is critical to keep reproducible state of all electrodes. Therefore, new regeneration method for electrodes was developed. For development of new express analytical methods, we utilized an electrochemical cell with three electrodes connected to a potentiostat/galvanostat. RDE working and counter platinum electrodes were used as well as Ag/AgCl double junction reference electrode. It has been found that multiple factors can reduce the speed of electrochemical analysis. One of the most potent parameters that affect speed of analysis was a scan rate. Fig. 1 shows the electrochemical responses of polarizer at different scan rates.

Other parameters and steps of analysis were also evaluated for potential reduction the analysis time. This presentation will provide more information related to the electrochemical behavior of organic additives at specific conditions when the analysis is performed with different speed. Results for other bath components will be also presented. The concentration of inorganic components and silver complexing agent can be determined via non-reagent and virtually real time spectroscopic analytical methods. The presentation will be concluded with a discussion of analytical results for several organic additives at different concentration levels.

References:

1. A. Foyet, M. Clauss, W. Zang-Beglinger, J. Woertink, Y. Qin, J. Prange, P. Lopez, , “Electroplating baths of silver and tin alloys”, US Patent, US9512529B2

2. M. Pavlov, D. Lin, E. Shalyt, I. Tsimberg, “Electrochemical Behavior and Analysis of Organic Additives in Modern Lead-Free Wafer Level Packaging Plating Baths, AiMES, ECS and SMEQ Joint International Meeting, September 30 – October 4, 2018, Cancun, Mexico

3. M. Pavlov, D. Lin, E. Shalyt, I. Tsimberg, “Electrochemical Analysis of Semiconductor Plating Baths”, MAM Conference, Milano, Italy, March 18-21,

Figure 1