SMTA International Conference Proceedings

The Impact of Deposition Thickness on High Speed Shear Test Results Specifically Related to Electroless Palladium and Semi Autocatalytic Gold

Authors: Rick Nichols, BSc (HONS), Sandra Heinemann, MSc
Company: Atotech
Date Published: 9/17/2017   Conference: SMTA International

Abstract: Increasingly it is becoming evident to the electronics market that form factor and signal integrity requirements are rendering electroless nickel obsolete as a diffusion barrier for future final finishes. In response a novel Electroless Palladium and Semi-autocatalytic Gold (EPAG) finish is gaining traction. EPAG is a universal finish in that it is both solderable and wire bondable. This paper will focus on the solder joint reliability (SJR) and investigate the merit of solder joint embrittlement being attributed to thick precious metal deposits. High Speed Shear (HSS) testing, to artificially induce mechanical failure, coupled with high resolution microscopy are the evaluation methods that are employed. The HSS test provides empirical data in terms of Total Energy, (TE), in mJ and semi empirical data represented as Fracture Modes (FM). The microscopy will be utilized to study the character of the Inter-metallic Compound, (IMC) to see if a correlation can be established.

To evaluate a realistic operation spectrum the impact of the following factors will be considered in the analysis: • Ageing • Solder ball size and solder alloy type • Palladium and gold deposit thickness

The versatile EPAG finish is expected to enhance the intrusive medical, high frequency, flexible and ultra-dense circuitry fields. In this capacity the deposit thickness are expected to vary according to application and OEM preference. The scope of this paper will be to evaluate the implications of layer thicknesses on SJR The matrix has been designed to include recommended thickness as optimum values for EPAG within a reasonable operating window (refer to Error! Reference source not found.).

In order to verify the viability of the variables as meaningful, a ‘fit model analysis’ was carried in jmp®. In Figure 1, the ‘Predicted P’ which represents probability is less than 0.0001and the ‘RSquared’ value is 0.77. These values suggest that the fit to model is very good. In fact the R squared value infers that almost 80% of the results agree with the software derived model. In essence, the implication is that the testing was controlled and that the results are significant.

This paper found that as the deposition thickness of the palladium and gold increased the solder joint reliability was detrimentally impacted. It must be stressed that even at the higher thicknesses, (150nm palladium and 150 nm gold), the EPAG finish out performed many of the existing mainstream selective finishes, (refer to Figure 26).

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