Control of Electroless Nickel Baths

This paper reports the authors’  work on developing methods to analyze in-situ key parameters, including pH, nickel concentration and reducing agent concentration of electroless nickel baths. The purpose is to enable production line automated control of the deposition process. The analytical techniques developed within the scope of this work are discussed and their inclusion into an automated chemical monitoring system described. Thousands of data points have been collected to evaluate the system’ s performance. The corresponding results are presented in the context.

 

Introduction

As PCB manufactures comply with the requirements of lead-free regulations, alternative finishes such as ENIG (Electroless Nickel/Immersion Gold), immersion tin, and immersion silver have become widely adapted. Among those, ENIG provides a highly solderable surface that does not tarnish nor discolor – ensuring a relatively long storage time as compared to other alternative finishes. In addition, ENIG is known as an effective barrier preventing copper diffusion and maintaining solderability of the PCB pads.

One possible drawback of the ENIG finish is the probable nickel corrosion during immersion gold deposition – a defect commonly referred to as “black nickel” or “black pad” [1]. Black pads typically cause solderability failure and therefore need be avoided. The structure and the phosphorous content of the nickel layer are among the key factors in determining the subsequent formation of black pads. Those factors are, in turn, related to the composition and pH of the bath during electroless nickel deposition. Control of the electroless nickel bath is therefore key to defect-free ENIG processes. Bath properties change as solution ages, through consumption of components as well as creation of byproducts. To be able to control the bath, one must know the properties of the bath, and then make adjustment accordingly. Being able to analyze bath properties is thus the first step towards effective control of baths.

Unlike electroplating in which an external circuit provides the electrons to reduce metal ions into metal deposits, an electroless process must use reducing agents to provide the electrons. The most commonly used reducing agent in an electroless nickel bath is sodium hypophosphite. This paper reports the authors’ approaches to analyze divalent nickel (Ni2+) and sodium hypophosphite concentrations as well as pH in the bath.

 

Analytical Methods and Results

Analyses of pH and nickel concentration were conducted by a Quali-Stream inline chemical monitoring system (ECI Technology, Inc.), Figure 1.

Quali-Stream inline bath analyzer used in this work for controlling electroless nickel baths.
Figure 01. Quali-Stream inline bath analyzer used in this work for controlling electroless nickel baths.

The system samples and analyzes solutions alternately from two production tanks based on pre-set schedules, and the solutions are automatically returned to the original production tanks after analysis. Solution inlets and outlets for multiple tanks are located on the left side panel of the system.

Analysis of nickel concentration is by spectroscopic method, based on Beer’s Law. Incoming light is partially absorbed by the solution under analysis. The higher the nickel concentration, the stronger the absorbance is, resulting in a weaker outgoing optical signal in the corresponding wavelength ranges. The outgoing light is collected by fiber optics and brought to an internal high-performance detector for analysis to ensure sensitivity and accuracy. A calibration curve is built by measuring the absorbance of solutions of known different nickel concentrations (carefully prepared ahead of time), Figure 2.

Calibration curve of nickel concentration
Figure 02. Calibration curve of nickel concentration

Absorbance of tank solution is measured and the corresponding nickel content is determined by mapping the absorbance to the calibration curve (an automatic process performed by software, while eliminating contribution from other species). Figure 3 shows the performance test results of the system measuring divalent nickel concentration.

Performance check of nickel concentration analysis
Figure 03. Performance check of nickel concentration analysis

More than 4,000 data points were collected over a 3-day period, with the spectrometer automatically calibrating itself periodically. As can be seen from the figure, while analyzing the same standard solution of 6.0 g/l nickel concentration, the analytical method achieved very high accuracy - with the highest reading during the test period being of 6.094 g/l and the lowest 5.922 g/l. Statistical analysis of this data set further affirmed the high reliability of the method, with relative standard deviation at 0.86%.

Measurement of pH was conducted by a pH meter that has been built into the Quali- Stream analyzer. Figure 4 shows the long-term performance test of the system on measuring pH.

Performance check of pH analysis
Figure 04. Performance check of pH analysis

More than 4,000 data points were taken at the same time as the aforementioned nickel concentration test was performed. The pH output reading had been maintained in a very narrow range throughout the 3-day period, with max at 4.727 pH unit, and min at 4.740 pH unit. Statistical analysis of this data set also showed a small relative standard deviation of only 0.06%. The accuracy of the system’s pH measurement was further affirmed by conducting an additional set of test. In this 2nd set of performance test, pH of one buffer solution was measured at several different temperatures. It’s been well documented that pH readings, even for the same solution, changes with the solution’s temperature. The pH vs. temperature results of this work, presented in figure 5 (blue data set), matched very closely with published data (pink data set), affirming the performance of the system.

Results of measuring pH buffer at multiple temperatures
Figure 05. Results of measuring pH buffer at multiple temperatures

Analysis of reducing agents was conducted by CVS (Cyclic Voltammetric Stripping) technique, the most widely adapted method to determine organic components concentrations in a copper electroplating bath [2]. The system used in this work to analyze sodium hypophosphite concentration was a Qualilab QL-5 plating bath analyzer (ECI Technology, Inc.). CVS technique applies a cyclic potential onto a platinum working-electrode that is immersed in the working solution (containing copper ions as well as precisely diluted bath sample from the process tank under analysis). The cyclic potential swings between pre-determined positive and negative limits. Copper is deposited onto the working electrode during the negative potential portion of the cycle and then completely stripped away during the positive potential portion of the cycle. The concentrations of additives in the solution affect the rate of copper plating onto the working electrode. When measuring reducing agents, the authors found that the deposition rate of copper in the working solution (note that Cu is the working metal used in the CVS analysis, though the reducing agent concentration in electroless nickel solution is being analyzed) increases monotonically with the increase of reducing agent concentrations.

Figure 6 illustrates the effect of hypophosphite concentration on voltammogram (I-V diagram monitoring the progress of CVS).

Effect of hypophosphite concentration (#1 < #2 < #3 < #4) on Voltammogram
Figure 06. Effect of hypophosphite concentration (#1 < #2 < #3 < #4) on Voltammogram

Four carefully as-prepared test solutions of different hypophosphite concentrations gave distinct I-V curves during voltage scan. The enclosed areas under the curves, referred to as ‘peak area’ or Ar, correspond to the integration of current against the applied voltage and are therefore proportional to how fast plating/stripping occurred. A calibration curve, figure 7, plotting peak area vs. hypophosphite concentration can thus be built to compare with the peak area of an unknown solution and accordingly determine the hypophosphite concentration of the unknown solution.

Calibration curve of CVS peak area vs. hypophosphite concentration in the solution
Figure 07. Calibration curve of CVS peak area vs. hypophosphite concentration in the solution

Long-term statistics showed that using CVS to measure hypophosphite concentration could achieve better than 4% relative accuracy and 3.5% repeatability.

Summary and Conclusion

Methods for analyzing pH and nickel concentration in electroless nickel baths have been developed. Engineering efforts based on instrumentation know-how’s have integrated the developed methods into one automated system, enabling PCB production environments to analyze tank solutions in real time. The corresponding long-term results demonstrated both high accuracy and repeatability of the measurements. Closed-loop dosing based on the analytical results can ensure the stability of the electroless nickel bath and give production engineers full control of their parts quality. Additionally, reducing agent in the electroless nickel solution can be measured by CVS technique using a separate lab analyzer, although in this case sampling from the tank needs be performed manually.

The authors have established similar analytical approaches to analyze palladium activation solution, electroless copper solution and electroless cobalt solution, achieving comparable accuracies. Discussions of some of those topics have been published elsewhere [3].

Authored by:

Eugene Shalyt, Semyon Aleynik, Michael Pavlov, Peter Bratin, Chenting Lin
ECI Technology, Totowa, New Jersey, USA

Reference

1. George Milad and Jim Martin, “Electroless Nickel/Immersion Gold, Solderability and Solder Joint Reliability as Functions of Process Control,” CircuiTree, October 2000.

2. D. Tench and C. Ogden, “A New Voltammetric Stripping Method Applied to the Determination of the Brightener Concentration in Copper Pyrophosphate Plating Baths,” J. Electrochem. Soc. n. 125, p. 194 (1978).

3. P. Bratin, et. Al., “Development of Chemical Metrology for Electroless Deposition Baths,” ISTC Proceedings, March 2006.