A thermal rollercoaster: Unraveling temperature dependence in CVS determinations

Manufacturing printed circuit boards (PCBs) is a complex process. The PCB layout requires precisely controlling the organic additive concentrations during the copper plating step. Cyclic Voltammetric Stripping (CVS) analysis is used to measure and quantify the concentration of these additives. Temperature variations can affect the accuracy of CVS analysis. Therefore, it is important to monitor and control the temperature of the copper plating bath. 

This article introduces the methodology to determine organic additives in copper plating baths and explains how temperature affects CVS measurements. In addition, a straightforward and effective way for groundbreaking improvement of precision in the analysis of organic additives is demonstrated.

Overview of PCBs, copper plating, and organic additives

Electronic devices continue to shrink in size while expanding in functionality and performance. For this reason, every millimeter of space on a printed circuit board is a precious commodity. Modern PCB layouts push the boundaries—increasing the number of connecting vias while simultaneously reducing interconnection distances [1]. This growing complexity places stringent requirements on the production process, where precision is paramount.

Among the crucial steps in PCB manufacturing, galvanic copper plating of drill holes and the board surface takes center stage. This process uses organic additives like suppressors, brighteners, and levelers to achieve precise control over the physical properties of plated copper. It is imperative to keep the concentration of these organic additives within a very narrow concentration range.

How to measure and quantify the concentration of organic additives

The complex interplay between the organic additives and the copper plating process itself is investigated using Cyclic Voltammetric Stripping. CVS utilizes one of the simplest principles in electrochemistry: the electroplating rate. This is the speed at which a copper layer is deposited onto a substrate surface.

To perform CVS analysis, an electrochemical cell equipped with a three-electrode system is employed. One of these is a rotating platinum disk electrode, precisely controlled by the instrument (Figure 1).  

Effects of organic additives on the copper plating process

In general, the suppressor reduces the quantity of plated copper when added to plating solution (Figure 2A). When introduced to a copper solution saturated with suppressor (also known as the «intercept solution»), the brightener increases the quantity of plated copper (Figure 2B). The addition of a leveler to a plating bath reduces the copper peak height. However, the leveler's effect on the deposition rate of copper is less efficient than that of the suppressor [2].

Sources of temperature variation during sample measurement

It is worthwhile to consider any temperature variations that may occur when determining organic additives in copper plating baths. Sample temperature can differ significantly from the temperature of other solutions used for measurement (e.g., intercept solution). This can be attributed to various factors, such as:

1. use of an air conditioning unit or vent in proximity to the device
2. diurnal temperature fluctuations: calibration is performed in the morning (lower temperatures) while measurements are conducted in the afternoon at higher temperatures
3. actual process conditions vs. lab settings: the difference between the operating temperature of the bath (e.g., 50 °C) and the ambient laboratory temperature (20–25 °C)

Although these situations are commonly encountered, they are often overlooked. All of these can negatively influence the accuracy when determining organic additives with CVS.

Unraveling the influence of temperature on suppressor determination 

The influence of temperature differences on the accuracy of suppressor determination was investigated using the Dilution Titration (DT) technique. To simulate realistic and relevant conditions, four calibration curves at different solution temperatures (20, 24, 28, and 32 °C) were recorded. 

As the calibration temperature (T_c) was increased, significant changes were observed in the slope of the DT calibration curve (Figure 3). This points to a positive correlation between solution temperature and the inhibiting effect of the suppressor additive. Increasing the solution temperature results in an enhanced inhibiting effect of the suppressor additive. Ultimately, a lower concentration of suppressor is required to decrease the plating rate to the same level (Figure 3, dashed line).

Next, four additional determinations were conducted using the same temperatures (determination temperature, T_d) as for the calibration curves in Figure 3. These determinations were then crosswise recalculated with the different calibration curves. This was done to investigate the effect of temperature difference between calibration and sample solutions on the precision of suppressor determination. The results of this crosswise recalculation are shown in Table 1.

Accurate results with a recovery rate between 90–110% can be obtained when T_d is within ± 8 °C of T_c. These findings strongly support common understanding of the intricate relationship between temperature and suppressor efficacy. They also explain inaccuracy in results obtained by some users and justify reasoning for improved solution temperature control during CVS determination.

Exploring the temperature effect on the brightener behavior

The quantification of the brightener concentration relies on the Modified Linear Approximation Technique (MLAT). MLAT assumes a linear relationship between concentration and signal. The effect of temperature on this correlation was explored using aliquots of brightener.

Standard addition curves were recorded across a brightener concentration range of 0–12 mL/L. Various temperatures (20, 25, 30, 35, and 40 °C) were evaluated for each brightener measuring solution. Standard addition curves recorded at these temperatures are presented in Figure 4. 

Higher signals result from increasing the temperature of the measuring solutions. However, when the temperature of the measuring solution exceeded 30 °C, no linear correlation between signal and concentration was evident (Figure 4, dashed lines).

The temperature of the measuring solution is influenced by both the auxiliary solution (intercept solution) and the added sample. To investigate the influence of sample temperature on the results, different mixing ratios of the sample and intercept solution were tested at temperatures from 20–40 °C. The temperature of the auxiliary solution remained constant at 25 °C. The effect of sample temperature on the recovery rate is shown in Table 2.

This table shows that if there is more than a 10 °C temperature difference between the intercept solution and the sample, and the sample fraction surpasses 48% of the entire measuring solution, then the recovery rate of a standard solution is greater than 110%.

Metrohm solutions for tackling temperature challenges during CVS determinations

Metrohm is committed to helping customers achieve the utmost precision and accuracy in their lab work. This extends to the determination of organic additives in copper plating baths. The result is the development of straightforward, convenient CVS solutions to overcome problems originating from temperature differences. 

The 894 Professional CVS (Figure 1), coupled with the Pt1000 temperature sensor (Figure 5), allows real-time temperature monitoring during CVS determinations. This simple and effective integration ensures optimal conditions for every analysis. The Pt1000 temperature sensor temperature can identify changes of 0.1 °C. With just a small adjustment in the viva software, a fully automated and temperature-controlled determination of organic additives is possible.

Complementing this powerful duo, the measuring vessel with thermostat jacket (Figure 6) adds an additional layer of control and stability. This thermostated environment for the intercept solution and sample eliminates temperature differences that may affect the accuracy of CVS determinations. 

Consistent and reliable results are achievable when determining organic additives in copper plating baths. This is possible by utilizing the 894 Professional CVS (or 884 Professional VA) equipped with the Pt1000 temperature sensor, along with a measuring vessel with thermostat jacket connected to any thermostated water bath circulator. 

Summary

Metrohm offers several solutions that can improve the precision and reliability of CVS analysis, ensuring that PCB manufacturing processes reach their full potential. By using a highly sensitive temperature sensor and a measuring vessel with thermostat jacket, CVS determinations can be performed reliably and reproducibly, eliminating errors from temperature differences.

References

[1] lesley. How to Avoid the Negative Effects of Vias in High-Speed PCB Design. PCBWAY.

[2] Ming-Yao Yen; Ming-Hung Chiang; Hsu-Hsin Tai; et al. Next Generation Electroplating Technology for High Planarity, Minimum Surface Deposition Microvia Filling. In 2012 7th International Microsystems, Packaging, Assembly and Circuits Technology Conference (IMPACT); IEEE: Taipei, Taiwan, 2012; pp 259–262. DOI:10.1109/IMPACT.2012.6420290

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