Measurement of Lead in Lead-Free Solder by ICP-AES, FAAS and EDX

Significance of the Topic

The restriction of lead content in electronic solder by the RoHS directive has driven the adoption of lead-free alloys such as Sn-Ag-Cu and Sn-Zn-Bi. Accurate determination of residual lead at low levels is essential for regulatory compliance, quality control and environmental safety. This study demonstrates analytical approaches spanning rapid screening to high-precision quantitation.

Study Objectives and Overview

The primary goal was to compare three analytical techniques—ICP-AES, FAAS and EDX—for measuring lead in a candidate reference material (NMIJ CRM 8202-a) of lead-free solder. Specific aims included method validation, assessment of detection limits, and evaluation of ease of use for routine and screening applications.

Methodology and Instrumentation

Sample preparation for ICP-AES and FAAS followed JIS Z 3910-2008. Approximately 0.2 g of solder chips was digested with 25 mL of aqua regia, heated at 150 °C for 30–60 minutes, then diluted to 50 mL with 1 + 2 HCl. Calibration employed external standards enriched with yttrium as an internal standard for ICP-AES, and matrix-matched standards for FAAS. EDX analysis was performed non-destructively on intact chips protected under a thin polypropylene film, with shape correction applied.

Instrumentation Used

• ICP-AES: Shimadzu ICPE-9000
• FAAS: Shimadzu AA-7000F
• EDX: Shimadzu EDX-GP

Main Results and Discussion

Analytical values for lead in the CRM sample were:

• ICP-AES: 197 mg/kg (DL 2 mg/kg)
• FAAS: 200 mg/kg (DL 20 mg/kg)
• EDX: 204 mg/kg (DL 25 mg/kg; 300 s measurement)

The certified reference value is 197.3 ± 3.3 mg/kg. ICP-AES and FAAS provided close agreement with the certified value and acceptable precision. EDX yielded slightly higher values but remains useful for rapid screening due to its non-destructive workflow.

Benefits and Practical Applications of the Method

Each technique offers distinct advantages:

• EDX: Quick, non-destructive screening without chemical pretreatment.
• FAAS: Low operational cost and straightforward operation, ideal for routine one- to five-element quantitation.
• ICP-AES: Multi-element capability, high throughput and internal standard correction for robust quantitation.

Future Trends and Potential Applications

Advances in laser ablation for direct solid sampling, micro-plasma sources and portable X-ray systems are expected to broaden on-site screening capabilities. Coupling high-resolution optics and machine-learning-based spectral deconvolution may further reduce detection limits and analysis time, enhancing real-time quality control in electronics manufacturing.

Conclusion

The comparative study confirms that while ICP-AES and FAAS deliver high accuracy and precision for low-level lead quantitation, EDX serves as an effective preliminary screening tool. A tiered analytical strategy combining EDX for rapid checks and ICP-AES or FAAS for confirmatory measurements ensures regulatory compliance and efficient laboratory workflows.

Reference

• Shimadzu Application Note No. J96, First Edition June 2012