Measurement of Lead in Lead-Free Solder

According to the RoHS directive, the concentration of lead in electrical and electronic equipment must be restricted to less than 1000 ppm. Hence, lead-free solder is used to be compliant with this directive, and lead-free solder such as SnAgCu and SnZnBi are being used instead.

Here, examples of analysis by ICP emission spectroscopy (ICP-AES), flame atomic absorption spectroscopy (FAAS), and energy dispersive X-ray fluorescence spectroscopy (EDX) of lead (Pb) in a lead-free solder sample (candidate standard NMIJ CRM 8202-a) have been introduced that is used for proficiency testing by the National Metrology Institute of Japan (NMIJ) of the National Institute of Advanced Industrial Science and Technology. EDX enables non-destructive, quick and simple analysis of samples, is used for screening analysis, and FAAS and ICP-AES are used for exact and accurate analysis of low concentration samples. Distribution of the standard lead-free solder chip NMIJ CRM 8202-a low Pb concentration (Sn96.5Ag3Cu0.5), which complies with green procurement guidelines, commenced in July 2011.

Sample Preparation

Sample preparation for ICP-AES and FAAS was done based on the Japan Industrial Standard JIS Z 3910-2008 "Methods for chemical analysis of solder." Figure 1 shows the sample decomposition flow. The sample was submitted as a chip for EDX analysis.

Sample Decomposition Flow Chart.

Figure 1. Sample Decomposition Flow Chart.

Analytical Method

The instruments that were used for measurement are shown in Table 1. The calibration curve method was utilized for measurement with all of the instruments. In the case of ICP-AES, the stock solution was spiked with yttrium (Y), and measurement was conducted using the intensity ratio (internal standard method). For the calibration curve samples used in ICP-AES and FAAS, decomposition was done in the same way as for the high-purity metallic tin (Sn), and was then added to obtain approximately the same concentrations as the measurement samples. The calibration curve standard samples are shown in Table 2 and Table 3.

For measurement by EDX, a lead-free solder standard sample from MBH Analytical was used as the calibration curve standard. Measurement was done with the sample placed in a sample container covered with 5 µm thick polypropylene film. In that case, correction was done based on the shape. Figure 2 shows a photograph of the samples used for EDX measurement.

Table 1. Measurement Instruments.

Analysis Method Instrument Name
ICP-AES ICPE-9000
FAAS AA-7000F
EDX EDX-GP

Table 2. Standard Solution for ICP-AES.

Std-1 Std-2 Std-3 Std-4 Std-5
Pb 0 0.5 1.0 1.5 2.0
Sn 0.44 %
Y 1
Unit: mg/L

Table 3. Standard Solution for FAAS.

Std-1 Std-2 Std-3 Std-4 Std-5
Pb 0 0.5 1.0 1.5 2.0
Sn 0.44 %
Unit: mg/L

Photograph of Samples for EDX.

Figure 2. Photograph of Samples for EDX.

Analysis Results

Table 4 shows the analysis results obtained for Pb in lead-free solder. The calibration curve for Pb using ICPAES is shown in Figure. 3 and the measurement solution peak profiles are shown in Figure. 4. The calibration curve using FAAS is shown in Figure. 5, and the signal generated from FAAS measurement of the Pb solution is shown in Figure. 6. Sample peak profile using EDX is shown in Figure. 7.

Calibration Curve of Pb by ICP-AES.

Figure 3. Calibration Curve of Pb by ICP-AES.

Table 4. Analytical Results for Lead-Free Solder.

Analysis Method Analysis Value Detection Limit
ICP-AES 197 mg/kg 2 mg/kg
FAAS 200 mg/kg 20 mg/kg
EDX 204 mg/kg* 25 mg/kg*

NMIJ CRM 8202-a certified value: (197.3 ± 3.3) mg/kg (numeric extension following "±" expresses extended uncertainty).

* Measurement time: 300 sec.

Peak Profiles of Pb by ICP-AES.

Figure 4. Peak Profiles of Pb by ICP-AES.

Calibration Curve of Pb by FAAS.

Figure 5. Calibration Curve of Pb by FAAS.

Pb Signal Using FAAS.

Figure 6. Pb Signal Using FAAS.

Peak Profile of PbLß1 by EDX..

Figure 7. Peak Profile of PbLß1 by EDX.

Conclusion

EDX is suitable for screening analysis as it permits nondestructive, quick and simple analysis. Although FAAS and ICP-AES require pre-treatment procedures such as the preparation of solutions, they permit more accurate analyses to be conducted. FAAS has the benefits of low running cost and easy operation, making it suitable for routine analysis of 1–5 elements. IC-AES permits multi-element simultaneous analysis as well as qualitative analysis, and further extends its applicability beyond routine analysis to research and development.

This information has been sourced, reviewed and adapted from materials provided by Shimadzu Scientific Instruments.

For more information on this source, please visit Shimadzu Scientific Instruments.

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