Analyzing Pb-Free Solder and Solder Films with XLNCE SMX-BEN XRF Analyzer

With the introduction of the European RoHS/ELV regulations, Pb-free solders became very important in electronics manufacturing. X-ray fluorescence spectrometry, also known as XRF, is a non-contact, non-destructive atomic technique, which is suitable for determining the thickness of solder films as well as the composition of trace, minor, and major components in Pb-free solders.

As per the RoHS directive, the concentration of Pb in solder is restricted to below 1000 ppm. During the development of Sn-based Pb-free solders, a number of other alloying elements are used such as Cu, Ni, Ag, Bi, Sb and In at trace and minor levels, typically in the range of 1000 to 5000 ppm to optimize solder characteristics.

Pb can be easily measured according to RoHS directive by using XRF spectrometry. In addition, XRF spectrometry can be used to measure other solder alloying elements for failure analysis and quality control.

SMX-BEN Instrument

The SMX-BEN XRF analyzer benchtop instrument includes a number of components such as a 50 kV, 50 W X-ray tube; five programmable primary beam filters, and six programmable collimator sizes. The analysis is carried out in air.

SMX-BEN Measurements

Three standards were measured whose thickness and composition are listed in Table 1. Measuring roughly 5 mm in diameter, these standard samples were placed onto a 2-pinhole metal frame.

Table 1. composition and thickness of PB-free solder samples measured

Standard # Sn [wt%] Ag [wt%] Cu [wt%] Pb [PPM]* Thickness [µm] **
1 96.17 3.1 0.61 1220 “infinite”
2 97.1 2.4 0.5 NR 10.97
3 97.2 2.4 0.4 NR 18.99

* NR indicates Not Reported
** Infinite indicates the samples were bulk samples with respect to XRF measurement depth

The following conditions were used to measure the samples:

  • 2 mm collimator
  • 30 seconds measuring time
  • 47 kV, 0.960 mA

The physical computer modeling called Fundamental Parameters (FP) was used for quantification. Often referred to as the “No Standards” technique, the FP quantitative technique can be easily calibrated using a range of “standards”, including a pure element, such as copper (Cu), up to corresponding type standards for ultimate precision.

The term “No Standards” was derived from the fact that type standards are not needed to obtain quantitative data, although with reduced precision. In this article, the measurement accuracy will be evaluated between a calibration utilizing a type standard (standard #2), and a calibration utilizing matching bulk, pure elements of Pb, Cu, Sn and Ag, which are not only cost-effective but are also readily available.

Results

In order to acquire measurement statistics, samples #1 and #3 were measured 30 times. Table 2 shows the results where bulk and pure elements (Pb, Ag, Sn and Cu) were used to perform FP calibration. This is typically carried out when it is not possible to access type standards, and the accuracy limits can be loosened.

Table 2. Statistics on 30 repeat measurements using pure, infinite elements for calibration of the FP quantification method.

Standard#   Sn [wt%] Ag [wt%] Cu [wt%] Pb [PPM] Thickness [µm]
1 Mean 96.52 3.04 0.35 899.62  
  Max 96.59 3.12 0.38 1077.37  
  Min 96.44 2.97 0.33 767.11  
  Std. Dev. 0.033 0.031 0.011 57.129  
  RSD % 0.03% 1.02% 3.20% 6.35%  
3 Mean 97.06 2.91 0.03   14.67
  Max 97.14 3.03 0.03   14.86
  Min 96.94 2.83 0.03   14.46
  Std. Dev. 0.042 0.042 0.001   0.099
  RSD % 0.04% 1.44% 3.24%   0.67%

In this kind of calculation, the composition is generally standardized to 100 wt%. Errors are likely to impact the precision on trace elements (elements having below 1 wt%) more significantly, due to low concentrations of the traces.

An interesting fact is that this quantification made on sample #1 would suggest that the sample would probably be flagged for additional testing. This is because as per RoHS XRF testing protocols, samples with XRF results surpassing 700 PPM for Pb are flagged for additional testing.

The measuring time can be increased to enhance the repeatability. However, a factor of 4 increase in measurement time, that is, measuring for only 2 minutes can enhance the accuracy by a factor of 2. The results of these 30 measurements made by utilizing type standards for calibration of the FP technique are shown in Table 3.

Table 3. Statistics on 30 repeat measurements using type standards for calibration of the FP quantification method

Standard#   Sn [wt%] Ag [wt%] Cu [wt%] Pb [PPM] Thickness[µm]
1 Mean 96.11 3.16 0.61 1278.82  
  Max 96.18 3.22 0.64 1445.06  
  Min 96.05 3.11 0.58 1125.82  
  Std. Dev. 0.036 0.030 0.016 83.037  
  RSD % 0.04% 0.94% 2.66% 6.49%  
3 Mean 97.20 2.40 0.40   18.94
  Max 97.27 2.47 0.42   19.34
  Min 97.11 2.33 0.37   18.67
  Std. Dev. 0.038 0.035 0.012   0.123
  RSD % 0.04% 1.44% 2.96%   0.65%

For the sake of standard regulatory issues and quality control, type standards can be bought either from commercial vendors or can be made in-house by verifying standards via destructive testing. Utilizing a type standard in the range of 10 µm thick, precision on the trace elements as well as the accuracy of thickness measurement are improved significantly.

By utilizing a single type thickness standard, the FP quantification can be calibrated with a high level of precision for a restricted range of thickness. Thicker layers are usually harder to measure accurately with the FP technique, as errors in the physical modeling can possibly accumulate over the thicker layer.

While these errors can be corrected with a thick, single type standard, the application of this thick, single type standard calibration model will negatively impact the outcomes for thinner layer measurements. If any thickness measurements have to be made across a “thin” to “thick” range, where thick and thin are defined by the energy of the XRF signal peaks and the absorptive properties of the materials to be quantified, then it would be best to have several type thickness standards which cover the thickness range to be determined.

Utilized by the XLNCE SMX family of XRF instruments, the FP quantification routine has a number of non-linear and linear correction functions to measure the coefficients of internal calibration as a function of coating thickness (Figure 1).

The XLNCE SMX-BEN XRF analyzer

Figure 1. The XLNCE SMX-BEN XRF analyzer

If there is a thickness range of coating standards, the FP quantification can be calibrated in a way that calibration coefficients also become a function of the coating thickness being quantified. This offers excellent precision in terms of composition and thickness measurements across a relatively wider range of composition and thickness from a calibration model.

Conclusion

The XLNCE SMX-BEN XRF elemental analyzer performs non-contact, non-destructive measurements to determine the composition and thickness of layers, in addition to performing a compositional analysis of bulk samples. With the help of the SMX-BEN unit, Pb traces in Pb-free solder can be measured in accordance with RoHS directive and also Pb content in Pb containing solders can be determined for high reliability applications like aviation and military.

The SMX-BEN is also suitable for determining the composition of other trace and minor alloying elements that are generally used in Pb-free solders, such as Sb, In, Ag, Bi, Cu, and Ni. The unit can be utilized for quality control in the lab and also in process control measurements with software having supervisor, operator, as well as maintenance access levels to guarantee that the right job is executed at the right time.

This information has been sourced, reviewed and adapted from materials provided by EDAX Inc.

For more information on this source, please visit EDAX Inc.

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