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X-Ray Inspection of Lead and Lead-Free Solder Joints
July 31, 2017 | Glen Thomas, Ph.D., and Bill Cardoso, Ph.D., Creative Electron Inc.Estimated reading time: 13 minutes
where Emax is the maximum energy in the spectrum, which is 150keV for the X-ray machine used in this paper.
TIN-LEAD SOLDER VS. LEAD-FREE SOLDERS
There are a number of lead-free solder compounds in the market today. For examples in this analysis we used some of the solder compounds discussed in [5]. In particular, we compare eutectic tin-lead solder (63Sn/37Pb), tin-bismuth (42Sn/58Bi), and a variety of predominantly tin based solders, including tin-silver (98Sn/2Ag), tin-antimony (95Sn/5Sb), tin-indium-silver (77.2Sn/10In/2.8Ag), and tin-silver-copper (96.3Sn/3.2Ag/0.5Cu).
Solder compounds can be classified according to those he deems most likely lead-free alternatives, all of which have at least 95% Sn content[5]. In our analysis, we consider three of these (98Sn/2Ag, 95Sn/5Sb, and 96.3Sn/3.2Ag/0.5Cu). The others are provided for comparison purposes. Equation 4 must be discretized for evaluation. Using a Creative Electron’s proprietary emulation and simulation software, we computed the energy spectra and attenuation coefficients for 16 bands of 10keV width spanning 10keV to 160keV. Equation 4 in discrete form then becomes:
Using this data and accounting for filtration due to the X-ray source anode materials, we obtain values for all p(Ei ). The results are summarized in Figure 5. In Figure 6 we provide the stopping power (i.e., 1-I/I0) for five lead-free solder compounds and eutectic tin-lead solder at thicknesses of 2 mil, 5 mil, 10 mil, and 20 mil. The stopping power indicates the relative amount of X-rays that are attenuated in the material. For example, a stopping power of 0.75 means that three-quarters of the incident radiation is attenuated. Higher numbers consequently imply higher attenuation.
Figure 5: Energy spectrum of a TruView FUSION with a 150kV microfocus X-ray source.
Figure 6: Attenuation of lead-free solders and eutectic tin-lead solder. Note that the amount of attenuation at different material thicknesses is similar.
X-RAY IMAGE ANALYSIS
To validate the analysis presented in the previous session our Advanced Technology Group inspected a wide range of samples with both lead and lead-free solder composites. We compared eutectic tin-lead (63Sn/37Pb) and tin-bismuth (42Sn/58Bi) solders for their imaging profile using a TruView FUSION running at a maximum of 150kV. As expected, the X-ray images obtained from the samples using both types of solder did not present a notable difference. Please note that it is not the focus of this paper to compare the overall performance between lead and lead-free solders. Instead, the objective of this analysis is to validate that the X-ray machine settings work for both lead and lead-free solders.
Figure 7: Gullwing solder joints using both lead and leadfree solder paste. Note that the lead-free solder is slightly lighter (less dense) than the lead solder joint.
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