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X-ray Inspection of Lead-free Solder Joints
December 31, 1969 |Estimated reading time: 6 minutes
As lead-free solder increasingly becomes a reality, inspecting lead-free solder joints must be investigated.
By Holger Roth Figure 1. Function principle of microfocus X-ray inspection. The geometric magnification is M=FDD/FOD. The resolution (i.e., the remaining fuzziness) is determined by the size of the X-ray source, which is mainly given by the focal spot size of the tube.
Microfocus X-ray inspection is a standard inspection method for hidden solder joints in area array packages such as BGAs, CSPs and flip chips. Due to increasing reliability requirements, it also is applied to traditional through-hole and SMT solder joints, since it reveals internal voids and visualizes optically obstructed solder menisci. Previously, solder joints could be imaged easily with excellent contrast due to X-ray absorption of lead, which is relatively high compared to that of printed circuit board (PCB) and component materials such as copper and epoxy. But now, facing the elimination of lead from electronics production and introduction of lead-free solder materials, for many X-ray users the question arises as to whether such lead-free interconnections will be accessible by X-ray testing methods.
Fundamentals Figure 2. Contrast in the X-ray image. Though object A is of the same thickness as object B, it shows higher absorption due to higher density or atomic number of its material. Object C consists of the same material as object B but absorbs less, since it is thinner than B.
Suitability of X-ray systems for inspecting lead-free interconnections should be considered in light of the physical fundamentals of X-ray imaging. In microfocus X-ray systems, the specimen is penetrated by a conical X-ray beam, which generates a magnified X-ray shadow image on the detector (an image intensifier, in most cases). The achievable resolution, or image sharpness mainly is determined by the focal spot size of the X-ray tube, typically in the range of a few microns but can amount to significantly less than 1 µm for novel nanofocus tubes. The magnification is a product of geometry (Figure 1).
Apart from magnification and resolution, detectability of a given object feature depends on the contrast in the X-ray image caused by this feature. Contrast is the result of varying levels of X-ray absorption of different areas within an object. As illustrated in Figure 2, those differences in absorption can be due to variations in thickness, such as shape of a solder joint or in material (e.g., solder on a copper land).
As a rule of thumb for the image intensifier, the absorption difference must amount to at least 2 percent for it to be clearly detected.
Physically, an exponential law rules X-ray absorption:
I/I0 is the ratio of the X-ray intensities before and after passing through the specimen. The material's influence is described by the mass absorption coefficient µ/ρ, which also depends on the energy E of the radiation quanta. The ρ is the density of the material of thickness χ.
Figure 3. X-ray images of two BGA solder joints. At the left is eutectic tin-lead; at the right, lead-free solder. The images have been taken from assemblies with identical layout and at exactly the same X-ray parameters, and are not processed. In the center of each solder joint, the land engulfed by the solder shows as a brighter circle.
Further, in the X-ray energy range of 30 to 160 keV, the typical range used in microfocus X-ray inspection, the mass absorption coefficient µ/ρ strongly depends on the atomic number Z (it increases approximately proportional with Z3).
Lead-free Solder Joints
Most lead-free solders are alloys of bismuth or, much more frequently, tin, such as SnAgCu. For bismuth (Z=83), an X-ray absorption comparable to tin-lead solders (SnPb) can be expected, whereas for tin solders (Z=50), a much lower absorption and, hence, a remarkably lower contrast might be expected. However, inspection applications show that, for example, SnAgCu solder joints nearly yield the same contrast as SnPb solder joints (Figure 3).
Figure 4. Mass absorption coefficient of lead (Pb), tin (Sn) and bismuth (Bi) as functions of X-ray quantum energy. Due to the advantageous level of the K-absorption edges at 30 and 88 keV, absorption of tin is similar to that of lead or tin-lead in the range of 30 to 80 keV.
This can be understood immediately by looking at the course of the mass absorption coefficients µ/ρ (E) (Figure 4). The K-absorption edge of tin is located as low as 30 keV so that its mass absorption coefficient is even higher than that of lead or tin-lead alloys up to the lead K-absorption edge at 88 keV. A numerical simulation (Figure 5) shows that tin solder joints can transmit a little more radiation within the image intensifier's most sensitive energy range (30 to 100 keV). Indeed, in Figure 3 the lead-free solder joint on the right appears slightly brighter than the SnPb solder joint on the left.
Practical Experience
In inspection service applications, all established test criteria for solder joint integrity and signatures of the soldering process were clearly visible and valid for lead-free solder joints as well. Moreover, no restrictions exist concerning automatic inspection and evaluation of lead-free BGA and CSP solder joints. This has been proven in particular within the scope of an investigation of void development in lead-free PBGA solder joints (Sn96Ag4 and Sn95.5Ag4Cu0.5). Due to the small grey value differences between the lead and lead-free solder, even mixed lots could be evaluated by the auto setup BGA evaluation software without changing thresholds or X-ray parameters. In a recent study of the impact of lead-free solder on testing of printed circuit board assemblies by the VDE/VDI committee, different manufactures produced assemblies using standard production using SnPb as well as lead-free solders (SnAgCu alloys) and subjected them comparatively to various electrical, optical and functional tests. In this study as well, none of the solder joint types, including BGA, through-hole and QFP, showed any significant change in appearance of the X-ray image.
Other Lead-free Interconnections
The first X-ray inspections on adhesive interconnections have been successfully performed with stud-bumped flip chips attached to the lands by isotropic silver-filled glue. The interconnections could be inspected for such factors as misregistration, adhesive distribution and short circuits. However, to evaluate those interconnections proficiently with regard to reliability and functionality, more trials would be useful, and standards should be developed.
Figure 5. Transmission of the continuous X-ray spectrum (Tungsten anode) at a typical tube voltage of 100 kV through 1 mm layers of tin (blue) and lead (brown). The integrated intensities in the image intensifier's sensitive energy range (ca. 30-100 keV) differ by a factor less than 2 (numerical simulation).
For low-contrast interconnection media, new digital detectors typically provide a contrast resolution four times better than that of the image intensifier.
Conclusion
Lead-free and in particular, tin-based solder joints are easily imaged using X-ray inspection. Using auto setup software, inspection routines need only minor modification, if any.
To what extent the customary test criteria have to be changed for lead-free solder joints is yet to be determined in future applications. Up to now, at least the appearance of Sn-based solder joints is quite similar to that of SnPb ones, and systematic deviations in factors such as thickness or shape have not yet been observed.
To find X-ray inspection solutions for alternative interconnection methods, such as adhesive bonding, capabilities of X-ray inspection should be evaluated experimentally using the latest available X-ray equipment. In doing so, good results can be achieved, as in the case of silver-filled adhesives.
References
For a complete list of references, please contact the author.
Holger Roth, applications engineer, may be contacted at phoenix|x-ray Systems + Services GmbH, Stuttgart, Germany, E-mail: hroth@phoenix-xray.com.