Lead-free alloys have been developed over the past 10 years to comply with legislation reducing hazardous material usage in electronics. The ideal properties of tin-lead alloys which made them suitable for soldered connections were considered when proposing proper lead-free alloys as alternatives. Some of these requirements include the following physical properties:

• Lower toxicity
  
• Low melting temperature
• Low wetting angle to surface finish

The first requirement relates to the legislation as well as integration of lead-free with leaded materials. The last two requirements are key factors to consider for ideal solder connections. With tin-lead solder, reflow temperature is typically 35 degrees higher than melting temperature to ensure reliable solder joints. Good wetting is needed to satisfy good solder joint integrity as well as product yield. Several elements have been used in place of lead (Pb), based on its toxicity level. The list below shows the alternative elements in order of increasing toxicity.1

• Bismuth (Bi)
  
• Zinc (Zn)
  
• Indium (In)
  
• Tin (Sn)
  
• Copper (Cu)
  
• Antimony (Sb)
  
• Silver (Ag)

Transition between leaded and lead-free technology has resulted in careful examination of manufacturing practices. X-ray fluorescence (XRF) provides a useful tool, both qualitative and quantitative, to determine whether or not a lead alloy was used in manufactured articles. The composition of the lead-free components and solder can be qualitatively determined using XRF, while the thickness of thin films can be quantitatively measured indirectly if the composition of the coating and substrate is known.

The basic principle behind XRF is when a sample is irradiated by x-rays, an electron can be ejected from its atomic orbit. To fill this space, an electron from a higher orbit (energy level) drops to the lower orbit and emits a secondary x-ray. The energy (wavelength) of this fluorescent x-ray is characteristic of a particular element, providing a means to qualitatively establish the elemental composition and quantitatively measure the concentration of these elements.

Capabilities of XRF instruments are dependent on the applied voltage, and while they have no trouble with detection of the lead-free elements listed previously, very light and very heavy elements may be difficult to detect depending on the instrument. Figure 3-1 is an example of XRF spectra for electroless nickel immersion gold (ENIG) over copper substrate.
In addition, XRF can indirectly measure the thickness of thin films, as well as multilayer films, by knowing the film and substrate material properties. Accuracy in the thickness measurement is strongly dependent on the uniformity of the film. Therefore, good wetting is necessary for accurate measurements. With lead-free applications, this property is also a requirement for good solder joint quality.

This thickness measurement capability with XRF allows for non-destructive and simple thickness measurements of surface finish and surface plating. Following guidelines from IPC-6012B (Qualification and Performance Specification for Rigid Printed Boards), XRF allows board manufacturers to pass/fail boards based on thickness criteria for Class 1, 2, or 3 products. Some surface platings/finishes specified in IPC-6012B are electroless nickel immersion gold (ENIG), immersion silver, and immersion tin finish.

Another useful tool for examining lead-free technology is energy dispersive x-ray spectroscopy (EDS) with an electron microscope system. In an electron microscope, an electron gun generates a focused beam of electrons that irradiates the sample. This interaction results in an emission of secondary and back-scattered electrons which can be processed into an image. In addition to electrons, x-rays are emitted which have an energy characteristic of the parent element. Moseley discovered that this energy transition was related to the atomic number (Z) by the relationship shown below; where E is the x-ray line energy and constants A and C are specific for each x-ray series.2



The difference between EDS and XRF is the type of radiation hitting the sample. EDS uses an electron beam while XRF uses an x-ray beam. Due to the small beam size possible with electrons, elemental analysis can be obtained for volumes as small as one µm in diameter.

EDS specimens must be sputtered with a conductive coating to avoid charging issues with the electron microscope. Specimens are often cross-sectioned at the area of interest on the component-board to provide an internal view of solder joint at both the component and board level. The elemental composition can be analyzed for the solder joint and the surface finish/plating thicknesses can be measured. Also, the intermetallic layer can be analyzed for composition as well as thickness. Thicknesses can be directly measured by electron microscopy. Figure 3-2 shows an EDS analysis of the intermetallic layer for a Sn96.5Ag3.0Cu0.5 solder joint cross-section. The bottom, darker region is the copper foil and the top, lighter region is the solder. The middle grey region between the foil and lead-free solder is the intermetallic layer.

In lead-free technology, both XRF and EDS can assist manufacturers in performing qualitative and quantitative analysis. XRF can provide non-destructive qualitative and quantitative analysis but is limited to surface analysis and by coating uniformity for thickness measurement. EDS with electron microscopy provides enhanced x-ray microanalysis, direct thickness measurement, and imaging under higher magnification for examining bulk solder and intermetallic layer. Unfortunately, this method is destructive since specimens are coated with conductive layer.

The EMPF facilities are well equipped to assist with both leaded and lead-free analysis and manufacturing issues. Contact the Helpline at 610.362.1320 with any electronics manufacturing-related questions.

1Ganesan, S. and M. Pecht. Lead-free Electronics. Hoboken: John Wiley & Sons, 2006: 83-84.
2Goldstein, J. and D. Newbury. Scanning Electron Microscopy and X-Ray Microanalysis. New York: Springer, 2003: 279.