| |
The EMPF recently presented a paper at the SMTA conference in Boston in June and also at the Joint Group of Pollution Prevention (JG-PP, a cross service DoD and NASA group) on microsectional analysis of lead-free solder joints. The information presented here is an excerpt from that paper. The changeover from tin-lead alloys to lead-free alloys introduces significant changes in surface mount processes. The changes are caused by differences in solderability, compatibility, material properties, higher reflow temperatures, and flux chemistries. Either alone or when coupled with optical and X-ray inspection methods, microsections can provide crucial information needed to make smooth transitions to lead-free solders. Because of this, the need is growing for cross-sections of solder joints, components, laminates, and substrates to identify issues that accompany the changeover from tinlead. The EMPF recently performed a study to investigate many of the issues of implementing lead-free soldering. While the study focused on processing changes and issues, the results yielded information about the microsectioning and evaluation of lead-free solder joints. The samples used for this study were fine pitch quad flat packs (QFPs). Atotal of six different alloys on nickel-gold and organic solder preservative (OSP) surface finishes were investigated.
The lead-free solder alloys investigated included:
• Sn63/Pb37
• Sn96.5/Ag3.5
• Sn95.5/Ag3.8/Cu0.7
• Sn95.5/Ag4/Cu0.5
• Sn96.2/Ag2.5/Cu0.8/Sb0.5
• Sn77.2/Ag2.8/In20
Samples were reflowed in nitrogen using an Electrovert Omniflow 7 Reflow Oven set to the profiles recommended by the paste manufacturers. The primary focus of the study included: solderability, joint
geometry, and flux residues. Scanning Electron Microscopy (SEM), Optical Microscopy, and Energy Dispersive Spectroscopy (EDS) were employed to examine the cross-sections of the lead-free solder joints. It was quickly determined that the  preparation and evaluation of lead-free joints deviated from the standard procedure used for typical tin-lead solder joints. In order to produce microsectioned samples of the same quality as the tin-lead samples usually produced in our materials laboratory, we had to alter the microsectioning process. Changes included the order and grit sizes of the grinding and polishing steps and changes in etching solutions (Table 1) in order to elucidate microstructure.
Microstructure
Interpretation of microstructure is extremely important when evaluating solder joint cross-sections. The grain structure and intermetallic regions of reflowed joints are often used for failure analysis and assembly process optimization. When electron microscopy and energy dispersive X-ray spectroscopy techniques are employed, the presence of contaminants and foreign materials within the microstructure can also be determined. The difference in microstructure between tin-lead and tin-based lead-free solder joints is vast. Tin-lead microstructures consist of Sn-rich and Pb-rich lamella grains. When using metallographic polarized light microscopes, the Pb-rich grains appear as dark regions while the Sn-rich grains appear significantly lighter in color. Using backscatter electron microscopy, where there is contrast between elements and compounds with different atomic numbers, Pb-rich regions appear as light areas, while the
|
Figure 1. Secondary electron SEM micrograph of
SnAgCu solder microstructure. Although it is difficult to differentiate,
both Cu6Sn5 and AgSn4 intermetallics are present
in the bulk microstructure. |
Sn-rich appear as dark areas. This effect can been seen in Figure 1. The size, shape, and general location of these grains can be utilized to determine the environmental conditions in which the tin-lead joint was subjected. Generally, grain growth and grain coarsening indicates exposure to elevated temperatures.
With proper etching, copper-tin intermetallic regions formed between the solder and the pad and lead, are disguisable from the bulk solder materials. Nickel-tin intermetallics are more difficult to resolve and require special preparation and high magnification. Like grain size, intermetallic dimensions can be used to interpret process, testing, and service conditions.
Tin-based lead-free alloys exhibit vastly different bulk solder microstructure. Where tin-lead alloys exhibit distinguished Snrich and Pb-rich grains, the majority of tin-based lead-free alloys exhibit intermetallic structures within the tin-matrix. These intermetallic structures are composed of a ratio of tin and some other elemental constituent of the alloy (e.g. Ag3Sn).
|
Figure 2. Internal tin structure adjacent
to the CuSn intermetallic layer within
the bulk solder of a Lead-free joint. |
Because of the relatively low percentage (3-5 weight %) of alloying elements, these intermetallic structures comprise a small percentage of the total volume within the solder joint. The intermetallic structures do not take on a lamellar form; rather, the morphology varies, exhibiting a round, lathlike, blocky, or needle-like structure. An example of SnAgCu microstructure is displayed in Figure 2.
The intermetallics within the bulk solder are easy to detect when etched with the chemicals listed in Table 1. These chemicals attack the tin matrix resulting in intermetallic structures that appear raised. These raised structures can be viewed using optical or scanning electron microscopy. While identification of the presence of an intermetallic structure using optical or electron microscopy is straight forward, identifying the elemental composition of that structure requires the use of EDS.
 |
Figure 3. Land lift of
through hole mounted lead soldered
with SnA/CuSb solder. |
There is little difference in the size, shape, location and color of the various intermetallic structures, even when observed using backscatter electron microscopy. For alloys with multiple intermetallic constituents, such as SnAgCu, EDS X-ray imaging can provide the elemental contrast required for proper evaluation. Otherwise, it becomes extremely difficult to distinguish Ag3Sn intermetallic structures from Cu6Sn5 intermetallic structures. In addition to intermetallic structures, other new microstructural phenomena can be characterized, such as the pure tin formation growing out of a solder joint interface in Figure 2.
 As lead-free solders are implemented, the industry can expect to observe new failure mechanisms and revisit some older ones due to the implementation of these new lead-free solders. Some examples can be seen in Figures 3 and 4.
|
