The use of lead free solder brings about a number of simple and complex problems that did not arise with tin lead (SnPb) solders. The ability to analyze and diagnose these problems is required for smooth transitioning into lead free solder processing. Fortunately, many of the analysis techniques used for tin lead solders are the same for lead free boards, components, and assemblies. With slight modifications, traditional evaluation techniques, such as scanning electron microscopy (SEM) and transmission X-ray, can be used to evaluate processes, perform materials analysis, and conduct failure analysis of lead free circuit boards.

Scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS)
Surface anomalies with immersion tin-plated components and circuit boards typically bring about fear of tin whiskers. Tin whiskers are single-crystal, metallic growths that form on the surface of highly stressed tin-plated surface finishes. The concern over tin whisker formation has grown recently because of the reemergence of tin plating as a lead free alternative finish. Whiskers that become dislodged from the surface are free to create short circuits, particularly in fine pitch, sealed components and assemblies.

Because of the tin whisker’s relatively small dimensions, the use of low power optical microscopy, like that typically found at a manufacturing facility, has been relatively ineffective at positively identifying them. The arduous task of identifying tin whiskers with low power microscopes can be further complicated by the presence of other plating defects, such as bumps or scrapes in the plating surface. To properly identify a defect as a tin whisker, three characteristics must be present:

1. Striations on the surface
2. High aspect ratio
3. 100% tin composition

Scanning electron microscopy with energy dispersive spectroscopy (EDS) is the best tool for positively identifying tin whiskers. The SEM’s high magnification capability creates images that easily distinguish striations on the surface. Elemental analysis by EDS can confirm the pure tin composition of a whisker.

Figure 2-1 shows two anomalies that appeared to be tin whiskers when viewed by low power optical microscopy. SEM/EDS evaluation revealed that only the left image was a true tin whisker. It displayed striations on the surface and was composed of 100% tin. The anomaly on the right displayed a cylindrical shape, a smooth surface, and was mainly composed of silicon. This contaminate was thought to be a glass fiber from an assembly or component laminate.

Sequential electrochemical reduction analysis (SERA)
Electroless nickel immersion gold (ENIG) is subject to nickel oxidation and black pad failures. Oxidation of underlying nickel layers will result in poor solderability and premature solder joint failure. Because the nickel layer is obscured by the protective immersion gold layer, visual inspection for nickel oxidation is not possible. In most cases, oxidized nickel is not detected until solderability problems arise during reflow or wave soldering. One of the best methods of testing for nickel oxidation is to evaluate the integrity of the immersion gold layers. SEM evaluation of the gold surface will reveal porosity in the gold surface which allows oxygen to reach the nickel surface. Sequential electrochemical reduction analysis (SERA) can also be used to quantify gold surface porosity.

Poor solderability and mechanically weak solder joints can also be attributed to black pad failures. High phosphorous levels in the nickel layer, associated with black pad related failures, are also challenging to characterize beneath the gold overcoat. SEM is commonly used to evaluate the cross-section and surface of the nickel pad prior to reflow. The gold layer is removed chemically, and the nickel surface is examined for corrosion spikes and other defect-revealing indicators. (Recent experiments have also shed light on chemical methods for detecting contaminated nickel .)

SERA can be used to detect the presence of silver sulfides and tin oxides on lead free bare board surfaces. In excess, the sulfide and oxides cause solderability problems. UV-Vis spectroscopy has proven to be an effective method for evaluating copper oxidation beneath organic solder preservative (OSP) coatings.

Many component manufacturers have converted to lead free compatible finishes such as immersion tin and silver (Sn and Ag). Often, a change in surface finish from lead-based to lead free will occur without notification or changes in the data sheets or procurement documents. To ensure material compatibility and proper inventory control, elemental analysis of the component surface finish may be required. EDS on the component lead surface is fast, accurate, and requires minimal surface preparation to perform. Elemental analysis is recommended for each lot of components
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Solder joint analysis
Cross-sections of reflowed and wave solder joints provide information about process and material compatibility. The formation of intermetallic layers at the lead and pad interfaces is a prime indicator of a reliable solder joint. The intermetallic dimensions are also used to characterize the soldering processes. Large intermetallics at the interfaces indicate longer heat exposure and vice versa. Intermetallic dimensions can be used to fine tune the higher-temperature lead free profiles, which often operate within a tighter process window than their tin lead counterparts.

Solder joint geometries (wetting angles) are viewed easily in cross-section. Heel and toe wetting angles, as well as hidden wetting defects, are effortlessly observed. Voiding and lead contamination are other anomalies identified by cross-sectioning lead free solder joints. Lead contamination can increase the intermetallic growth rate and can also create localized eutectic tin lead. The localized eutectic tin lead areas have a lower melting point than most lead free solders and may create additional stresses at the pad and lead interfaces. The amount of voiding detected in a cross-section is often used as a process indicator and can help when adjusting reflow profile, board solderability, or quantity of flux.

Evaluations of the cross-sections are conducted with polarized light microscopy and/or scanning electron microscopy. With proper sample preparation techniques, contamination, intermetallics, and grain structures are easily distinguished.

Wave solder analysis
Controlling the composition of the solder alloy used in wave solder operations is crucial to proper solder joint formation. High-tin solders are corrosive to many mild steels and may dissolve the lining of the solder pot, thus contaminating the solder bath. This may result in reduced solderability, changes in mechanical properties, and increased or decreased melting temperatures. Additionally, the compositions of lead free solders are delicate. In some alloys, as little as 0.2 wt. % difference in composition can create significant changes in the physical properties. Frequent monitoring of wave solder alloy is a critical quality control requirement. Solder samples taken from a solder pot are often analyzed using emission spectroscopy. This technique can resolve composition to extremely low levels (ppm) and has better accuracy than SEM/EDS, which can produce as much as 1 wt. % error.

Total lead analysis
Measurement of the total lead content of a circuit board ensures compliance to proposed lead free legislation abroad. Total lead composition could also be used as a measure of the manufacturer’s ability to produce lead free product. In measuring the total lead composition, all materials from the board, with the exception of large heatsinks, braces, backplanes, and other passive elements, are included in the analysis. After the removal of the passive elements, the entire assembly is milled into a fine powder, placed in solution, and analyzed. Atomic absorption (A.A.) spectroscopy is used to detect the presence of lead in the solution. The percentage weight of lead in the circuit board is reported.

To be most effective, these lead free analytical techniques should be coupled with normal quality and process control methods, such as cleanliness testing, bare board inspection, and component solderability testing.

Lee, BabHui, Implementing a Simple Corrosion Test Method to Detect “Black Pad” Phenomenon in Electroless Nickel/Immersion Gold Plating, Circuitree, November 2003.

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