SMT Electrolytic Capacitor Solder Joint Criteria and Integrity Investigation

Estimated reading time: 10 minutes

Figure 5: Assembly showing SMT electrolytic capacitor locations.

Figure 6: Typical SMT electrolytic capacitors. (Photo courtesy of Conrad Dubilier)

Assembly

The test vehicles were assembled at the Rockwell Collins Coralville, IA production facility. The solder paste was Henkel MP-218 tin lead eutectic solder deposited with a 0.005 inch thick stencil. The components were placed using a Universal Advantis automated placement machine. The assemblies were reflowed in a Heller 1912EXL Reflow Oven. Assemblies were cleaned in the Electrovert Aquastorm 200 inline cleaner. The majority of the solder joint is obscured due to the package configuration, with only the outer end of the component lead being visible for optical inspection (Figure 8). The portions of the solder joints that were visually inspectable were considered non-compliant as they did not meet a 1T minimum side fillet height. The typical placement/spacing of the SMT electrolytic capacitors makes solder joint touch-up challenging and creates a potential opportunity for component or laminate damage.

Thermal Cycle Testing

The test vehicles were placed into a thermal chamber set for a temperature range of -55°C to 125°C. The ramp rate was set for 5°C–10°C per minute with 10-minute dwells at each temperature extreme. Figure 9 shows the thermal profile.

Figure 7: Typical SMT electrolytic capacitors. (Photo Courtesy of Panasonic).

After completing a total of 532 thermal cycles, the components were removed from the thermal cycle chamber for metallographic cross-sectional analysis.

Shear Testing

Prior to thermal cycle testing, a set of electrolytic capacitors were carefully removed from the assembled test vehicles for shear testing. This data set documented the typical as-soldered, not thermal cycled condition to provide a measurement of the strength of the solder joint wetting/geometry configuration. An Instron test system using a shear rate of 0.15 inches/minute was used for the test.

Figure 10 illustrates the test components/shear test setup. The measured mean peak load for the shear testing was 40.3 pounds with a standard deviation of 5.4 pounds (Figure 11). There are no industry acknowledged minimum shear force values, however, 30 pounds would be considered very adequate for solder joint integrity.

For comparison, the typical component pad peel strength is 6-12 pounds.

Figure 8: Optical view of solder joints after automated reflow soldering.

Optical examination of the failed solder joints after shear testing revealed minor solder joint voiding, good wetting/solder joint geometry and complete metallurgical attachment for both the solder joint/component lead interface and the solder joint/component pad interface (i.e. cohesive failure).

Figure 12 illustrate the typical failed solder joint interfaces resulting from the shear testing.

Metallographic Analysis

A number of the SMT electrolytic capacitors were removed from the test vehicle after the completion of the thermal cycle conditioning for metallographic analysis. Some very minor cracking was observed in the solder joint heel fillet region but there were no observed anomalies in the solder joint microstructure or cracking that would indicate degradation of the solder joint integrity due to thermal cycle conditioning (Figure 13).

It was noted that a “1T” solder joint heel fillet or side fillet was not achieved using the standard automated reflow soldering process. The solder joint heel fillet height was found to be in the 1/2T height range.

Figure 14 through Figure 16 illustrate the solder fillet heights achieved with the standard automated reflow soldering process.

Figure 14: Cross sectional toe view of SMT electrolytic capacitor illustrating ~ 63% T side fillet height.

Figure 15: Cross sectional toe view of SMT electrolytic capacitor illustrating ~ 54% T side fillet height.

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