Soldering Influence on Passive Performance

Binning, or sorting of manufacturing tolerance will affect capacitor performance in different ways. Soldering introduces stress in the capacitors, and especially in surface-mount parts. That stress can cause piezoelectric voltages with vibration and even crack the capacitor so it later fails. Proper reflow soldering is impressive. The surface tension of the melting solder causes the parts to rotate into alignment as if by magic. But poor solder-temperature profiles can really damage the device.

Have you seen capacitors stand up on one end like a tombstone? This can happen if the solder temperature ramp is wrong. Always follow the manufacturer’s solder-profile recommendations. Some components are more sensitive to temperature, so the board assembly may require two or more solders with different melting temperatures.

Most components in a circuit are soldered first with the highest melting point solder and then any “sensitive” component is soldered at lower temperatures. The solders must be used in the right order so those parts soldered early in the process are not unsoldered later.

Summary

When we discuss passives like capacitors, we must remember that these devices all contain parasitic portions that can change a signal. The impact of this, of course, depends on the signal strength. If we want to measure microvolts, then everything counts: grounding (star points), shielding decoupling capacitors, guarding, layout, the Seebeck effect, cable construction, and soldering connectors. Our schematics usually gloss over this, which is acceptable until we are looking for small noise or voltages.

Remember that a passive capacitor is just one component and really more active than it appears. There are subtle effects of component parasitics, tolerance, calibration, temperature, aging, and even assembly methods and practices that will influence the device’s performance. Knowing that, we need to understand the potential errors that can accumulate with numerous capacitors. In future articles we will discuss other so-called passive components, resistors, potentiometers, switches, and, surprisingly, the lowly PCB.

Finally, AVX and Kemet are capacitor companies that specify parasitic components and provide free Spice tools. These tools allow us to graph the actual performance of the capacitors. The application notes on both their Web sites are also very informative.