Spectrum Control Filters

Gideon Analytical Laboratories received four Spectrum Control Pi filters  N49927 Bus A (capacitor components FL1, FL2, FL3, and FL4). Capacitor-input filters (also known as Pi filters) are a type of electronic filter used to remove unwanted frequencies from a signal. Pi filters are primarily constructed of two capacitors and an inductor. They are used in situations where the ripple of a signal or the AC and DC currents within the signal are found to interfere with each other. Components FL1 and Fl3 had resistance measurements indicating that they were shorted. The goal was to determine the cause of the short.

The filters did not exhibit catastrophic expulsion of dielectric debris from the middle of the capacitor to the filter activity. On FL2, Resistance between body and pin measured 1.7k ohms. FL1 was cross-sectioned and electrical overstress (EOS) damage sites were observed. Many stress-related semiconductor failures are electrothermal in nature microscopically; locally increased temperatures can lead to immediate failure by melting or vaporizing metallization layers, melting the semiconductor or by changing structures. If the EOS was allowed to remain, the capacitor will explode due to dead short causing the melting of electrodes, cracking of the ceramic, and expulsion of the shorted area.

There were several issues with these capacitors. The termination solder was sufficient, but the solder holding it into place on the board had many voids. Cross-sectioning of the capacitors revealed cracks near the EOS failure sites. The origin of the failure was within the capacitor and was a result of the dielectric breakdown between the two plates. On both FL1 and FL3, resistance was infinite; it was open. These capacitors had poor contact areas for the pins because they were not “wetted” properly. There was an over voltage spike that caused these failures and the origin site was plain to see; there was cracking involving the first and second electrodes. A voltage spike greater than the dielectric strength between the two electrodes of the capacitor was the cause of failure for these Spectrum Control Pi filters.

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