A customer called the EMPF Helpline with questions regarding electrical shorts on a component. The component was experiencing check sum errors and was needed for a high-performance product for military and aerospace applications.

The customer’s initial observations determined that a crystalline residue was present between the leads of the 44-pad windowed ceramic leadless chip carrier (LCC). If they used a pick to abrade the area between the shorted leads, the short would be removed. In another instance, removal of the component from the board by mechanically clipping the leads caused two of the three shorts to disappear.

The EMPF visually examined the electrical short areas indicated by the customer. The foreign residue between the leads was examined and was determined to consist of the following elements: magnesium, aluminum, silicon, and oxygen. Some charging from the electron beam was also present, and the particle was determined to be non-conductive. This indicated that the particles were similar in nature to common sand. Sand is both similar in composition and electrically insulating; therefore, these particles could not be the cause of the electrical short. It was possible that the particles were stray remnants from the hermetic ceramic seal formation process. Fillers, also known as “frit”, are used in this process.

Further analysis showed that conglomerates composed of lead and oxygen, in the ratio of PbO2, were formed between the lead frames (Figure 3-1). The electrical properties of PbO2 (resistivity as low as 40 to 50 x 10-10 ohm-cm) show that it is a better electrical conductor than many metals and a much better conductor than carbon or graphite. Backscatter Electrons (BSE) showed the difference in elemental composition of the particles. This mode revealed the significant presence of a bright material that indicated high atomic number (Pb) – a conductive material which could be the source of the intermittent failures.

Graphite Substrate Lead Dioxide (GSLD) anodes are used in high amperage cells in the production of chlorates, perchlorates, and other inorganic preparations like bromates and iodates. They are manufactured from electrochemical deposition of beta PbO2 from lead nitrate baths. When lead nitrate alone is used, a dendrite form of lead is also deposited onto the cathodes, resulting in a short of the electrodes. Experiments have shown that the addition of 2 to 3 percent copper salt to the plating process prevents lead deposition onto the cathode. The copper, because it is more electronegative than lead, deposits preferentially onto the cathode.

The presence of dendrites (Figure 3-2) indicates a chemical process. However, the component manufacturer’s specification required a molten process solder dip. Because the manufacturer of the component was overseas and the components were no longer produced, information about the manufacturing process was difficult to acquire. For the manufacturer to use a molten SnPb dip process is probable, since the process is common; however, the dendrite formation indicates remnants of a deposition process.

The customer asked for suggestions regarding the removal of the PbO2. The EMPF suggested mechanical abrasion, and the customer suggested the use of a diamond probe. An air gun with SiC abrasive set at 10 psi would most likely work, but a very fine tip would be necessary. Chemically, PbO2 is insoluble in cold water and dissolves in hot water. It also dissolves in acids to the conjugate salt; however, acid is difficult to control and would be left behind for further corrosion.

The EMPF is currently working with the customer to determine if the component can tolerate a hot water rinse. If so, the rinse will be applied, and the EMPF will then inspect for removal of the dendrite formations.

1. “Preparation and Applications of Graphite Substrate Lead Dioxide (GSLD) Anode,” K.C. Narasimham and H.V.K. Udupa, Journal of Electrochemical Society, September 1976.