Silver migration in thick film conductors and chip attachment resins

The rates of silver migration of silver alloy thick film conductors and silver-loaded resins have been examined under conditions of 98% relative humidity at 25°C and 85% relative humidity at 85°C, and various d.c. power loadings. The value of a current limiting resistor in series with the sample under test was shown to have a considerable effect on migration. The presence of debris in the gap between conductor tracks and on the substrate surface was shown to increase the rates of migration. The nature and composition of the substrate surface also affected the migration of conductors printed and fired under identical conditions. Overglazes over conductors and cross-over dielectrics between conductors slowed moisture penetration to the conductor surfaces but did not prevent dendritric growth from occurring. Migration always occurred from the cathode to the anode but it was necessary for silver ions to move from the anode to the cathode to provide the dendrite source material. It was concluded that care was necessary in interpreting data from silver migration tests but all silver loaded materials will migrate in the presence of moisture, and the only sure solution would be improved encapsulation or dry atmosphere sealing.