Morten Stendahl Jellesen

Electrochemical Migration on Electronic Chip Resistors in Chloride Environments

Electrochemical migration behavior of end terminals on ceramic chip resistors (CCRs) was studied using a novel experimental setup in varying sodium chloride concentrations from 0 to 1000 ppm. The chip resistor used for the investigation was 10-k¿ CCR size 0805 with end terminals made of 97Sn3Pb alloy. Anodic polarization behavior of the electrode materials was investigated using a microelectrochemical setup. Material makeup of the chip resistor was investigated using scanning electron microscopy (SEM)/energy dispersive spectroscopy and focused-ion-beam SEM. Results showed that the dissolution rate of the Sn and stability of Sn ions in the solution layer play a significant role in the formation of dendrites, which is controlled by chloride concentration and potential bias. Morphology, composition, and resistance of the dendrites were dependent on chloride concentration and potential.

Impact of NaCl Contamination and Climatic Conditions on the Reliability of Printed Circuit Board Assemblies

The effect of climatic conditions and ionic contamination on the reliability of printed circuit board assembly has been investigated in terms of leakage current (LC) and electrochemical migration susceptibility. The change in LC as a function of relative humidity (RH) and temperature was measured using single components (size 0805) and surface insulation resistance comb patterns precontaminated with sodium chloride at levels adjacent to levels used in the IPC J-STD-001 standard. The potential bias of 5-10 V was applied during experiments, and the climatic conditions were in the range 60%-98% RH and 15 °C-65 °C. The variation of RH at the surface of the test specimens was imposed by 1) increasing the RH of the surrounding air and 2) reducing the temperature of the printed circuit boards using a cooling stage, while maintaining a constant climatic condition in the chamber. A considerable increase in LC was observed at sodium chloride concentrations above 1.56 μg·cm-2, as the RH on the surface was close to the critical RH for sodium chloride. Prolonged exposure to conditions close to condensation resulted in the formation of tin whiskers and hillocks growing from the component electrodes due to corrosion of the electrodes freeing the whisker growth.

Investigation of Electronic Corrosion at Device Level

This work presents device level testing of a lead free soldered electronic device tested with bias on under cyclic humidity conditions in a climatic chamber. Besides severe temperature and humidity during testing some devices were deliberately contaminated before testing. Contaminants investigated are ionic or airborne contaminants likely to be introduced by production or service conditions. The effect of changes in processing parameters as a result of production shift to lead free solder (e.g. higher soldering temperature) has also been investigated. Analysis have shown that one printed circuit board assembly (PCBA) in the device is more prone to corrosion reliability and this was further analysed using thermography to detect areas that have high risk of condensation due to lower temperature under working condition. Tested PCBAs are subjected to detailed investigation before and after testing using high resolution photography, detailed optical microscopy and SEM/EDS.

Effect of No-Clean Flux Residues on the Performance of Acrylic Conformal Coating in Aggressive Environments

The influence of no-clean flux residues on the performance of acrylic conformal coating used for printed circuit board (PCB) assemblies was investigated under aggressive exposure conditions using plain coated laminates and PCBs with comb shaped surface insulation resistance (SIR) pattern. Plain laminate substrate and SIR PCBs were cleaned or dosed with no-clean flux residues prior to coatings. Performance studies were carried out by total immersion in deionized water at 60°C for 10 days followed by microscopic investigations and potentiostatic studies on coated SIR patterns at 12 V potential bias and by measuring the resulting leakage current. Foe both plain laminates and SIR patterns, detailed investigation of the corrosion morphology was carried out. The amount of water intake by the coating was calculated by the weight-gain method. The amount of no-clean flux residue resulting at various temperatures was quantified using ion chromatography by extracting the residue, and surface morphology of the residues was investigated using optical microscopy. The flux residue in general consists of both resin and activator components such as carboxylic acids. Coated samples with flux residues after exposure showed blisters all over the surface and reduction in adhesion strength. The coated SIR PCBs with flux residues also showed failure due to electrochemical migration more quickly than on the clean PCBs.