The effects of design and environmental factors on the reliability of electronic products

The reliability of electronic products is fast becoming of major importance with the demand for increased safety, especially in the automotive industry. Tracks, pads and vias on printed circuit boards can suffer a variety of problems if circuits are contaminated with electrical-conducting substances. Electrochemical migration, especially dendrite growth, has long been a concern in safety critical and durable systems, and current preventative methods tend to focus on various styles of printed circuit board protective coatings. These measures have a number of disadvantages, mainly process and material costs with extreme scepticism on their overall efficacy. Any design related developments that can minimise the impact of dendrite growth on reliability can lead to a more economic, durable and safer product. The work in this thesis provides a thorough literature search of the field of electrochemical migration on printed circuit boards. This study then develops a novel circuit-designorientated model, based on a multilevel full-factorial design, to study the effects of temperature, voltage and electrode gap on dendritic growth under saturated conditions. Preparation of several DC-biased copper-comb patterned printed circuit boards placed in temperature-controlled water-filled cuvettes enables the specific monitoring of dendrite activity, and detects a sharp current increase that accompanies a dendritic short circuit condition. A high R2 polynomial correlation-model is derived and it is noted that increased voltage and temperature and reduced track spacing increases the impact of dendritic growth on reliability. At voltages between 3 and 4V, gas bubble formation at the electrodes has the effect of increasing reliability by destroying the dendrite fuses. It is shown that dendrites may not grow below 1.25V, which coincides with the theoretical onset voltage for the decomposition of water. It was also demonstrated that electrically biased, watercontaminated printed circuit boards form extreme acid and alkaline regions close to the anode and cathode terminations, respectively, which can cause corrosion. The thesis proposes a novel approach, termed ‘design contingency’, for preventing dendritic growth through design optimisation