Parametric study of interior climate in electronic device enclosures and corrosion reliability

Abstract

The current PhD work is supported in part by the Danish Council for Independent Research, Technology and Production Sciences through the ICCI (improved climate control inside electronic equipment) project and in part by the Innovation Fund Denmark through the IN-SPE (innovation consortium for sustained performance of electronics) project at DTU. The aim of this project is to obtain a deeper understanding and improvement of climatic reliability of electronic devices by performing parametric studies of interior climate in electronic enclosures. Electronic control systems are used in all sorts of modern technological devices be it outside or indoors. Uncontrolled local climate inside the enclosures for these systems causes several humidity related failures, especially on the printed circuit board assemblies (PCBAs) placed inside. This PhD project focusses on detailed parametric studies of the influencing factors, such as outdoor climatic profiles and enclosure design, but also of humidity control solutions and materials and corrosion humidity related failures. Paper 1 focusses on printed circuit board surface finish and effects of chloride contamination, electric field, and humidity on corrosion reliability. Papers 2 and 3 are related to the contamination profile on typical PCBAs caused by soldering processes, and on the decomposition of no-clean solder flux systems and their effects on the corrosion reliability of electronics. Paper 4 concerns water absorption in electronic materials such as PCBA laminates and solder masks, while Paper 5 focusses on the humidity build-up in typical electronic enclosures exposed to constant conditions, investigating the effect of parameters such as hole size, hole shape, casing material, and the resistor-capacitor (RC) approach for modelling was attempted. Paper 6 is a study of the humidity build-up in a typical electronic enclosure exposed to cycling conditions and studies the effect on corrosion reliability specifically investigating the “pumping effect” due to delay in temperature change in presence of heavy thermal mass. Paper 7 is about the study carried out during external research stay at Robert Bosch, Germany and concerns the dynamics of moisture ingress in first and second level electronic housings with in-situ measurement of T and RH in an epoxy mold compound. Paper 8 is a study of climate profiling of different geographical locations, with RC modelling of humidity build-up in electronic enclosures and their reliability prediction. Paper 9 concerns the use of desiccant as a humidity control in electronic enclosures. Overall, the investigations clearly showed the importance of the presence of contamination of flux residues and sodium chloride on the surface of the PCBAs, which will reduce the humidity thresholds at which failures (corrosion and electrochemical migration) appear on the PCBAs. The solder mask material absorbs more moisture than the laminates of the PCBAs, however the presence of moisture in laminate as well as solder mask material will increase the dielectric properties and can create voids and blisters during the reflow soldering process. The moisture ingress into the electronic enclosures will arise by any presence of leaks or cracks in the casing walls, while diffusion and solubility of the polymeric casing material have to be considered, especially when exposed to cyclic conditions. Electronic products are used in varying geographical locations, where climate conditions can be very different, and the moisture inside a fully hermetic polymeric enclosure can reach a saturation level in few weeks due to temperature change. The presence of thermal mass inside the device can also act as a site for local humidity build-up due to its high thermal capacity, and enhances the moisture uptake inside the electronic device. Due to their high ability to absorb water, desiccant can be used as humidity control inside electronic enclosures, but a regeneration process is needed in order to avoid desiccant water saturation and even humidity increase in inside the enclosures.