Roumen Ratchev

Absorption and diffusion of water in printed circuit boards

When exposed to harsh and humid environments, electronic systems can show electrochemical corrosion. One of the possible defects is Conductive Anodic Filament (CAF). This defect needs the presence of humidity. Though the moisture uptake of printed circuit boards has been studied thoroughly, a complete understanding of the diffusion processes and paths in printed circuit boards has not yet been achieved. This work deals with the anisotropy of moisture diffusion in the laminate. According to this purpose samples have been produced and tested. The results show a different absorption behavior according to sample type and base material as well as to storage temperature and humidity. The diffusion constants can be extracted from the measured mass changes of the samples. Since the different sample designs allow diffusion only in specific directions, a determination of the anisotropy is feasible. Furthermore, information on the storage location of the moisture in printed circuit boards can be gained. Therefore, samples have been produced to investigate the absorption behavior of different components of printed circuit boards such as resin and glass weave.

Vibration reliability of SMD Pb-free solder joints

This study presents a method to predict lifetime of SMD lead free solder joints submitted to vibrations. An experimental method is presented to test chip resistors under vibrations. A specially designed testboard allows testing several specimens simultaneously. The time to failure is electrically detected. The experimental method is simulated using the Finite Element Method (FEM) in order to correlate the stress level calculated on solder joints with the experimentally observed time to failure. The obtained results show that the Printed Circuit Board (PCB) has critical zones where the components fail at first. It is noticed that the damage on solder joint is mainly caused by the warpage of the PCB. On the contrary, the damage caused by the high local acceleration levels on the testboard is insignificant compared with the stress level caused by the warpage of the board.

Thermal shock fatigue enhancement of solder joints by using novel reinforcing fabrics

To meet the increasing lifetime requirements of solder joints in thermal cycling, the coefficient of thermal expansion (CTE) of the printed circuit board (PCB) has to be conformed to the CTE of the surface mounted devices (SMD). Novel reinforcing fabrics were successfully used to reduce the CTE of the PCB. Thereby, the mismatch in the CTE of PCB and SMD was reduced. In order to check the expected effect of the improvements, chip resistors were mounted to the PCBs. The samples were aged in a temperature shock test between −40 °C and 125 °C. The characterization of the ageing behavior was done by measuring the required force to shear SMDs off the PCB as well as analysis of crack lengths. The results with the improved CTE showed significant improvement in lifetime of the solder joints.