Analysis of surface mount technology solder joints

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The factors determining the quality of surface mount technology (SMT) solder joints are numerous, and complex. The exploration of these factors, and how they may affect the reliability and quality of the joints can only be achieved through continuous research. In this project, essential areas of SMT joints were selected for study and analysis, with the intention of providing additional design and process guidelines for the production of quality SMT joints. In the infrared reflow process, one of the common defect phenomena is the occurrence of tombstoning; that is after soldering only one end of the component is soldered while the other is lifted up, assuming a position like a tombstone. The initiation of tombstoning during reflow was analysed based on the forces acting on the component. A model was developed to predict the initiation of this phenomenon. The model shows that, under vibration-free conditions, the surface tension of the molten solder is the source of the force causing the initiation of tombstoning. The contact angle, which varies with the length of the printed circuit board solder land, has a significant effect on the value of the surface tension acting as a force pulling upward on the component. The model further shows that tombstoning initiation is due to the combined effects of the surface tension; the weight of the component; the dimensions of the component; the length of the solder underneath the component; and the length of the solder protruding from the end of the component. Selected components were used as examples for predicting the conditions of initiation, and these conditions were further substantiated by a series of experiments. Another area of study was a method which directly pulled the components off printed circuit boards and this was used as a means for testing the bond quality of surface mount technology leadless chip solder joints. Components D7243, CC1206, RC1206, RC121O, and CC1 812 were selected for this study. It was found that the ultimate tensile force which breaks a component off the printed circuit board has the potential to be used as a parameter for measuring the quality of the solder joint. The effect of solder thickness on the strength of a joint has also been investigated. The shape of joints soldered by two methods, wave soldering and infrared reflow, were compared. Joints at the two ends of a component produced by infrared reflow were found more uniform than the ones produced by wave soldering. A recommendation is made here for the wave soldering approach in achieving uniform solder joints. The effects of solder shape on the joint strength were further investigated by finite element analysis. A convex joint was found marginally more robust than a concave joint. Two aspects of the internal structure of SMT solder joints were investigated, void content and copper/tin intermetallic compounds. The voiding conditions of wave-soldered and infrared reflow joints were compared. No voids were found in all specimens that were produced by wave soldering. However, there were always voids inside joints produced by infrared reflow. Microhardness tests indicated that the hardness of compounds at the copper/solder interface of infrared reflowed joints is lower than that in the wave-soldered joints. It is considered that the lower hardness of the interfacial region of the infrared reflowed joints is due to the presence of voids. Scanning electron microscopy was used to study the formation of copper/tin intermetallic compounds for joints produced by infrared reflow. The results show that Cu 6 Sn5 was the only compound with a detectable thickness. Other compounds such as Cu3 Sn, were virtually not found at all. Aging of the joints at 100°C, shows that both the Cu 6Sn5 and the overall interfacial thickness grew with time. One of the important areas which had been overlooked previously and was studied in some details was the effects of solder paste exposure on the quality of solder paste. The characteristic changes of solder paste due to exposure were investigated in three areas, weight loss, tackiness, and rheology. The evaporation of low boiling point solvents was considered as the main contribution to the loss in the weight of the solder paste. The weight loss against exposure time was found to follow an exponential behaviour. A method was designed to evaluate the tackiness changes of solder paste due to exposure. It was found that the decay of tackiness against exposure time can be expressed by a power law. It is recommended that solder paste manufacturers should provide the necessary characteristic constants so as to enable the characteristics to be calculated after a specific exposure. The rheological changes of the solder paste as a result of exposure were also investigated. The implication on the printability of the solder paste due to these changes was studied and discussed

Survey of oral hygiene behaviour, knowledge and oral hygiene status among Hong Kong adults : a pilot study

Objectives: To study the correlation between oral health behaviour and knowledge with respect to the oral hygiene status of Hong Kong Chinese adults. Materials and methods: Subject selection was by convenience sampling. A total of four outreach visits were arranged in March 2015. The participants’ oral health behavior and knowledge were evaluated through a self-reported questionnaire, while existing oral conditions were recorded following clinical examination using Visible Plaque Index (VPI) and Gingival Bleeding Index (GBI). Data analysis was carried out using SPSS on results obtained from the questionnaire as well as clinical examination. Results: A total of 147 subjects participated in this research project, of which 72% (103/147) were female while 28% (44/147) were male. Male subjects had statistically significantly higher mean VPI scores compared to female subjects interproximally, buccally and lingually (t-test, p<0.05). Furthermore, there exists a statistically significant negative correlation between oral health knowledge score (mean = 9.3, SD = 3.1) and VPI score (Pearson correlation test, p=0.025). Subjects who agreed accumulation of plaque or bacteria as a contributing factor to caries and periodontal diseases are statistically significantly lower than subjects who disagreed this statement in terms of mean VPI scores (53% vs 63%, t-test, p<0.05). Conclusion: Participants with better oral health knowledge who also recognized accumulation of plaque or bacteria as one of the contributing factors to dental caries and periodontal disease had better oral hygiene levels in terms of VPI.published_or_final_versio

AI is a viable alternative to high throughput screening: a 318-target study

: High throughput screening (HTS) is routinely used to identify bioactive small molecules. This requires physical compounds, which limits coverage of accessible chemical space. Computational approaches combined with vast on-demand chemical libraries can access far greater chemical space, provided that the predictive accuracy is sufficient to identify useful molecules. Through the largest and most diverse virtual HTS campaign reported to date, comprising 318 individual projects, we demonstrate that our AtomNet® convolutional neural network successfully finds novel hits across every major therapeutic area and protein class. We address historical limitations of computational screening by demonstrating success for target proteins without known binders, high-quality X-ray crystal structures, or manual cherry-picking of compounds. We show that the molecules selected by the AtomNet® model are novel drug-like scaffolds rather than minor modifications to known bioactive compounds. Our empirical results suggest that computational methods can substantially replace HTS as the first step of small-molecule drug discovery