Daniel Rigler

Investigating of electrochemical migration on low-ag lead-free solder alloys

Among the conditions of high humidity, elevated temperature, and voltage applied, metals and alloys in electronic components and assemblies can cause insulation failures by forming so called dendrites due to the electrochemical migration (ECM) failure phenomenon. This effect causes short-circuit failures of the electronic circuits, which might lead to catastrophic failure. The ECM behavior of lead-free micro-alloyed low Ag content solder alloys (SAC0307 and SAC0807) were compared with common used SAC305, SAC405 and with the traditionally lead-bearing solder alloys: Sn63Pb37 and Sn62Pb36Ag2 as references. In order to carry out the ECM investigations, water drop (WD) tests were made on standard comb patterns in 1 mMol NaCl and 1 mMol Na2SO4 solutions as well. The results have shown that the SAC0807 low Ag content micro-alloyed solder alloy could have a relative weak resistance against ECM.

Electrochemical migration of copper in pure water used in printed circuit boards

Electrochemical migration (ECM) behaviour investigations were carried out on “oxide-free” and highly oxidized copper surfaces in pure water. It was shown in our previous paper that the oxidation number of copper ion mainly depends on the oxide layer thickness of the copper surface; in the case of “oxide-free” copper surface the Cu1+ ions dominated, while Cu2+ ions were mainly formed in the case of highly oxidized copper surface, which modify the standard electrode potential and also the solubility product constant of copper-hydroxides, thus can effect on the mean time to failure (MTTF) caused by ECM. In order to verify the effect of different copper ions (Cu1+ vs. Cu2+) on MTTF, water drop (WD) tests were carried out on both copper surfaces. The results showed that in the case of “oxide-free” copper the MTTF value was lower than in the case of highly oxidized copper.

[Carbon/carbon implants in oral and maxillofacial surgery--Part 2].

In their previous report, the authors presented observations regarding the long-term application of carbon/carbon implants. After evaluating the good functional and aesthetic results, the effect of the human body on the structure and morphology of the implants was investigated with state of the art methods. An implant retrieved from the body after eight years was compared to implants which were sterilized but not implanted (reference). Carbon and oxygen were the main components of both implants, however, as a result of the interaction with the human body the amount of oxygen increased 3-4 times and phosphorus, sulphur, calcium and iron were detectable as trace elements on the surface. The width of the carbon fibres (5-7 µm) building up the implants was not changed during the interaction with the human body. The surface of the implant retrieved from the human body was covered with a 15-17 µm thick layer, not present on the reference implant, having a similar composition to that of the carbon fibres (high amount of calcium that is typical to bone tissue was not detected). According to these results, the structure and the morphology of the implants were not altered notably by the human body.

Laser soldering of small pitch BGAs on Plexiglas substrate PCBs

The paper presents the results of the study and the procedure of soldering 200µm pitch BGAs on circuits on PMMA (Poly-methyl-methacrylate or Plexiglas) substrate, starting from the paste deposition process, fine placement of the BGAs and finally soldering. The novelty in this research is that it had to face two restrictions. The combination of PMMA substrate, transparent to laser, and SnBi, ecologic solder paste, which required a low and very well-controlled soldering temperature, and the small size of the BGAs which needed a punctual soldering. These particularities led the team to use laser for soldering, due to the possibility of highly accurate energy calculation and very small size of the focused beam.

Failure analysis results systematization

At the Department of Electronics Technology high amount of failure analysis inspections have been carried out during the last five years, thus an outstanding number of results and reports have been accumulated by the Failure Analysis Group. The results and experiences gained in the past by these analyses can provide useful information in the present and future in further cases which show similarities to former problems. Such knowledge can only be exploited if the analysis results are organized in a suitable structure and the researchers do not have to rely on their own memory. A suitable system should handle queries based on various approaches i.e. type of failure, involved component, during which process the failure occurred, applied analysis method, etc. So the analysis result storage system has to provide a possibility to find any earlier analysis result which can be in connection with current cases to provide us the previously gained experiences and conclusions. In case a lecturer wants to illustrate a presentation with images of a certain failure type or of a given analysis method then the time needed to find the right pictures can be significantly reduced by the help of the analysis database.

Electrochemical migration investigations on Sn-Sb solder alloys using 3.5 wt% NaCl solution

ECM (electrochemical migration) is sort of short failure mechanism that can lead to series defect in case of loaded circuits, when moisture exist on/in a conductor-dielectric-conductor system. Water drop (WD) test and scanning electron microscopy — energy disperse spectroscopy (SEM-EDS) methods were used to investigate the ECM behavior on different Sn-Sb alloys and pure Sn was the reference. The results show that all of the samples have similar ECM risk comparing to the pure Sn. However, in case of Sn95-Sb5 solder alloy antimony was found in the dendrites next to Sn content, which was dominated during the ECM processes in all cases.

Morphological and composition study on lead-free micro-alloyed solders after corrosion test

Electroanalytical tests were carried out to investigate the corrosion behavior of various Pb-free solder alloys in NaCl bulk solution under room temperature. Scanning electron microscope (SEM) method was applied to investigate the corroded surface structure and corrosion depth using cross-sectional samples. Furthermore, energy dispersive spectroscopy (EDS) method was also applied to identify the elemental composition of the corrosion products of the solders. The results showed that one of the tested Pb-free μ-alloyed solder have lower corrosion resistance compared to other μ-alloyed Pb-free solders and to the widely used SAC305 solder alloy as well. The different corrosion resistance was explained by the different content and volume of the corrosion products, which can lead back to the differences of the original alloying components.

Evaluation of solder joints formed by different Vapour Phase Soldering systems

As Vapour Phase Soldering (VPS) has become a significant alternative method for reflow soldering, several investigations started to determine the possibilities of the re-introduced technology. In this particular case, experiments were made using two different offline VPS systems - Oven A and Oven B - in order to determine differences in soldering quality. The main aim of the experiments was to produce samples using the default linear soldering profile of each machine. The default settings are not only reflecting the different setup of the control parameters, but the differences in the oven constructions as well. The results were compared based on the quality of the joints. Additionally, plateau shaped soldering profiles were set up for Oven B. Test boards containing 0603 sized chip resistors and fine pitch BGAs were soldered using each heating profile. The mechanical quality of the solder joints was investigated with shear tests. X-ray inspection was used to determine the tendency of voiding regarding BGAs, while cross sections of each sample were analysed by SEM. The results show that there is significant difference between the mechanical qualities of samples soldered with different VPS machines. Correlation was found between the shape and parameters of the reflow profile and solder joint quality. The paper also highlights the importance of possible non-linear reflow profiles during vapour phase soldering.