Visweswara Chakravarthy Gudla

Humidity Build-Up in a Typical Electronic Enclosure Exposed to Cycling Conditions and Effect on Corrosion Reliability

The design of electronic device enclosures plays a major role in determining the humidity build-up inside the device as a response to the varying external humidity. Therefore, the corrosion reliability of electronic devices has direct connection to the enclosure design. This paper describes the internal humidity build-up in a typical enclosure prescribed for electronic applications as a function of external humidity conditions and enclosure-related parameters. Investigated parameters include external temperature and humidity conditions, the temperature and time of the internal heating cycle, thermal mass, and port/opening size. The effect of the internal humidity build-up on corrosion reliability has been evaluated by measuring the leakage current (LC) on interdigitated test comb patterns, which are precontaminated with sodium chloride and placed inside the enclosure. The results showed that the exposure to cycling temperature causes significant change of internal water vapor concentration. The maximum value of humidity reached was a function of the opening size and the presence of thermal mass inside the enclosure. A pumping effect was observed due to cycling temperature, and the increase in the level of absolute humidity at each cycle led to condensation, which caused a sudden increase in LC.

Interface strength and degradation of adhesively bonded porous aluminum oxides

For more than six decades, chromic acid anodizing has been the main step in the surface treatment of aluminum for adhesively bonded aircraft structures. Soon this process, known for producing a readily adherent oxide with an excellent corrosion resistance, will be banned by strict international environmental and health regulations. Replacing this traditional process in a high-demanding and high-risk industry such as aircraft construction requires an in-depth understanding of the underlying adhesion and degradation mechanisms at the oxide/resin interface resulting from alternative processes. The relationship between the anodizing conditions in sulfuric and mixtures of sulfuric and phosphoric acid electrolytes and the formation and durability of bonding under various environmental conditions was investigated. Scanning electron microscopy was used to characterize the oxide features. Selected specimens were studied with transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy to measure resin concentration within structurally different porous anodic oxide layers as a function of depth. Results show that there are two critical morphological aspects for strong and durable bonding. First, a minimum pore size is pivotal for the formation of a stable interface, as reflected by the initial peel strengths. Second, the increased surface roughness of the oxide/resin interface caused by extended chemical dissolution at higher temperature and higher phosphoric acid concentration is crucial to assure bond durability under water ingress. There is, however, an upper limit to the beneficial amount of anodic dissolution above which bonds are prone for corrosive degradation. Morphology is, however, not the only prerequisite for good bonding and bond performance also depends on the oxides’ chemical composition.Surface coating: interface strength and degradationChromic acid anodizing has been the dominant electrochemical process used to create a thin aluminum oxide layer upon aluminum alloys. Such thin layers are critical to the corrosion protection of alloys that enable many of our daily expectations, including aerospace applications. However, such a chromic acid treatment is being forced to be soon phased out as a result of the associated environmental and health concerns. Now a team led by Arjan Mol from Delft University of Technology in the Netherlands, with co-workers from Denmark and Belgium, are working on alternative treatment processes and reveal the fundamental adhesion and degradation mechanism at the interface between the surface oxide and an accompanying resin. Aided by both imaging and spectroscopic characterization, this study provides fresh insights into the interplay between the anodizing conditions and the formation and durability of bond strength, showing that the morphology and chemistry of the surface oxide are the two factors that should be considered in the selection of chromium-free surface treatments.

Band gap tuning of amorphous Al oxides by Zr alloying

The optical band gap and electronic structure of amorphous Al-Zr mixed oxides with Zr content ranging from 4.8 to 21.9% were determined using vacuum ultraviolet and X-ray absorption spectroscopy. The light scattering by the nano-porous structure of alumina at low wavelengths was estimated based on the Mie scattering theory. The dependence of the optical band gap of the Al-Zr mixed oxides on the Zr content deviates from linearity and decreases from 7.3 eV for pure anodized Al2O3 to 6.45 eV for Al-Zr mixed oxides with a Zr content of 21.9%. With increasing Zr content, the conduction band minimum changes non-linearly as well. Fitting of the energy band gap values resulted in a bowing parameter of ∼2 eV. The band gap bowing of the mixed oxides is assigned to the presence of the Zr d-electron states localized below the conduction band minimum of anodized Al2O3.

Investigation of moisture uptake into printed circuit board laminate and solder mask materials

Presence of moisture in a printed circuit board (PCB) laminate, typically made of glass fibres reinforced epoxy polymer, significantly influences the electrical functionality in various ways and causes problems during soldering process. This paper investigates the water uptake of laminates coated with different solder mask materials and exposed to saturated water vapour and liquid water. The solder masks are characterised for their microstructure and constituent phases using scanning electron microscopy and X-ray diffraction. The observations are correlated with the moisture absorption characteristic such as diffusivity, permeability, and solubility. In addition, the effect of a reflow soldering simulation on microstructural changes and on increase of water uptake of the materials has been analysed.