Corrosion of Copper Alloys in Consumer Electronics Environments
C Results of corrosion testing on eight alloys from five copper alloy families common in the consumer electronics industry—copper nickel tin (CuNiSn), copper beryllium (CuBe), copper titanium (CuTi), phosphor bronze (CuSnP), and Corson alloys (CuNiSiX)—are presented. Short-term exposures were conducted in simulated consumer environments such as a salted carbonated cola beverage, artificial human perspiration, and phosphate-buffered saline. Longer-term, conventional salt fog testing, up to 1,000 h, was also conducted. In all of these tests, CuNiSn showed the lowest corrosion rate followed closely by CuBe alloys. Corrosion rates of CuTi alloys were consistently the highest after shortterm testing, while CuSnP showed the highest corrosion rate after salt fog testing.
SOLDER FLUX RESIDUES AND ELECTROCHEMICAL MIGRATION FAILURES OF ELECTRONIC DEVICES
Corrosion reliability is a serious issue today for electronic devices, components, and bare printed circuit boards (PCBs) due to factors such as miniaturization, globalized manufacturing practices, and global usage. The result is reduced life span for electronic products and heavy economic loss due to failures. Miniaturization at all levels is one of the key factors reducing corrosion reliability. Over the last 10 years, size of the electronics has been reduced by over 70%. For flip chip ICs, miniaturization amounts to ~ 90%. The closer spacing increases the electric field (E = V/d), which makes corrosion cell formation easier during local condensation under humid environments. Process related residues (contamination) on PCB surfaces results from all stages of manufacturing process starting from base PCB production to components mounting, soldering, inspection and testing, device assembly, and packing are all process that will have great influence on corrosion. A particularly important factor is the residue resulting from no clean flux especially from the wave soldering process as this is one of the step in the PCB manufacturing processes. Such residues can easily absorb water under humid conditions, and can accelerate the corrosion problems by providing conducting ions, participating in corrosion reaction, or as a site for entrapment of dust. This paper focuses on mechanistic investigation of the role of no-clean flux residues on electrochemical migration failures, which is a dominant corrosion failure mode in electronic devices. During electrochemical migration, a suitable metal ion such as Sn, Pb, Cu etc. dissolves in to the condensed water layer from a positively biased point on a PCB and migrates through the water layer to a nearby negatively charged part to deposit there in the form of dendrites. As it progress, dendrites will bridge the gap between two points, which will lead to an electric short. This paper investigate in detail how no-clean flux residues are present on the PCBs, their composition, morphology, interaction with humid environments, and their electrochemical behaviour in aqueous environments. A novel Single Component Electrochemical Migration set up (SCECM) was used for investigating electrochemical migration under various flux residue conditions on tiny electronic components, while spectroscopic studies and SEM/EDX were used for understanding the residue composition and morphology. Detailed analysis of the dendrite morphology and composition was carried out using FEG-SEM. Results shows that the organic acids in the flux residue can some time act as corrosion inhibitors to reduce migration problem, however the exact effect depends on combination of factors such as amount, morphology, and presence of other ions such as chlorides. Composition of the dendrite, and therefore its conductivity was a function of stability of the corresponding metal ions in the respective environments.
Routine quantitative phase analysis of niobium-bearing lateritic ores
Routine quantitative analyses by Rietveld full-pattern fitting have been carried out on more than one thousand niobium ore samples originating from four pyrochlore deposits: Lueshe and Bingo (D.R. Congo), Mabounie (Gabon), and Sokli (Finland). The ore samples were characterized by a highly variable mineral content, the presence of mineral phases with a wide range of compositions and a high amorphous content. An analytical procedure was established which addressed several problems: creation of an appropriate crystallographic parameter database, specimen preparation, check for preferred orientation and measurement of the amorphous content by the internal standard method.