EFFECT OF FLUX SYSTEMS ON ELECTROCHEMICAL MIGRATION OF LEAD-FREE ASSEMBLY
The reliability concerns regarding surface insulation resistance (SIR) and electrochemical migration (ECM) have been significantly raised in the printed circuit board (PCB) assembly with wide adoption of lead-free assembly materials and process. Moreover, the miniaturization trend of the electronic industry also increases the sensitivity of the functional assemblies to surface contamination and flux residues. There are numerous SIR reliability testing methods and standards, such as IPC, JIS, Bellcore, and Bono, whose predictive power will depend on the field of application including temperature, relative humidity and electric field. Each standard is limited in essence, because the failure mechanism is strongly affected by the chemistry of the assembly materials, e.g. soldering flux, paste, and substrate materials, which will perform differently under various conditions. This paper presents a summary and comparison of the major SIR reliability standards currently in use in the industry, and discusses the impact of the different flux systems on the failure mechanisms.
Effects of Solder Mask on Electrochemical Migration of Tin-Lead and Lead-Free Boards
Electrochemical migration (ECM) is the growth of conductive metal filaments on a printed circuit board (PCB) through an electrolyte solution under a DC voltage bias. ECM can cause a reduction in surface insulation resistance (SIR) between adjacent conductors, generate a path of leakage current, and lead to intermittent or catastrophic circuit failures. Solder mask has been widely used on printed circuit boards to define wettable surfaces, reduce moisture access, control outer layer impedance, and improve corrosion resistance. The mechanical and thermal properties of solder mask have been widely reported, but systematic studies of their influence on ECM have been few. This paper presents the results of temperature-humidity-bias (THB) testing of more than 1000 hours duration at 40V, 65 ̊C, and 88% relative humidity for comparative evaluation of ECM on circuit boards with and without a solder mask. The boards were HASL finished and wave soldered using a no-clean, low solids flux. Besides primarily assessing the effects of using a solder mask on ECM, the effects of solder alloy composition (eutectic SnPb versus Sn-3.0Ag-0.5Cu) were also investigated. In situ monitoring of SIR was performed throughout these tests. Optical microscopy and scanning electron microscopy were employed to examine the correlation between the physical attributes of dendrites and the measured SIR, as well as to evaluate the effects of solder mask and solder alloy on ECM. Ion chromatography (IC) was conducted to measure contaminant levels on the surface of the PCBs. Elemental mapping by energy dispersive X-ray spectroscopy was employed to identify the migrating species and their distributions and morphologies within the dendrites. As expected, the use of a solder mask resulted in higher SIR, but a dramatic difference was observed in its effect on dendritic growth and characteristic life.
A rapid on-filter immunoassay screen for dexamethasone in equine urine
A simple on-filter immunoassay screen has been developed for the detection and semi-quantification of dexamethasone in equine urine samples. The assay consists of an indirect competitive ELISA in which dexamethasone in standards or samples competes with an immobilised dexamethasone–protein conjugate for binding to a complex of sheep anti-dexamethasone antibodies complexed with alkaline phosphatase-labelled second antibody. The drug–protein conjugate is immobilised as a spot on the surface of a cellulose nitrate filter and the assay is performed with the filter retained within a filtration port. The assay involves two incubation steps, firstly a ten minute competitive step and secondly a five minute step to generate via an enzyme-catalysed reaction an insoluble coloured dye as a spot on the filter at the site of the immobilised drug–protein conjugate. Rapid washing is achieved through use of the filtration system. The assay has a limit of detection of 10–6 M (390 ng ml–1) for a visual end point in which the colour intensity of spots developed in the presence of samples is compared with those of standards. Twelve filters can be processed in a single batch consisting of two standards and ten samples. The assay has been optimised for equine samples and dexamethasone in the urine of a race horse following injection of 20 mg of drug has been detected for 23 h post administration.