In-situ non-destructive ultrasonic rheology technique for monitoring different lead-free solder pastes for surface mount applications

This paper investigates the application of a non-destructive ultrasonic technique for characterising the rheological properties of solder paste through the use of through-mode microsecond ultrasonic pulses for evaluation of viscoelastic properties of lead-free solder paste containing different types of flux. Ultrasonic techniques offer a robust and reliable form of non-destructive testing of materials where access to the sample is restricted or when sample handling can interfere with the monitoring or analysis process due to externally incorporated changes to the material’s physical state or accidental contamination during the removal or testing process. Ultrasonic based techniques are increasingly used for quality control and production monitoring functions which requires evaluation of changes in material properties for a wide range of industrial applications such as cement paste quality, plastic/polymer extrusion process, dough and even sugar content in beverage drinks. In addition, ultrasound techniques are of great interest for their capability to take rapid measurements in systems which are optically opaque. The conventional industry approach for characterising the rheological properties of suspensions during processing/packaging stage is mainly through the use of viscometer and some through the use of rheometer. One of the potential limitations of viscometer and rheometer based measurements is that the collection and preparation of the solder paste samples can irreversibly alter the structure and flow behaviour of the sample. Hence the measurement may not represent the actual quality of the whole production batch. Secondly, rheological measurements and the interpretation of rheological data is a very technical and time consuming process, which requires professionally trained R&D personnel. The ultrasound technique being proposed provides simple, yet accurate and easy to use solution for the in-situ rheological characterisation of solder pastes which will benefit the materials suppliers (who formulate and produce solder pastes) and solder paste consumers (especially, contract electronics manufacturers). The results from the work show that the technique can be used by R&D personnel involved in paste formulation and manufacture to monitor the batch-to-batch quality and consistency.

The development of dynamic CFD model of the magnetic semi-levitation of liquid metals

In the casting of reactive metals, such as titanium alloys, contamination can be prevented if there is no contact between the hot liquid metal and solid crucible. This can be achieved by containing the liquid metal by means of high frequency AC magnetic field. A water cooled current-carrying coil, surrounding the metal can then provide the required Lorentz forces, and at the same time the current induced in the metal can provide the heating required to melt it. This ‘attractive’ processing solution has however many problems, the most serious being that of the control and containment of the liquid metal envelope, which requires a balance of the gravity and induced inertia forces on the one side, and the containing Lorentz and surface tension forces on the other. To model this process requires a fully coupled dyna ic solution of the flow fields, magnetic field and heat transfer/melding process to account for. A simplified solution has been published previously providing quasi-static solutions only, by taking the irrotational ‘magnetic pressure’ term of the Lorentz force into account. The authors remedy this deficiency by modelling the full problem using CFD techniques. The salient features of these techniques are included in this paper, as space allows.