Thermal-mechanical analysis of flexible substrates during lead-free solder reflow

This paper presents modeling results about the performance of flexible substrates when subjected to higher lead-free reflow temperatures. Both adhesiveless and adhesive types of polyimide substrates were studied. Finite element (FE) models of flex substrates were built, two copper tracks located in the centre of the substrate was considered. The thermal induced shear stress in the flex substrate during the lead-free reflow process was studied and the effect of the design changes including the track thickness, flex thickness, and copper width were studied. For both types of flexes, the one of most important variables for minimizing damage to the substrate is the height of the copper tracks. The height of flex and the width of copper track show less impact. Beside of the geometry effects, the increase in reflow peak temperature can also result in a significant increase in the interfacial stress between the copper track and flex. Higher stresses were identified within the adhesive flex due to the big CTE mismatch between the copper and adhesive/dielectric

Numerical simulation of heat transfer in rectangular microchannel

Numerical simulation of heat transfer in a high aspect ratio rectangular microchannel with heat sinks has been conducted, similar to an experimental study. Three channel heights measuring 0.3 mm, 0.6mmand 1mmare considered and the Reynolds number varies from 300 to 2360, based on the hydraulic diameter. Simulation starts with the validation study on the Nusselt number and the Poiseuille number variations along the channel streamwise direction. It is found that the predicted Nusselt number has shown very good agreement with the theoretical estimation, but some discrepancies are noted in the Poiseuille number comparison. This observation however is in consistent with conclusions made by other researchers for the same flow problem. Simulation continues on the evaluation of heat transfer characteristics, namely the friction factor and the thermal resistance. It is found that noticeable scaling effect happens at small channel height of 0.3 mm and the predicted friction factor agrees fairly well with an experimental based correlation. Present simulation further reveals that the thermal resistance is low at small channel height, indicating that the heat transfer performance can be enhanced with the decrease of the channel height.

Solder paste characterisation: towards the development of quality control (QC) tool

Purpose – The purpose of this paper is to develop a quality control tool based on rheological test methods for solder paste and flux media. Design/methodology/approach – The rheological characterisation of solder pastes and flux media was carried out through the creep-recovery, thixotropy and viscosity test methods. A rheometer with a parallel plate measuring geometry of 40mm diameter and a gap height of 1mm was used to characterise the paste and associated flux media. Findings – The results from the study showed that the creep-recovery test can be used to study the deformation and recovery of the pastes, which can be used to understand the slump behaviour in solder pastes. In addition, the results from the thixotropic and viscosity test were unsuccessful in determining the differences in the rheological flow behaviour in the solder pastes and the flux medium samples. Research limitations/implications – More extensive rheological and printing testing is needed in order to correlate the findings from this study with the printing performance of the pastes. Practical implications – The rheological test method presented in the paper will provide important information for research and development, quality control and production staff to facilitate the manufacture of solder pastes and flux media. Originality/value – The paper explains how the rheological test can be used as a quality control tool to identify the suitability of a developmental solder paste and flux media used for the printing process.