Modelling the performance of lead-Free solder interconnects for copper bumped flip-chip devices

Traditionally, before flip chips can be assembled the dies have to be attached with solder bumps. This process involves the deposition of metal layers on the Al pads on the dies and this is called the under bump metallurgy (UBM). In an alternative process, however, Copper (Cu) columns can be used to replace solder bumps and the UBM process may be omitted altogether. After the bumping process, the bumped dies can be assembled on to the printed circuit board (PCB) by using either solder or conductive adhesives. In this work, the reliability issues of flip chips with Cu column bumped dies have been studied. The flip chip lifetime associated with the solder fatigue failure has been modeled for a range of geometric parameters. The relative importance of these parameters is given and solder volume has been identified as the most important design parameter for long-term reliability. Another important problem that has been studied in this work is the dissolution of protection metals on the pad and Cu column in the reflow process. For small solder joints the amount of Cu which dissolves into the molten solder after the protection layers have worn out may significantly affect solder joint properties.

Modeling the dynamics of electromagnetically agitated metallurgical flows: levitation, cold crucible, aluminium electrolysis, etc.

We present practical modelling techniques for electromagnetically agitated liquid metal flows involving dynamic change of the fluid volume and shape during melting and the free surface oscillation. Typically the electromagnetic field is strongly coupled to the free surface dynamics and the heat-mass transfer. Accurate pseudo-spectral code and the k-omega turbulence model modified for complex and transitional flows with free surfaces are used for these simulations. The considered examples include magnetic suspension melting, induction scull remelting (cold crucible), levitation and aluminium electrolysis cells. The process control and the energy savings issues are analysed.

Study of the thermal stress in a Pb-free half-bump solder joint under current stressing

The thermal stress in a Sn3.5Ag1Cu half-bump solder joint under a 3.82×108 A/m2 current stressing was analyzed using a coupled-field simulation. Substantial thermal stress accumulated around the Al-to-solder interface, especially in the Ni+(Ni,Cu)3Sn4 layer, where a maximal stress of 138 MPa was identified. The stress gradient in the Ni layer was about 1.67×1013 Pa/m, resulting in a stress migration force of 1.82×10-16 N, which is comparable to the electromigration force, 2.82×10-16 N. Dissolution of the Ni+(Ni,Cu)3Sn4 layer, void formation with cracks at the anode side, and extrusions at the cathode side were observed

Modelling the reliability of components on flexible substrates

The use of flexible substrates is growing in many applications such as computer peripherals, hand held devices, telecommunications, automotive, aerospace, etc. The drive to adopt flexible circuits is due to their ability to reduce size, weight, assembly time and cost of final product. they also accommodate flexibility by allowing relative movement between component parts and provide a route for three dimensional packaging. This paper will describe some of the current research results from the Flex-No-Lead project, European Commission sponsored programme. The principle aim of this project is to investigate the processing, performance and reliability of flexible substrates when subjected to new environmentally friendly, lead-free soldering technologies. This paper will discuss the impact of specific design variables on performance and reliability. In particular the paper will focus on copper track designs, substrate material, dielectric material and solder mask defined joints

Enlightenment and mission in early nineteenth century South Africa

In the early 19th century the London Missionary Society’s activities in South Africa were the subject of great scandal and a source of disrepute. The behaviour and attitudes of the first wave of LMS missionaries had challenged, and caused outrage, to both the political and moral norms of the colony. The radical attitudes and unconventional private lives of many of the early missionaries had also clearly shocked the Directors in Europe. In these controversies, and in the manner that the Society dealt with them, there can be read a contestation about not only the character, but also the purpose of mission activity. Was the Missionary task to work for political stability, to spread European values and help prepare a compliant and educated workforce? Or was it to save ‘lost souls’ and turn people away from idolatry and sin? Or, again, was it to fight for the oppressed, to liberate slaves and oppose tyranny? These debates were framed in complex and contradictory ways by a larger discussion that was informed by the new ideas and agendas that had emerged in the 18th century, commonly referred to as ‘The Enlightenment’. This paper traces the contours of an engagement between ‘Evangelical’ values and ‘Enlightenment’ principles through an exploration of the issues of the day such as: abolitionism, women’s rights, civilization and savagery. [From the Author]

A methodology for characterising new lead-free solder paste formulations used for flip-chip assembly applications

The paper reports on the investigation of the rheological behaviour new lead-free solder pastes formulations for use in flip-chip assembly applications. The study is made up of three parts; namely the evaluation of the effect of plate geometry, the effect of temperature and processing environment and the effect of torsional frequencies on the rheological measurements. Different plate geometries and rheological tests were used to evaluate new formulations in terms of wall slip characteristics, linear viscoelastic region and shear thinning behaviour. A technique which combines the use of the creep-recovery and dynamic frequency sweep tests was used to further characterise the paste structure, rheological behaviour and the processing performance of the new paste formulations. The technique demonstrated in this study has wide utility for R & D personnel involved in new paste formulation, for implementing quality control procedures used in paste manufacture and packaging and for qualifying new flip-chip assembly lines

Effect of long-term aging on the rheological characteristics and printing performance of lead-free solder pastes used for flip-chip assembly

Stencil printing of solder pastes is a critical stage in the SMT assembly process as a high proportion of the solder-related defects can be attributed to this stage. As the trend towards product miniaturization continues, there is a greater need for better understanding of the rheological behaviour and printing performance of new paste formulations. This fundamental understanding is crucial for achieving the repeatable solder paste deposits from board-to-board and pad-to-pad required for more reliable solder interconnections. The paper concerns a study on the effect of ageing on the rheological characteristics and printing performance of new lead-free solder pastes formulations used for flip-chip assembly applications. The objective is to correlate the rheological characteristics of aged paste samples to their printing performance. The methodology developed can be used for bench-marking new lead-free paste formulations in terms of shelf life, the potential deterioration in rheological characteristics and their printing performance.