Thermal and electrical modelling of polymer cored BGA interconnects

Polymer cored BGA/CSP balls have been proposed as a more reliable alternative to solid solder balls for demanding application environments. Their potential advantages are dependant on their increased compliance compared with a solid solder ball, thereby reducing the level of stress imposed on the solder joints during exposure to cyclic thermal loads and impacts. The latter is of particular importance for hand held products assembled using lead free solders, which are much more brittle than traditional tin-lead alloys, but this may also be important for harsh environment applications where tinlead solders are still being used, such as in aerospace and defence electronics applications. The increased compliance of a polymer cored ball may reduce the requirement for underfilling of components in hand held products, and allow adoption of BGA/CSP for safety critical applications in harsh environments. Such polymer cored interconnects are however likely to provide a reduced thermal and electrical conductivity and it is important to ensure any such effects do not impact upon the thermal and electrical performance of the product. This paper utilises analytical and computational modelling techniques to achieve an understanding of the effect of conductor particle geometry and properties on thermal and electrical performance. Such models offer a route to appropriate materials selection for the polymer spheres and their conductive coatings, and for establishing optimum design parameters such as ball diameter, conductive coating thickness, solder pad diameter, and solder volumes. The results confirm that the introduction of polymer cored BGA balls will result in some increases in thermal and electrical resistance, but that these changes will have minor impacts on the overall performance of products

Thermal and electrical modelling of polymer cored BGA interconnects

Polymer cored BGA/CSP balls have been proposed as a more reliable alternative to solid solder balls for demanding application environments. Their potential advantages are dependant on their increased compliance compared with a solid solder ball, thereby reducing the level of stress imposed on the solder joints during exposure to cyclic thermal loads and impacts. The latter is of particular importance for hand held products assembled using lead free solders, which are much more brittle than traditional tin-lead alloys, but this may also be important for harsh environment applications where tinlead solders are still being used, such as in aerospace and defence electronics applications. The increased compliance of a polymer cored ball may reduce the requirement for underfilling of components in hand held products, and allow adoption of BGA/CSP for safety critical applications in harsh environments. Such polymer cored interconnects are however likely to provide a reduced thermal and electrical conductivity and it is important to ensure any such effects do not impact upon the thermal and electrical performance of the product. This paper utilises analytical and computational modelling techniques to achieve an understanding of the effect of conductor particle geometry and properties on thermal and electrical performance. Such models offer a route to appropriate materials selection for the polymer spheres and their conductive coatings, and for establishing optimum design parameters such as ball diameter, conductive coating thickness, solder pad diameter, and solder volumes. The results confirm that the introduction of polymer cored BGA balls will result in some increases in thermal and electrical resistance, but that these changes will have minor impacts on the overall performance of products

Novel processes to enable deposition of metal coated polymer micro-spheres for flip-chip interconnections

Electronics packaging with Anisotropic Conductive Adhesives (ACAs) has been successfully implemented in flat screen assembly and smart cards for more than two decades, but novel processes are required to enable any significant further increases in interconnection density. This paper will report work to further develop two promising methods which can potentially extend the application of the types of conductor particles used in ACAs to enable ultra-fine pitch interconnections by selectively depositing these metal-coated polymer particles onto targeted bond pads. These two methods are electrophoretic deposition (EPD) and magnetic deposition (MD). To allow EPD the particles were positively charged by being immersed in a HCl solution, and then were selectively deposited onto the bumps on a silicon test chip. However the HCl immersion results in etching of the nickel layer, which is believed to significantly impact the conductivity and reliability of the interconnections formed, even though a connection forms between the particles and pads which is quite strong. In the MD process the Ti/Ni/Au bond pads on the test chips were permanently magnetized so that the particles were attracted to the pads, and the results proved that a sufficient density of the Ni/Au coated polymer particles became adhered to the pads providing a simple, low cost, and ultra-fine pitch method

Electrical properties of an isotropic conductive adhesive filled with silver coated polymer spheres

In the present study the electrical performance of newly developed epoxy resin based Isotropic Conductive Adhesives (ICAs) filled with silver coated mono sized polymer spheres have been investigated and compared with conventional solid silver particle/flake filled ICAs. The effects of particle size on the volume resistivity and percolation threshold of the new ICA have been studied. Two different diameters, i.e. 30μm and 4.8 μm, of silver coated mono sized spherical polymer particles have been used in this study. The results show that, for the same volume fraction, the volume resistivity of the adhesive with 4.8μm particles is lower than that with 30μm particles. The adhesive formulated with 4.8μm particles also exhibits a lower percolation threshold than that with 30μm particles. The resistivity of the adhesive containing 4.8μm particles was found to be of same order as that of currently commercially available ICAs, but with a significantly reduced silver content