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Title: Experimental and modelling studies of electronic packaging interconnections formed with lead-free materials
Author: Rizvi, Mohammed Jahir
ISNI:       0000 0001 3520 3586
Awarding Body: University of Greenwich
Current Institution: University of Greenwich
Date of Award: 2007
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Both the experimental and the modelling techniques have been investigated and used to investigate factors that influence the formation, quality and reliability of electronic packaging interconnections formed using solder alloys and anisotropic conductive films (ACFs). The wetting behaviours of new lead-free solders (i.e. Sn-2.8Ag-0.5Cu-1.0Bi and Sn-0.7Cu-0.3Ni) have been evaluated using the wetting balance test. This assessment ahs been performed for three soldering temperatures with three different types of fluxes. The results have been compared with the conventional Sn-Pb, Sn-Ag-Cu and Sn-Cu solders. It has been found that the wettability of the lead-free solders is not as good as that of the Sn-Pb solder. The additions of Bi into the Sn-Ag-Cu solder and Ni into the Sn-Cu solder improve the wettability that is strongly dependent on the type of the flux and the soldering temperature. In general, NC-flux is suitable for Cu-substrate whereas Ws-flux is suitable for Ni substrate, but for the Sn-2.8Ag-0.5-Cu solder on Ni substrate, good wettability has been observed with both the NC and the R-type fluxes. Computational modelling of this test has revealed that the increase in the depth and the radius of the solder bath has little effect on the wetting force, but the meniscus height decreases when the bath radius exceeds 14 mm. Dissolution of solid metals into liquid solders has been investigated through experiments and computer modelling. Microstructural studies have been carried out and the growth behaviours of the intermetallic compounds (IMCs) during wetting, solidification and isothermal ageing have been investigated. It has been found that the addition of Bi into the Sn-Ag-Cu solder reduces the consumptions of the substrates and suppresses the growth of IMCs during wetting and ageing. Similarly, the addition of Ni into the Sn-Cu solder reduces both the consumptions of the substrates and the growth of IMCs during wetting and short term ageing but enhances the growth of IMCs during long term ageing. Experimental and computer modelling techniques have been used to measure the temperature in the ACF during bonding. The temperature in an ACF joint becomes very close to the required maximum bonding temperature within the first 1 s of bonding time. The impact of this temperature on the cure process and on the ACF physical properties such as loss modulus, storage modulus, and glass transition temperature has been investigated. It has been found that the higher the bonding temperature the more the curing degree of ACF is. Rapid changes occur in the physical properties of ACF at temperatures above the glass transition point. When the ACF is cured for a long time at a high temperature, the physical properties may degrade. The adhesion strength of ACF joint increases as the curing degree increases. However, when the ACF joint undergoes a thermal aging treatment, the adhesion strength increases for the samples bonded at lower temperatures, but decreases for the samples bonded at higher temperatures. The rate of increase in the contact resistance is dramatically higher for the samples bonded at lower temperatures than for those bonded at higher temperatures. Computer modelling of the isothermal ageing of ACF joint confirms that the thermal load causes the expansion of the adhesive matrix and generates high stresses on the conductive particle. This may result in the permanent damage of the outermost conductive metallic layers as well as electrical failure. The effect of external bending loads on the electrical reliability of ACF-based interconnection has also been studied through computer modelling. The analysis reveals that ACF thickness increases at the corners of the chip-ACF joint more than that of the middle position. This causes a gap between the chip and the substrate results in the failure of the electrical interconnections.
Supervisor: Bailey, Christopher ; Lu, Hua Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QA Mathematics ; TK Electrical engineering. Electronics Nuclear engineering