Studies of Doping Distribution and Carbon Nanotube Reinforcement in Low Temperature Solder Joints for Advanced Electronic Packaging


Student thesis: Doctoral Thesis

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Award date6 Oct 2017


Because of concerns over the toxicity of lead and environmental protection policies, lead-free solder alloys are now extensively used in the electronic manufacturing organizations to replace the environmentally harmful tin-lead (Sn-Pb) solder alloys. Among these alternatives, the low temperature solder alloys, such as Sn58Bi, have received much attention. The most obvious advantage of these solder alloys is the significant reduction of reflow process cycle time and energy consumption. Considering the development of energy-conservation society around the world, low temperature solder alloys should have a larger potential market.

As the miniaturization of electronic products, the interconnecting solder joints play more critical roles as electronic and thermal connectors, signal transmitters, and mechanical supporters. Due to their week mechanical and electrical properties, the low temperature lead free solder joints are badly in need of improvement. Nanoparticle doping, as the most effective method, promises to have an important role in reinforcing properties of the lead free solder joints. A variety of metallic and non-metallic particles have been studied as the dopants in solder joints over the past decade years. According to recent research, carbon allotropes, such as graphite, carbon nanotubes (CNTs) and graphene, are some of the hottest objects of study as dopants in solder joints. These particles are well known because of their prominent electrical, thermal and mechanical properties. However, some natural differences in physical and chemical properties, in contrast to those of the solder alloys, have restricted the use of their remarkable properties in solder joints.

In this thesis, the properties of the Sn57.6Bi0.4Ag low-temperature solder alloy have been studied. This near-eutectic Sn57.6Bi0.4Ag solder alloy derives from the traditional Sn58Bi eutectic solder alloy that has been used in the electronic manufacture industries. In addition, CNTs and Ni-modified CNTs have been studied as reinforcing dopants to improve the performance of Sn57.6Bi0.4Ag solder joints for advanced electronic packaging.

First of all, the function of the element silver (Ag) in the Sn-Bi solder joints has been investigated. There are two methods for the introduction of Ag element into the Sn-Bi solder matrix. In the first approach (Approach I), Ag particles are blended with solder powders together and this process is then followed by pressure forming, sintering, cooling, crystallization and serial machining methods under an inert atmosphere to manufacture the solder paste. In the second approach (Approach II), the Ag particles are directly doped into the solder paste by sufficient mechanical-stirring. The mechanical and electrical properties and reliability performance of Sn57.6Bi0.4Ag (prepared by Approach I), Sn58Bi+0.4Ag (prepared by Approach II), and Sn58Bi solder composites have been compared. For the mechanical properties, before the thermal ageing testing, the doped Sn58Bi+0.4Ag solder joints shows better mechanical performance than Sn57.6Bi0.4Ag solder joints. However, Sn57.6Bi0.4Ag solder joints exhibited better resistance to thermal stress. The degradation of the reinforcing effect of Ag nanoparticle occurs seriously in the doped Sn58Bi+0.4Ag solder joints. For the electrical properties, Sn57.6Bi0.4Ag solder joints also perform better under high-density current stressing. The distribution of Ag-Sn inter-metallic compounds (IMCs) in the solder matrix plays an important role in influencing the properties of solder joints.

Secondly, the movement of CNTs during reflow solder process and the distribution of CNTs in solder joints have been investigated. The final distribution of CNTs in the solder matrix has a decisive influence on the performance of solder joints. The non-uniform distribution of CNTs can lead to fatal defects and have negative effects on the properties of solder joints. The inducing factor that causes the re-distribution of CNTs has been analyzed in detail. The effect of drag forces that caused by the outward flow of the solder flux and the effect of buoyancy forces that caused by the density gap are the main inducing factors for the movement of dopants. Furthermore, an innovative experiment has been initiated to study the distribution and movement of CNTs in the solder block. By using mathematical transformation, the experimental result can be used to predict actual conditions of the distribution of CNTs in the ball grid-array (BGA) solder joints.

Thirdly, the CNTs have been studied as the reinforcing dopants in the Sn58Bi solder joints. The reflow soldering induced re-distribution of CNTs was analyzed and its impacts on the mechanical properties of solder joints have been investigated. With 0.100 wt.% of CNTs, the solder matrix become saturated. Doping with more CNTs hardly increases the final weight percent of CNTs. Furthermore, the excess CNTs aggregate at the near surface region, making the surface of the solder composite darker and coarser. The distribution and movement of CNTs in solder joints are difficult to control. To find a better amount of doping, Sn58Bi solder joints with different content of CNTs have been prepared, in order to compare the ball-shear-force. According to the results, the highest shear-stress is 77.3 MPa for the solder joints with 0.050 wt.% of doping. Over 0.100 wt.% of doping, the shear-stress decreases sharply. In addition, via morphology observation, the aggregation of CNTs have been found at the bonding interface seriously in the solder joints with 0.200 wt.% of doping.

In the fourth part of this thesis, Ni modified CNTs have been introduced to improve the effect of CNTs in the Sn57.6Bi0.4Ag solder joints. A comparison has been carried out on the distribution performance and on mechanical and reliability reinforcements for these two dopants in solder joints. According to the findings, Ni-CNTs obtain more uniformly distribution in solder joints, due to the chemical force between Ni and Sn atoms. Before thermal shock testing, the highest ball-shear-force belongs to the solder joints with 0.07 wt.% of Ni-CNTs. The dopants aggregating-induced defects became more obvious as the number of cycles of thermal shock testing increases. Micro-cracks can be found in both kinds of solder joints. According to the thermal shock testing result, the suggested amount of doping should be revised to around 0.03 wt.%. On the other hand, Ni-coating can lead to the excessive CNTs in the solder matrix, which also have negative effects on the performance of solder joints.