Performance and Reliability of Polymer-based Sensor Packages at High Temperatures

Electronics is increasingly used in many applications. In addition to new consumer electronics, there is a trend to implement electronics and sensors in many industrial applications, which often requires improved reliability in very demanding conditions. This thesis concerns the effect of high temperature on electronics packages. High temperature tends to accelerate chemical reactions, aging of materials and thereby also failures. Properties of polymer materials, which are often used in electronic packaging, are heavily temperature-dependent. They are, however, very versatile and compatible materials which are readily available and easy to manufacture compared to expensive high-temperature speciality materials such as ceramics. Therefore, using polymers at high temperatures would be highly beneficial. However, it is crucial to understand the effects of high temperature on polymer materials before they are used.This work concentrates on the reliability of polymer-based sensor packages at high temperatures and the changes occurring in their materials at different temperatures. The structures were aged using several high temperature tests including thermal cycling, step stress and thermal storage tests at several temperatures around 200°C. The effects of high temperature on several commercial polymer-based printed circuit boards (PCB) and electrically conductive adhesives (ECA) were analysed from an electrical and mechanical perspective. Moreover, changes in material parameters due to aging were studied to achieve a more profound understanding of the effects of high temperatures.The temperature of 180°C seemed to be low enough not to cause reliability problems in the polymer-based packages studied. Additionally, a good performance in thermal cycling testing up to 180°C was achieved. At 200°C degradation was seen on the surfaces of the polymer materials and their mechanical properties gradually declined. Good electrical performance was nevertheless achieved with suitable material choices. A temperature of 240°C was shown to be too high for extended exposure of the materials studied. With careful material choices relatively good electrical performance was achieved, but FTIR showed dramatic and rapid degradation with most of the polymerbased materials at this temperature. The degradation was much more severe than at 200°C, and the mechanical properties also showed drastic impairment at 240°C.Material selection was shown to be absolutely critical for the reliability of the whole polymer-based package. With poor PCB material interconnections failed much earlier than with more stable PCB materials. Additionally, ECA selection was also important. This thesis showed that polymer-based electronic packages can withstand high temperatures, especially for limited exposure times. However, it is crucial that all materials present are able to withstand the selected temperatures.<br/

Novel fine pitch interconnection methods using metallised polymer spheres

There is an ongoing demand for electronics devices with more functionality while reducing size and cost, for example smart phones and tablet personal computers. This requirement has led to significantly higher integrated circuit input/output densities and therefore the need for off-chip interconnection pitch reduction. Flip-chip processes utilising anisotropic conductive adhesives anisotropic conductive films (ACAs/ACFs) have been successfully applied in liquid crystal display (LCD) interconnection for more than two decades. However the conflict between the need for a high particle density, to ensure sufficient the conductivity, without increasing the probability of short circuits has remained an issue since the initial utilization of ACAs/ACFs for interconnection. But this issue has become even more severe with the challenge of ultra-fine pitch interconnection. This thesis advances a potential solution to this challenge where the conductive particles typically used in ACAs are selectively deposited onto the connections ensuring conductivity without bridging. The research presented in this thesis work has been undertaken to advance the fundamental understanding of the mechanical characteristics of micro-sized metal coated polymer particles (MCPs) and their application in fine or ultra-fine pitch interconnections. This included use of a new technique based on an in-situ nanomechanical system within SEM which was utilised to study MCP fracture and failure when undergoing deformation. Different loading conditions were applied to both uncoated polymer particles and MCPs, and the in-situ system enables their observation throughout compression. The results showed that both the polymer particles and MCP display viscoelastic characteristics with clear strain-rate hardening behaviour, and that the rate of compression therefore influences the initiation of cracks and their propagation direction. Selective particle deposition using electrophoretic deposition (EPD) and magnetic deposition (MD) of Ni/Au-MCPs have been evaluated and a fine or ultra-fine pitch deposition has been demonstrated, followed by a subsequent assembly process. The MCPs were successfully positively charged using metal cations and this charging mechanism was analysed. A new theory has been proposed to explain the assembly mechanism of EPD of Ni/Au coated particles using this metal cation based charging method. The magnetic deposition experiments showed that sufficient magnetostatic interaction force between the magnetized particles and pads enables a highly selective dense deposition of particles. Successful bonding to form conductive interconnections with pre-deposited particles have been demonstrated using a thermocompression flip-chip bonder, which illustrates the applicable capability of EPD of MCPs for fine or ultra-fine pitch interconnection