Helge Kristiansen

Overview of conductive adhesive interconnection technologies for LCD's

In the field of flat panel displays, packaging technology has a significant influence on display performance. The electrical interconnect between the LCD and the LCD driver circuit is an area that needs improvement to achieve finer pitch, easier assembly and greater connection reliability. For LCD driver packaging, the ideal assembly process would possess the following characteristics: low processing cost; reliability suitable to the final application; high-density, fine pitch capability; low product profile; acceptable joint resistance; ease of inspection; and reworkability. The overall trend in LCD driver IC packaging has been to move the driver IC closer to the LCD itself. At present, TAB is the predominant packaging approach for large area LCDs. In most cases, the TAB is directly connected to the ITO traces on the glass using anisotropic conductive adhesive (ACA). In chip-on-glass (COG) technology, the driver LSI chips have moved all the way on to the LCD glass itself. COG is typically done by flip chip, also often with the use of conductive adhesives. ITO traces fan out from the IC to the display area, as well as to the point where a polyimide flexible circuit is connected to the glass substrate to supply power and picture information. COG mounting is currently being used in a number of products, in particular when the pixel density is high.

Overview of conductive adhesive joining technology in electronics packaging applications

This paper presents an overview of the current status of use of conductive adhesives in various electronics packaging applications. Strong emphasis is placed on recent developments in surface mount and flip-chip technology, as these methods in combination with conductive adhesives represent the latest developments in the area of electronics packaging. It is concluded that little practical use of conductive adhesives in surface mount has been found. In flip-chip applications, both isotropically and anisotropically conductive adhesives (ICAs and ACAs) have been used in real applications. Greater use is expected in the near future in this fast developing area.

Use of conductive adhesive for MEMS interconnection in ammunition fuze applications

A novel conductive adhesive is used to interconnect MEMS test structures with different pad sizes directly to a printed circuit board (PCB) in a medium caliber ammunition fuze. The fuze environment is very demanding, with a setback acceleration exceeding 60,000 g and a centripetal acceleration increasing radially with 9000 g/mm. The adhesive shows excellent mechanical and thermal properties. The mounted MEMS test structures perform well when subjected to rapid temperature cycling according to military-standard 883G method 1010.8 test condition B. The test structures pass 100 temperature cycles, followed by a firing test where the test structures are exposed to an acceleration of more than 60,000 g.

Mechanical properties of nanostructured polymer particles for anisotropic conductive adhesives

Abstract Metal-coated polymer particles are used as a critical component in anisotropic conductive adhesives. The mechanical properties of polymer particles are of crucial importance to both application and design. In this study, a nanoindentation-based flat punch test approach has been developed to determine the mechanical properties of two polymer particles. Particle A failed at an average deformation of 67.2 %, while no failure was observed for particle C when a maximum 10 mN normal load was applied. The contact stress – strain behavior and contact modulus were obtained from the flat punch test.

Characterization of mechanical properties of metal-coated polymer spheres for anisotropic conductive adhesive

Metal coated small (micron sized) polymer particles are used in developing anisotropic conductive adhesives (ACA). The mechanical properties of polymer particles are of crucial importance both to assembly process and the reliability of ACA. Tn this paper we present a method to determine the mechanical properties of polymer sphere particles by using inverse indentation test - soft elastic sphere against rigid flat. Finite element analyses have been carried out to study the large deformation contact between the sphere particle and a rigid flat. The classical Hertz solution works only for small sphere deformation. A modification has been made to the Hertz contact force-displacement solution and an approximate equation is presented. A capacitance based experimental setup for particle indentation has been built. The proposed method has been applied to determine the elastic properties of typical polymer particles used for conductive adhesives. For the metal plated polymer particles tested, it has been found that a linear elastic model seems to hold for a large range of deformation.

Spherical polymer particles in isotropic conductive adhesives a study on rheology and mechanical aspects

Isotropic conductive adhesive (ICA) filled with metal coated polymer spheres has been studied as a novel approach to increase the flexibility, and hence the reliability of the adhesive compared to traditional metal filled ICAs. In this paper, we have investigated the rheological properties of the novel ICA to evaluate its applicability in practical use. The current work also involves the investigation of the mechanical properties including shear strength of the novel ICA. Spherical polymer particles (SPP) of sizes Ø6 µm and Ø30 µm were investigated in the present study. The results show minor differences in the rheological properties and the adhesion strength for adhesives filled with particles in different sizes. Filling SPP into the adhesive matrix increases the viscosity of the system monotonically and continuously, in excellent accordance with model systems previously reported in the literature. Furthermore, the novel ICA exhibits high mechanical shear strength, being comparable to the traditional solder joint technology and twice higher than the traditional metal filled ICA.

Mechanical Performance of Polymer Cored BGA Interconnects

This paper presents the results from preliminary models comparing the mechanical performance of polymer cored BGA type interconnects with conventional solid solder BGA balls. The surface evolver was first used to predict the solder fillet shapes for use in the models. The mechanical behaviour of some candidate polymer spheres were also measured and the results of these measurements used to estimate the properties of the polymer. These results were then used together in the construction of preliminary elasto-plastic finite element models of the interconnects when subjected to cyclic shear loads. The results indicate that the polymer cored balls may potentially provide substantial improvements in solder joint fatigue life, but also indicate the significant effects that the design variables will have on the achievable benefits.

Size-dependent mechanical behavior of nanoscale polymer particles through coarse-grained molecular dynamics simulation

Anisotropic conductive adhesives (ACAs) are promising materials used for producing ultra-thin liquid-crystal displays. Because the mechanical response of polymer particles can have a significant impact in the performance of ACAs, understanding of this apparent size effect is of fundamental importance in the electronics industry. The objective of this research is to use a coarse-grained molecular dynamics model to verify and gain physical insight into the observed size dependence effect in polymer particles. In agreement with experimental studies, the results of this study clearly indicate that there is a strong size effect in spherical polymer particles with diameters approaching the nanometer length scale. The results of the simulations also clearly indicate that the source for the increases in modulus is the increase in relative surface energy for decreasing particle sizes. Finally, the actual contact conditions at the surface of the polymer nanoparticles are shown to be similar to those predicted using Hertz and perfectly plastic contact theory. As ACA thicknesses are reduced in response to reductions in polymer particle size, it is expected that the overall compressive stiffness of the ACA will increase, thus influencing the manufacturing process.

Die Shear Testing of a Novel Isotropic Conductive Adhesive—Epoxy Filled With Metal-Coated Polymer Spheres

Isotropic conductive adhesives (ICAs) filled with metal-coated polymer spheres (MPS) are introduced to improve the mechanical reliability compared with conventional ICAs filled with silver (Ag) flakes. This paper deals with the die shear performance of an MPS-based ICA; an epoxy filled with 45 vol% of Ø30 μm Ag-coated monodisperse polymer spheres. The curing kinetics of the ICA is also studied. The MPS-based ICA is compared with two ICAs filled with Ag flakes: an in-house prepared ICA (with the same epoxy matrix as the MPS-based ICA) and a commercially available ICA. Both ICAs with Ag flakes have a lower particle volume fraction than the MPS-based ICA. Loading 45 vol% of MPS into the epoxy matrix has no significant effect on the curing kinetics and the glass transition temperature (Tg) of the matrix. The MPS-based ICA has 140% higher die shear strength than the in-house prepared ICA with Ag flakes, and 23% higher die shear strength than the commercial ICA with Ag flakes. The MPS-based ICA also has higher shear strain at failure, and less scatter in shear strength between repeated tests. Hence, the MPS-based ICA has a good potential for demanding applications; it has better die shear performance and higher Tg than the commercial ICA with Ag flakes.

Current density simulations for polymer cored CSP interconnects

Polymer cored spheres have been proposed as a more flexible and therefore more reliable alternative to solid solder balls for use in ball grid array (BGA) and chip scale package (CSP) interconnects. However the use of such polymer cored interconnects will result in a significant increase in the average current density within the individual interconnects, due to the much smaller cross sectional area of metal through which conduction may take place. High current densities in metals are well known to lead to electro-migration and this may lead to reductions in their mechanical performance. The soft solders used in electronics manufacturing operate at high homologous temperatures and are therefore particularly vulnerable to electro-migration damage, particularly as miniaturization also leads to increasing current densities.