Rolf Johannessen

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.

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.

Au-Sn fluxless SLID bonding: Effect of bonding temperature for stability at high temperature, above 400 °C

Fluxless SLID (Solid-Liquid InterDiffusion) bonding based on Au and Sn is presented, using two different processes, and bonding temperatures in the range 300–350 °C. The decomposition of the bond was tested by applying shear force while heating the samples. No bond delamination was observed for temperatures up to 350–400 °C, with 95 % of the tested samples surviving 400 °C without bond delamination. This is more than 100 °C higher than the melting temperature of the commonly used eutectic Au-Sn bond (80 wt% Au, melting at 278 °C). The Au-Sn system is particularly interesting since it is oxidation resistant, allowing fluxless bonding. With the SLID process, the metal system is applicable for true high-temperature applications.

SnAg Microbumps for MEMS-Based 3-D Stacks

Fine pitch 100-mum electroplated SnAg microbump interconnection technology is presented and discussed for use in microelectromechanical systems (MEMS) based 3-D stacks. Electrochemical deposition (ECD) of copper top side metallization (TSM) is compared to performance of nickel-copper TSM on substrate chips. Nickel-copper was selected for under bump metallization (UBM) of die chips. Lead free Sn3.5%Ag was deposited on the die chip UBM and chip-to-wafer bonded by a standard SnAg reflow process in inert atmosphere. An automotive application module was selected as target application for investigating reliability and failure mechanism of the interconnection technology. Bonded units have been investigated by mechanical and physical analysis, visual inspection and electrical resistance measurements after assembly and subsequent environmental stress test including thermal cycling, elevated temperature, humidity, and high current. The fine pitch lead free microbumps displayed promising capability for 3-D stacking of silicon devices. Excellent performance under thermal cycling with no global thermal mismatch was demonstrated. The microbump interconnection technology proved to be tolerant to high temperature and extensive current exposure. TSM consumption and intermetallic compound (IMC) formation were less evident for the nickel-copper TSMs compared to those entirely made of copper. Only minor Kirkendall porosity was observed at the solder alloy interfaces in the present study.

Temperature dependence of mechanical properties of isotropic conductive adhesive filled with metal coated polymer spheres

An isotropic conductive adhesive (ICA) filled with metal-coated polymer spheres (MPS) has been studied as a novel approach to increase the flexibility, and hence the reliability, compared to the conventional metal-filled ICA. In this study, the effect of the metal coating on the die shear strength was investigated by comparing ICA materials with coated and uncoated polymer spheres. The other important part of the study was to assess the temperature dependence of the die shear strength of an MPS-based ICA, and also compare this with the behavior of a conventional ICA filled with Ag particles. The results showed that the metal coating does not have a critical effect on the die shear strength of ICA filled with MPS. The die shear strengths obtained for the MPS-based ICA and the conventional Ag-filled ICA have the same temperature dependence in the range of 20 °C to 120 °C. Furthermore, none of the ICA systems has experienced a critical drop in die shear strength at and above the glass transition temperature.

Isotropic conductive adhesive filled with metal-coated polymer spheres — Effects of metal coating on rheological and mechanical properties

An isotropic conductive adhesive (ICA) filled with metal-coated polymer spheres has been studied as a novel approach to increase the mechanical flexibility, and hence the reliability, compared to conventional metal-filled ICAs. In the present paper, effects of the metal coating on the viscosity and the die shear strength were investigated, by comparing ICA materials with metal-coated and uncoated polymer spheres. The sphere diameter is 30 μm, and the metal coating used on the spheres is silver (Ag). The results showed a minor effect of the Ag coating on the die shear strength of the ICA. On the other hand, the Ag coating had an evident effect on the viscosity of the ICA, particularly at low shear rates.

Use of conductive adhesive for MEMS interconnection in military fuze applications

A novel conductive adhesive has been used to interconnect MEMS test structures with different pad sizes directly to a 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 have been subjected to rapid temperature cycling according to MIL-STD 883G method 1010.8 test condition B and performed well. The test structures with the largest pad sizes passed 100 temperature cycles and firing test where the test structures have been exposed to an acceleration of more than 60 000 g.

Anisotropic conductive film interconnects for fine-pitch MEMS

The flip-chip interconnection technology based on anisotropic conductive films (ACFs) has recently become an attractive solution for the assembly of micro-electromechanical systems (MEMS) and application specific integrated circuits (ASIC) in MEMS packages. In the present work, we have studied the fine pitch capability of ACF interconnects for MEMS applications such as fingerprint sensors and capacitive micromachined ultrasonic transducers, in which interconnects spread around MEMS and ASIC surface. The silicon test chips and substrates with different interconnect pitch were assembled using a single layer ACF. The electrical performance of ACF interconnects with varying pitch from 110 to 200 μm was compared. Furthermore, the distribution of conductive particles and the electrical resistance of ACF interconnects at both peripheral and central parts of the chips were evaluated. Effect of thermal shock cycling test (−40 to +125 °C) on samples was investigated. The results showed insignificant difference in the electrical performance between ACF interconnects with pitch varying from 110 to 200 μm. The particle distribution and the electrical resistance of ACF interconnects at different chip regions were similar. No significant effect of the thermal shock cycling test was observed. No failures (open/short circuit) occurred, both before and after the thermal shock cycling test.