Astrid-Sofie B. Vardøy

High-temperature shear strength of solid-liquid interdiffusion (SLID) bonding: Cu-Sn, Au-Sn and Au-In

Solid-Liquid Interdiffusion (SLID) bonding is a promising bonding technique, particularly for high-temperature applications. Based on intermetallics as the bonding medium, the bonds are stable at temperatures far above the processing temperature which is in the range of normal solder temperatures. This work confirms experimentally this high-temperature stability through shear strength testing as function of temperature (room temperature to 300 °C) for three different SLID systems: Cu-Sn, Au-Sn and Au-In. All three systems remain solid within the tested temperature range, as expected, but they show remarkably different temperature dependence of mechanical strength: Au-Sn SLID bonds show strongly decreasing shear strength with temperature (but at 300 °C it is still well above the MIL-STD requirement); Cu-Sn SLID bonds show only small changes; whereas Au-In SLID bonds show increased shear strength at 300 °C, accompanied with a change in fracture mode from brittle to ductile. All three behaviours can be explained from the phase diagrams with the actual phases in use.

Characterization of hermetic wafer-level Cu-Sn SLID bonding

A flux-less copper-tin (Cu-Sn) solid-liquid inter-diffusion (SLID) bonding process, providing a cost-effective hermetic vacuum sealing at wafer-level, has been investigated. Observations have been made indicating that the storage time of Cu-Sn plated wafers before bonding is critical with regard to voiding. Growth of the intermediately formed intermetallic compound (IMC), Cu6Sn5, was investigated as a possible cause. Room temperature aging of Cu-Sn plated wafers prior to bonding was performed as well as annealing of un-bonded Cu-Sn plated wafers. The presence of large Cu6Sn5 and Cu3Sn crystallites which nearly depleted the Sn was observed by optical microscopy after annealing. If large Cu6Sn5 grains from opposite contact planes meet at the bond interface, voids are predicted to be formed during the subsequent stages of liquid inter-diffusion and solidification. Implications on the Cu-Sn bonding strategy based on the results are presented.

Compliant interconnects for reduced cost of a ceramic ball grid array carrier

The transition to lead free solders has accentuated the inherent reliability problem in electronic packaging caused by thermo mechanical mismatch between different parts of an assembled system. To mitigate this problem, polymer core solder balls (PCSBs) have been proposed as a mechanically more flexible and therefore more reliable alternative to solid solder balls normally used for such applications. In this paper we report results from testing of PCSBs used for connecting and attaching a low temperature co-fired ceramic (LTCC) Ball Grid Array (BGA) carrier to an FR-4 board. The results appear to be strongly dependent on the assembly process. However, the results also indicate that with a properly executed assembly process, these balls represent an alternative to traditional solder balls with remarkable resistance to thermal cycling.

All-Copper Flip Chip Interconnects by Pressureless and Low Temperature Nanoparticle Sintering

Flip chip interconnects purely made out of Cu, so-called all-Cu interconnects, have the potential to overcome the present current capacity limit of state-of-the-art solder based interconnects, while meeting the demand for ever decreasing interconnect pitches. Parasitic effects in solder based interconnects, caused by interdiffusion of various metals, are mitigated in all-Cu interconnects. In this work, all-Cu interconnects were formed by the use of low temperature and pressureless sintering of Cu nanoparticles. Thereby, a Cu paste material was applied between the Cu pillars of a silicon chip and the Cu pads on a silicon substrate by a dip transfer method. The electrical and mechanical properties of sintered Cu were characterized on films of the same Cu pastes. The porous films resulted in 4.4 times higher electrical resistivity and one order of magnitude reduced mechanical stiffness and tensile strength compared to bulk Cu. All-Cu interconnects with a diameter of 30 μm and a pitch of 100 μm were formed with an optimized Cu particle distribution and sintering procedure. Resistances down to 1.7 ± 0.5 mO were measured for these all-Cu interconnects which is comparable to solder based benchmark interconnects. However, the porosity of the sintered Cu interconnect results in lower shear strength compared to the solder benchmark.

Development of a wearable multisensor device enabling continuous monitoring of vital signs and activity

This paper reports on the development and testing of a wearable device intended as a component in an ambulatory system for health monitoring of physical rehabilitation patients. The device measures heart rate, skin temperature, activity level and posture on the users chest. The wearable device has been run through a set of verification tests and the accuracy has been validated in controlled environment on 12 healthy volunteers. A long term user pilot with 5 congestive heart failure patients and their nurses was performed to test the whole system. The conclusion from the performed tests is that the developed wearable multisensor monitoring device is reliable, accurate, easy to use and fit for the purpose.

Bending machine for testing reliability of flexible electronics

A novel bending machine has been designed and tested. It enables flexible electronics to be subjected to repeated bending with constant radius and tension. In-situ electrical characterization can give accurate analysis of lifetime distributions if sufficiently many samples are ran to failure, allowing reliability prediction models to be developed. Four sets of test samples with different combinations of substrate, routing, interconnect technology and components were examined. A poor level of reliability was observed when using anisotropic conductive paste to form interconnects, whereas a significantly higher level of reliability was observed when using a bismuth-tin solder paste. The assembly of larger components resulted in shortened time to failure, whereas increasing the bending radius prolonged the observed lifetimes.

Thermomechanical reliability of gold stud bump bonding for large volume MEMS devices

Reliability of gold stud bump bonding must be questioned once process parameters are tweaked and dimensions shrunk to minimize costs for high volume MEMS assembly. 3D mock-up stacks representing a 0.5 x 1.0 mm MEMS die and its read-out integrated circuit were built using thermosonic and thermocompression chip-to-wafer bonding (TSB and TCB) with parameters targeting an optimized throughput. Electrical test circuits, daisy chains and Kelvin structures, were included in the design. Mechanical and electrical characterization was performed on samples directly after bonding and after environmental stressing in the form of thermal cycling and high temperature storage. Bond strength in the range of 40-60 MPa was measured after bonding. No degradation was observed after thermal cycling and an increased strength was measured after high temperature storage. Tilt in the range of 2.2 - 6.6 μm measured using white light interferometry was concluded to be a root cause of a limited yield for TSB. Fractography after shear testing, visual inspection of cross sections and electrical testing supported the hypothesis. On the other hand, tilt was measured to be in the range of 0.3 - 1.3 μm for TCB and this was concluded to be acceptable based on the same inspections and also based on stability of electrical properties measured in situ during environmental stressing. Tilt can be reduced by engineering of bond heads.

Metal coated polymer spheres for compliant fine pitch ball grid arrays

The further development of heterogeneous 3D integration involving electrical connections between layers of various materials demand compliant fine pitch interconnects to allow for the varying expansion as a function of temperature of the different materials involved. However, most available fine pitch interconnects, like micro bumps and copper pillars, are not particularly compliant whereas available compliant interconnects, like plastic core solder balls, are not fine pitch. In this work we present a novel process for achieving a fine pitch ball grid array comprised of compliant interconnects using singular 30 μm metal coated polymer spheres (MPS) in conjunction with nano-particle silver conductive ink. Results show that when an MPS attached to a chip is flip chip assembled onto a substrate with a 1.3 μm thick layer of wet conductive ink, the ink is drawn up along the MPS surface to such an extent that it a forms a bonding neck between the MPS and the chip surface. It also created a coating up to 1.5 μm on the MPS. Shear testing of the assembly gave values of up to 0.67 grams per MPS. This strength can be increased further by optimizing the process and to utilize such an interconnect in combination with underfill can result in sufficient reliability for several applications. The volume of ink required to form such bonds is in the same range as industrial ink jet nozzles can deliver. MPS was applied onto a substrate which had been jetted with a regular pattern of ink droplets. It was found that the MPS were observed to have had a certain extent of self-alignment along the jetted pattern.

Protective jacket enabling decision support for workers in cold climate

The cold and harsh climate in the High North represents a threat to safety and work performance. The aim of this study was to show that sensors integrated in clothing can provide information that can improve decision support for workers in cold climate without disturbing the user. Here, a wireless demonstrator consisting of a working jacket with integrated temperature, humidity and activity sensors has been developed. Preliminary results indicate that the demonstrator can provide easy accessible information about the thermal conditions at the site of the worker and local cooling effects of extremities. The demonstrator has the ability to distinguish between activity and rest, and enables implementation of more sophisticated sensor fusion algorithms to assess work load and pre-defined activities. This information can be used in an enhanced safety perspective as an improved tool to advice outdoor work control for workers in cold climate.

Interconnects and substrates for thermal considerations

Novel and emerging packaging technologies expand the designers toolbox. Metal coated polymer spheres (MPS) for ball grid array (BGA) assembly is a promising interconnect technology improving reliability, while high thermal conductivity substrates, e.g. AlSiC and AlN, is interesting for enhanced thermal performance. But new tools bring along new challenges in the design phase of innovative packaging solutions. Knowledge of how these tools influence the system characteristics is therefore key; e.g. electrical, mechanical and thermal performance. This study reports on the thermal performance of several novel and more traditional interconnect and substrate technologies. It comprises a relative thermal impact study of individual technologies. The system consists of a power dissipating silicon die assembled onto different substrates with varying interconnect technologies. The study was performed with finite element analysis (FEA) assisted with a compact model and is scheduled for comparison with identical fabricated systems. The results show that it is possible to utilize FEA efficiently on a system scale during the design process with sufficient accuracy. It also reveals that the combination of interconnect and substrate technology should be chosen with care, especially regarding the systems thermal performance, disclosing potential reliability issues and illustrating cost-benefit tradeoffs.