Jorma K. Kivilahti

Morphology, kinetics, and thermodynamics of solid-state aging of eutectic SnPb and Pb-free solders (Sn–3.5Ag, Sn–3.8Ag–0.7Cu and Sn–0.7Cu) on Cu

Intermetallic compound (IMC) growth during solid-state aging at 125, 150, and 170 °C up to 1500 h for four solder alloys (eutectic SnPb, Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and Sn-0.7Cu) on Cu under bump metallization was investigated. The samples were reflowed before aging. During the reflow, the solders were in the molten state and the formation of the IMC Cu 6 Sn 5 in the cases of eutectic SnPb and Sn-3.5Ag had a round scallop-type morphology, but in Sn-0.7Cu and Sn-3.8Ag-0.7Cu the scallops of Cu 6 Sn 5 were faceted. In solid-state aging, all these scallops changed to a layered-type morphology. In addition to the layered Cu 6 Sn 5 , the IMC Cu 3 Sn also grew as a layer and was as thick as the Cu 6 Sn 5 . The activation energy of intermetallic growth in solid-state aging is 0.94 eV for eutectic SnPb and about 1.05 eV for the Pb-free solders. The rate of intermetallic growth in solid-state aging is about 4 orders of magnitude slower than that during reflow. Ternary phase diagrams of Sn-Pb-Cu and Sn-Ag-Cu are used to discuss the reactions. These diagrams predict the first phase of IMC formation in the wetting reaction and the other phases formed in solid-state aging. Yet, the morphological change and the large difference in growth rates between the wetting reaction and solid-state aging cannot be predicted.

Interfacial reactions between lead-free SnAgCu solder and Ni(P) surface finish on printed circuit boards

Transmission electron microscopy and scanning electron microscopy were employed to analyze the interfacial microstructure between Sn-Ag-Cu solder alloys and Ni(P)/Au metallizations. The intermetallic compound Cu/sub 6/Sn/sub 5/, containing a small amount of dissolved Ni, was found to form preferentially on the Ni coating. This compound layer served as a barrier for direct reaction of Sn with the Ni(P) coating. On the Ni(P) side, a nickel phosphide was identified. Thermodynamic evaluation of the Cu-Ni-Sn system was carried out to rationalize the enrichment of Cu at the solder/finish interface. Effects of the interfacial reactions on joint reliability are discussed.

Reactive sputter deposition and properties of Ta x N thin films

The aim of this work was to evaluate tantalum nitride thin films fabricated using reactive sputtering with adjusted deposition parameters. Thin TaxN films were deposited reactively on Si wafers using reactive RF magnetron sputtering at various N2/Ar gas ratios. The films were investigated by four-point probe sheet resistance measurement, profilometry, X-ray diffraction, scanning electron microscope, 2 MeV 4He+ backscattering spectroscopy, and atomic force microscopy. As the amount of nitrogen was increased, the phases in the as-deposited films were identified as β-Ta, Ta2N (5% N2-flow), hexagonal TaN (10% N2-flow) and f.c.c.-TaN (15% N2-flow) with resistivities of 166 µΩ cm, 234 µΩ cm, 505 µΩ cm and 531 µΩ cm, respectively. Only the phase obtained at 5% N2-flow showed a reasonable uniformity over the wafer suggesting suitability as thin film resistors. The value of temperature coefficient of resistance (TCR) determined for the Ta2N thin film resistor was - 103 ppm/°C.

Wetting reaction versus solid state aging of eutectic SnPb on Cu

The reaction kinetics of eutectic SnPb solder on Cu were studied and compared in the liquid state at 200 to 240 °C and in the solid state aged at 125–170 °C. The ternary phase diagrams of SnPbCu, the morphology of intermetallic compound (IMC), and the kinetics of growth of the intermetallics were used in the comparison. The temperature difference between these two reactions is only 30 °C, but the kinetics of reaction, as well as the morphology of IMC formation, are very different. The kinetics in the wetting reaction is four orders of magnitude faster than that in solid state aging. The Cu6Sn5 intermetallic morphology in solid state aging is a layer type, but it has a scallop-type morphology in the wetting reaction. The morphology strongly affects the kinetics. While the kinetic difference can be attributed to the difference in atomic diffusivity between the liquid state and the solid state, it is the morphology that determines the kinetic path in these reactions. We conclude that a fast rate of reaction,...

Reliability of Lead-Free Interconnections under Consecutive Thermal and Mechanical Loadings

To simulate more realistically the effects of strains and stresses on the reliability of portable electronic products, lead-free test assemblies were thermally cycled (−45°C/+125°C, 15-min. dwell time, 750 cycles) or isothermally annealed (125°C, 500 h) before the standard drop test. The average number of drops to failure increased when the thermal cycling was performed before the drop test (1,500 G deceleration, 0.5 ms half-sine pulse). However, the difference was not statistically significant due to the large dispersion in the number of drops to failure of the assemblies drop tested after the thermal cycling. On the other hand, the average number of drops to failure decreased significantly when the isothermal annealing was carried out before the drop test. The failure analysis revealed four different failure modes: (1) cracking of the reaction layers on either side of the interconnections, (2) cracking of the bulk solder, (3) mixed mode of component-side intermetallic and bulk solder cracking, and (4) voidassisted cracking of the component-side Cu3Sn layer. The assemblies that were not thermally cycled or annealed exhibited only type (1) failure mode. The interconnections that were thermally cycled before the drop test failed by mode (2) or mode (3). The drop test reliability of the thermally cycled interconnections was found to depend on the extent of recrystallization generated during the thermal cycling. This also explains the observed wide dispersion in the number of drops to failure. On the other hand, the test boards that were isothermally annealed before the drop testing failed by mode (4).

Impact of printed wiring board coatings on the reliability of lead-free chip-scale package interconnections

When lead-free solder alloys mix with lead-free component and board metallizations during reflow soldering, the solder interconnections become multicomponent alloy systems whose microstructures cannot be predicted on the basis of the SnPb metallurgy. To better understand the influences of these microstructure so n the reliability of lead-free electronics assemblies, SnAgCu-bumped components were reflow-soldered with near-eutectic SnAgCu solder pastes on Ni(P)Au- and organic solderability preservative (OSP)-coated printed wiring boards and tested under cyclic thermal shock loading conditions. The reliability performance under thermomechanical loading was found to be controlled by the kinetics of recrystallization. Because ductile fracturing of the as-soldered tin-rich colonies would require a great amount of plastic work, the formation of continuous network of grain boundaries by recrystallization is needed for cracks to nucleate and propagate intergranularly through the solder interconnections. Detailed microstructural observations revealed that cracks nucleate and grow along the grain boundaries especially between the recrystallized part and the non-recrystallized part of the interconnections. The thermal cycling test data were analyzed statistically by combining the Weibull statistics and the analysis of variance. The interconnections on Ni(P)Au were found out to be more reliable than those on CuOSP. This is due to the extensive dissolution of Cu conductor, in the case of the CuOSP assemblies, into molten solder that makes the microstructure to differ noticeably from that of the Ni(P)Au interconnections. Because of large primary Cu6Sn5 particles, the Cu-enriched interconnections enhance the onset of recrystallization, and cracking of the interconnections is therefore faster. The solder paste composition had no statistically significant effect on the reliability performance.

Failure mechanism of Ta diffusion barrier between Cu and Si

The reaction mechanisms in the Si/Ta/Cu metallization system and their relation to the microstructure of thin films are discussed on the basis of experimental results and the assessment of the ternary Si–Ta–Cu phase diagram at 700 °C. With the help of sheet resistance measurements, Rutherford backscattering spectroscopy, x-ray diffraction, a scanning electron microscope, and a transmission electron microscope, the Ta barrier layer was observed to fail at temperatures above 650 °C due to the formation of TaSi2, the diffusion of Cu through the silicide layer, and the resulting formation of Cu3Si precipitates. However, in order for the TaSi2 phase to form first, the Ta diffusion barrier layer must be thick enough (e.g., 50–100 nm) to prevent Cu diffusion into the Si substrate up to the temperature of TaSi2 formation (∼650 °C). Independent of the Ta layer thickness, Cu3Si was present as large nodules, whereas the TaSi2 existed as a uniform layer. The resulting reaction structure was found to be in local equil...

Fabrication and characterization of polymer optical waveguides with integrated micromirrors for three-dimensional board-level optical interconnects

This paper describes the fabrication and characterization of optical/electrical printed circuit boards (O/E-PCB) with embedded multimodal step index (MM-SI) waveguides and integrated out-of-plane micromirrors (IMMs) for three-dimensional (3-D) optical interconnects. Optical circuitry is built up on PCBs using UV lithography; 45/spl deg/ input/output (I/O) couplers are fabricated by inclined exposure. Commercial polymers are used as optical core and cladding materials. Critical mirror properties of angle, surface quality, reflectivity, and coupling efficiency are characterized experimentally and theoretically. Optical and scanning electron microscopy, white light interferometry, and fiber scanning method are used in the investigations. Sloping profiles measured as a function of the incident light showed the attainment of mirror angles of /spl alpha/=36/spl deg/-45/spl deg/ with /spl plusmn/2/spl deg/ consistency. Near-field optical imaging with a white light source showed that out-of-plane beam turning was achieved. Topography investigations revealed a rectilinear negative tapering shape regardless of the incoming beam angle or type of substrate. However, higher substrate reflectancy was observed to lower the mirror angle. The average propagation loss measured for 10-cm-long waveguides at /spl lambda/=850 nm by the cut-back method was 0.60 dB/cm; the excess loss calculated for the mirror coupling was 1.8-2.3 dB. The results showed that the IMMs can be incorporated in O/E-PCBs to couple light in and out of planar waveguides. Furthermore, the presented results indicate that optical waveguides with integrated micromirrors for optical 3-D wiring can be produced compatible with volume manufacturing techniques.

Effect of Ag, Fe, Au and Ni on the growth kinetics of Sn-Cu intermetallic compound layers

The effect of Ag, Fe, Au and Ni on the interfacial reactions between Sn-based solder and Cu substrate has been investigated in this paper. Based on the solubility of the alloying elements in the Sn–Cu intermetallic compound (IMC) layers these elements can be divided into two categories: (i) alloying elements that do not dissolve significantly in either Cu6Sn5 or Cu3Sn and (ii) elements that exhibit significant solubility in Cu6Sn5 and also to Cu3Sn. It is shown that the latter group of elements have stronger effect on the growth behaviour of IMC’s in the Sn–Cu system than those belonging to the first group. Of the investigated elements Ni had the most prominent effect on the growth kinetics. It reduced greatly the thickness of Cu3Sn and consequently also the total IMC layer thickness. Au had similar but markedly weaker effect. On the contrary, Fe and Ag only slightly decreased the total IMC layer thickness, and more importantly did not change the thickness ratio of Cu6Sn5 to Cu3Sn in comparison to the pure Sn–Cu system.

Reliability of CSP Interconnections Under Mechanical Shock Loading Conditions

Failure modes and mechanisms under mechanical shock loading were studied by employing the statistical and fractographic research methods, and the finite element (FE) analysis. The SnAgCu-bumped components were reflow-soldered with the SnAgCu solder paste on Ni(P)|Au-coated and organic solderability preservative-coated multilayer printed wiring boards with and without micro-via structure in the soldering pads. The component boards were designed, fabricated, assembled, and drop tested according to the JESD22-B111 standard for portable electronic products. The test data were analyzed by utilizing the Weibull statistics, and the characteristic lifetimes (eta) and shape parameters (beta) were calculated. Statistically significant differences in the reliability were found between the different coating materials and pad structures. The results on the failed assemblies showed good correlation between the failure modes and the FE calculations. Under high deformation rates the solder material undergoes strong strain-rate hardening, which increases the stresses in the interconnections as compared to those in the thermal cycling tests. Therefore, the failure mechanisms under high deformation rates differed essentially from those observed in thermal cycling tests