Publications

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We present ultra-thin single crystal mini-modules built with specific power of 450 W/kg capable of voltages of >1000 V/cm2. These modules are also ultra-flexible with tight bending radii down to 1 mm. The module is composed of hundreds of back contact microcells with thicknesses of approximately 20 μm and diameters between 500-720 μm. The cells are interconnected to a flexible circuit through solder contacts. We studied the characteristics of several mini-modules through optical inspection, evaluation of quantum efficiency, measurement of current-voltage curves, and temperature dependence. Major efficiency losses are caused by missing cells or non-interconnected cells. Secondarily, damage incurred during separation of 500 μm cells from the substrate caused material detachment. The detachment induced higher recombination and low performance. Modules made with the larger cells (720 μm) performed better due to having no missing cells, no material detachment and optimized AR coatings. The conversion efficiency of the best mini module was 13.75% with a total Voc = 7.9 V. © 2013 IEEE.

We present ultra-thin single crystal mini-modules built with specific power of 450 W/kg capable of voltages of >1000 V/cm2. These modules are also ultra-flexible with tight bending radii down to 1 mm. The module is composed of hundreds of back contact microcells with thicknesses of approximately 20 μm and diameters between 500-720 μm. The cells are interconnected to a flexible circuit through solder contacts. We studied the characteristics of several mini-modules through optical inspection, evaluation of quantum efficiency, measurement of current-voltage curves, and temperature dependence. Major efficiency losses are caused by missing cells or non-interconnected cells. Secondarily, damage incurred during separation of 500 μm cells from the substrate caused material detachment. The detachment induced higher recombination and low performance. Modules made with the larger cells (720 μm) performed better due to having no missing cells, no material detachment and optimized AR coatings. The conversion efficiency of the best mini module was 13.75% with a total Voc = 7.9 V. © 2013 IEEE.

Compression creep tests were performed on the ternary 91.84Sn-3.33Ag-4.83Bi (wt.%, abbreviated Sn-Ag-Bi) Pb-free alloy. The test temperatures were: -25 °C, 25 °C, 75 °C, 125 °C, and 160 °C (± 0.5 °C). Four loads were used at the two lowest temperatures and five at the higher temperatures. The specimens were tested in the as-fabricated condition or after having been subjected to one of two air aging conditions: 24 hours at either 125 °C or 150 °C. The strain-time curves exhibited frequent occurrences of negative creep and small-scale fluctuations, particularly at the slower strain rates, that were indicative of dynamic recrystallization (DRX) activity. The source of tertiary creep behavior at faster strain rates was likely to also be DRX rather than a damage accumulation mechanism. Overall, the strain-time curves did not display a consistent trend that could be directly attributed to the aging condition. The sinh law equation satisfactorily represented the minimum strain rate as a function of stress and temperature so as to investigate the deformation rate kinetics: dε/dtmin = Asinhn (ασ) exp (-ΔH/RT). The values of α, n, and ΔH were in the following ranges (±95% confidence interval): α, 0.010-0.015 (±0.005 1/MPa); n, 2.2-3.1 (±0.5); and ΔH, 54-66 (±8 kJ/mol). The rate kinetics analysis indicated that short-circuit diffusion was a contributing mechanism to dislocation motion during creep. The rate kinetics analysis also determined that a minimum creep rate trend could not be developed between the as-fabricated versus aged conditions. This study showed that the elevated temperature aging treatments introduced multiple changes to the Sn-Ag-Bi microstructure that did not result in a simple loss ("softening") of its mechanical strength. © 2012 by Sandia Corporation.

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The development of Pb-free solutions for the highreliability electronics community necessitates the consideration of hybrid microcircuit (HMC) products. This study used a test vehicle that included both plastic and ceramic packages as well as leaded and area-array solder joints on an alumina substrate. The conductor was a Ag-Pd thick film layer. The shear strength was measured for interconnections made with 63Sn-37Pb (wt.%, abbreviated Sn-Pb) and 95.5Sn-3.0Ag-0.5Cu (Sn-Ag-Cu) solders as a function of isothermal aging, thermal cycling, and thermal shock environments. The area-array packages indicated that solder joint fatigue was not altered significantly in a forward compatibility situation (i.e., Sn-Pb balls and a Sn-Ag-Cu assembly process). Local CTE mismatch fatigue strains are important for solder joints connecting ceramic area array packages to ceramic substrates. The gull-wing lead, SOT plastic package solder joints assembled with the Sn-Ag-Cu solder exhibit a greater strength loss under temperature cycling than did the corresponding Sn-Pb interconnections. Thermal shock is more detrimental to Sn-Pb HMC solder joints than are the equivalent number of thermal cycles. Copyright 2012 ASM International® All rights reserved.

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