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Modified soldering iron extracts a wrap-around solar cell that is attached with solder or adhesive to a substrate without destroying the cell removed or damaging adjacent cells. Heat, vacuum, and compressed air, operated from a special head attached to the soldering iron, loosen, extract, and protect the cell.
Ten different conductive inks used in the fabrication of thick-film circuits were evaluated for their physical and handling properties. Viscosity, solid contents, and spectrographic analysis of the unfired inks were determined. Inks were screened on ceramic substrates and fired for varying times at specified temperatures. Selected substrates were given additional firings to simulate the heat exposure received if thick-film resistors were to be added to the same substrate. Data are presented covering the (1) printing characteristics, (2) solderability using Sn-63 and also a 4 percent silver solder, (3) leach resistance, (4) solder adhesion, and (5) wire bonding properties. Results obtained using different firing schedules were compared. A comparison was made between the various inks showing general results obtained for each ink. The changes in firing time or the application of a simulated resistor firing had little effect on the properties of most inks.
Four types of solar cell contacts were given pull-strength tests at temperatures between -173 and +165 C. Contacts tested were: (1) solder-coated titanium-silver contacts on n-p cells, (2) palladium-containing titanium-silver contacts on n-p cells, (3) titanium-silver contacts on 0.2-mm-thick n-p cells, and (4) solder-coated electroless-nickel-plated contacts on p-n cells. Maximum pull strength was demonstrated at temperatures significantly below the air mass zero cell equilibrium temperature of +60 C. At the lowest temperatures, the chief failure mechanism was silicon fracture along crystallographic planes; at the highest temperatures, it was loss of solder strength. In the intermediate temperatures, many failure mechanisms operated. Pull-strength tests give a good indication of the suitability of solar cell contact systems for space use. Procedures used to maximize the validity of the results are described.
In the assembly of hybrid microcircuits, passive and active discrete chip components are attached by epoxy cements and by solders. This study was initiated to determine the integrity of such attachments. Alumina substrates were subjected to a group of thermal and mechanical test environments defined by test methods in MIL-STD-883. Lot samples of substrates with assembly variations were subjected to separately applied testing and to serial testing. These test methods were applied sequentially to the same lot of substrates. Epoxy cements were generally satisfactory with respect to separate tests, although sequential tests severely degraded epoxy bond strengths of all types. Bond strengths to unglazed substrates were slightly superior to those of the same components on glazed types. Solder joints were vulnerable to catastrophic failure in several environments, especially temperature cycling and thermal shock. Vibration and impact shock also produced solder joint failures.
Elements of an actively cooled structural panel for a hypersonic aircraft have been investigated for fatigue characteristics. The study involved a bonded honeycomb sandwich panel with d-shaped coolant tubes. The curved portion of these tubes was embedded in the honeycomb, and the flat portion was bonded or soldered to the inner surface of the outer skin. The elements examined were two plain skin specimens (aluminum alloy); two specimens with skins attached to manifolds and tubes (one specimen was bonded, the other soldered); and a specimen representative of a corner section of the complete cooled sandwich. Sinusoidal loads were applied to all specimens. The honeycomb sandwich specimen was loaded in both tension and compression; the other specimens were loaded in tension only. The cooling tubes were pressurized with oil throughout the fatigue tests. The most significant results of these tests follow: All specimens exceeded their design life of 20,000 cycles without damage. Crack growth rates obtained in the plain skin specimens were used to determine the crack growth characteristics of aluminum alloy. Cracks in skins either bonded or soldered to cooling tubes propagated past the tubes without penetration. The coolant tubes served as crack arresters and temporarily stopped crack growth when a crack reached a tube-skin interface. The honeycomb core demonstrated that it could contain leakage from a tube.
The electrical resistivity of silicon bronze and phosphor bronze was measured in magnetic fields from 0 to 14 T and at temperatures between 2 and 300 K. At any fixed temperature, the change in resistivity to 14 T was less than a few parts in 100,000. Thus, these bronzes are excellent for use in high magnetic fields where constant resistance is required. Welding leads to the sample was found to be superior to soldering. The soldered contacts were subject to spurious resistivity changes that resulted from superconducting transitions in the solder.
A machine is described for attaching solar cells to a flexable substrate on which printed circuitry has been deposited. The strip is fed through: (1) a station in which solar cells are elevated into engagement with solder pads for the printed circuitry and thereafter heated by an infrared lamp; (2) a station at which flux and solder residue is removed; (3) a station at which electrical performance of the soldered cells is determined; (4) a station at which an encapsulating resin is deposited on the cells; (5) a station at which the encapsulated solar cells are examined for electrical performance; and (6) a final station at which the resulting array is wound on a takeup drum.
Solar cells are oriented and fluxed automatically at first work station along solar-array assembly line. In under 2 seconds rotary drive rotates cell into proper position for applying solder flux to bus pad on collector side. When contact bus pad is in correct position, capstan drive is disengaged, and vacuum holddown beneath cell is turned on. Flux system lowers and applies preset amount of solder flux to bus pad. Two interconnect tabs are soldered to fluxed areas.
To reduce labor, industrial robot used for solar-cell positioning and soldering operations. Cassette unloader feeds solar cell to vacuum chuck, which rotates cell into proper orientation as determined by optical sensor. Solder paste and interconnect ribbons are applied to cell. Industrial robot then picks up cell, heats it by RF inductions, and solders it into position in solar cell.
Vacuum holddown fixtures hold photovoltaic solar cells during assembly into array. Tab attachment fixture holds single cell and interconnecting tabs during heat-lamp soldering. Array-layout fixture has computer-scribed layout template and holds cells (with attached tabs) while being placed in position and transferred to array-interconnection fixture, which holds cells during final soldering step. Group of heat lamps on trolley passes slowly over interconnection fixture to heat cells to soldering temperature.
It was concluded that systems requirements would force a reassessment of the conventional approach to interconnecting cells into blanket or array modules. Defense applications (hardening) were identified as the key requirement that would force a movement away from the standard method (solder) of forming array circuits. The panel also agreed that requirements associated with the impending NASA Space Station and in-bound missions would lead to alternative interconnecting approaches. It was concluded that the diverse requirements of future space missions (high temperature and extended thermal cycling) might not be met by one approach, such as parallel-gap resistance welding. The panel suggested that other options such as high temperature solders and brazing be considered for the various mission requirements that were anticipated. The panel agreed that blanket technology was potentially suitable for in-orbit annealing to temperatures of 200 C provided that conventional soldered connecting techniques were replaced by "welding".
A program was conducted to demonstrate the cycle life capability of welded solar cell modules relative to a soldered solar cell module in a simulated low earth orbit thermal environment. A total of five 18-cell welded (parallel gap resistance welding) modules, three 18-cell soldered modules, and eighteen single cell samples were fabricated using 2 x 4 cm silicon solar cells from ASEC, fused silica cover glass from OCLI, silver plated Invar interconnectors, DC 93-500 adhesive, and Kapton-Kevlar-Kapton flexible substrate material. Zero degree pull strength ranged from 2.4 to 5.7 lbs for front welded contacts (40 samples), and 3.5 to 6.2 lbs for back welded contacts (40 samples). Solar cell cross sections show solid state welding on both front and rear contacts. The 18-cell welded modules have a specific power of 124 W/kg and an area power density of 142 W/sq m (both at 28 C). Three welded and one soldered module were thermal cycle tested in a thermal vacuum chamber simulating a low earth orbit thermal environment.
Heat-sinking tool holds microcircuits for lead tinning while protecting circuits from heat of tinning solder. Microcircuit holder dips leads in molten solder. Holder shields microcircuit from solder heat while leads immersed and absorbs heat conducted through leads. Thus keeps microcircuit relatively cool. Application tool was developed for requires tinning not closer than 0.02 in. (0.5 mm) from package body or its glass seals.
The firmly adherent film of bondable metal, such as silver, is applied to the surface of glass or other substrate by decomposing a layer of solution of a thermally decomposable metallo-organic deposition (MOD) compound such as silver neodecanoate in xylene. The MOD compound thermally decomposes into metal and gaseous by-products. Sealing is accomplished by depositing a layer of bonding metal, such as solder or a brazing alloy, on the metal film and then forming an assembly with another high melting point metal surface such as a layer of Kovar. When the assembly is heated above the temperature of the solder, the solder flows, wets the adjacent surfaces and forms a hermetic seal between the metal film and metal surface when the assembly cools.
The purpose of this program is to develop a basic understanding of how a molten material front spreads over a solid that is below its melting point, arrests, and freezes. Our hope is that the work will contribute toward a scientific knowledge base for certain new applications involving molten droplet deposition, including the "printing" of arbitrary three-dimensional objects by precise deposition of individual molten microdrops that solidify after impact. Little information is available at this time on the capillarity-driven motion and arrest of molten contact line regions. Schiaffino and Sonin investigated the arrest of the contact line of a molten microcrystalline wax spreading over a subcooled solid "target" of the same material. They found that contact line arrest takes place at an apparent liquid contact angle that depends primarily on the Stefan number S=c(T(sub f) -T(sub t)/L based on the temperature difference between the fusion point and the target temperature, and proposed that contact line arrest occurs when the liquid's dynamic contact angle approaches the angle of attack of the solidification front just behind the contact line. They also showed, however, that the conventional continuum equations and boundary conditions have no meaningful solution for this angle. The solidification front angle is determined by the heat flux just behind the contact line, and the heat flux is singular at that point. By comparing experiments with numerical computations, Schiaffino and Sonin estimated that the conventional solidification model must break down within a distance of order 0.1 - 1 microns of the contact line. The physical mechanism for this breakdown is as yet undetermined, and no first-principles theory exists for the contact angle at arrest. Schiaffino and Sonin also presented a framework for understanding how to moderate Weber number molten droplet deposition in terms of similarity laws and experimentation. The study is based on experiments with three molten materials- molten wax on solid wax, water on ice, and mercury on frozen mercury- which between them span a considerable range of the deposition/solidification similarity parameters. Correlations are obtained for the spreading velocity, spreading time scales, the spreading factor (i.e. ratio of deposited drop's final footprint radius and the drop's initial radius), post-spreading liquid oscillation amplitudes and time scales, and bulk solidification time scales. Duthaler carried out an experimental and theoretical investigation of the relationship between the liquid's apparent contact angle and the Capillary number Ca=mu U/sigma based on contact line speed, for molten materials spreading over subcooled solids. This relationship is required for modeling of melt spreading. We have adapted Voinov's methodology to the molten contact line and formulated a theoretical model for the Ca vs. contact angle relationship, based Schiaffino and Sonin#s (1997a,b) wedge-like solidification front model. With the solidification front angle taken from Schiaffino and Sonin, the model is in good agreement with the experimental results for Ca vs. contact angle. Duthaler also extended the experimental investigation of droplet deposition and contact line freezing to more materials, including solder on glass, solder on solder, water on ice, and molten microcrystalline wax on wax. The latter also included tests on inclined targets. Deposition tests have also been done with molten octacosane (C28H58) on various targets. An important objective of our program has been the development of micron-scale sensors for measuring the transient temperature at a point on the substrate surface as a molten contact line moves over it. The expectation is that this temperature history will yield a better understanding of the thermal process in the contact line region. The sensors are of the thermistor type, either 2.5 microns or 1.5 microns square, microfabricated with silicon-based technology on either pure silicon or amorphous silicon dioxide chips. Each chip has 32 sensors distributed on its surface in arrays. The time response is better than 10 ms. At the time of writing, sensor calibration is in progress. Results on thermal transients during contact line passage will be discussed at the conference. While we expect that the data will provide information on the near-contact-line heat transfer process, we also foresee possible problems. First, the spatial resolution of the sensors may be insufficient to resolve the near-contact-line region. Second, the sensors protrude about 0.5 microns above the substrate surface, and may affect the contact line motion. Third, a sensor's temperature history depends on both the heat flux distribution into it from the fusion front and the thermal properties of the substrate below it and the solidified melt between it and the fusion front. The heat flux distribution in the contact line region must therefore be unfolded from computations of the overall system's transient thermal response.
The space-age materials that NASA employs in its spacecraft and satellites have different attributes than the building materials that can work for Earthly uses. These materials do not behave like the typical construction materials, and therefore, require new methods for construction. Work done at NASA s Langley Research Center in the realm of active solder joining has led to a new, self-bonding solder that enables high conductivity, as well as the metallic joining of carbon and ceramic materials to a wide range of metals. The original work involved evaluating high- and low-temperature joining technologies for joining carbon composite structures for use in thermal management and reusable launch vehicles. The initial plan for this innovation was to lower the weight of battery packs in satellites. It was a success. NASA scientists found use for this technology in fabricating a thermal management package for battery compartments in the Earth Observing System (EOS) satellites, but it is also being used by the Agency for space radiator panels. Because it is light, simple to use, and economical, NASA will likely find other uses for this solder, just as outside of the Space Agency, this unique bond is finding many practical applications.
In this work, results of multiple temperature cycling (TC) (up to 1,000 cycles) of different types of solid tantalum capacitors are analyzed and reported. Deformation of chip tantalum during temperature variations simulating reflow soldering conditions was measured to evaluate the possibility of the pop-corning effect in the parts. To simulate the effect of short-time exposures to solder reflow temperatures on the reliability of tantalum capacitors, several part types were subjected to multiple cycles (up to 100) between room temperature and 240 C with periodical measurements of electrical characteristics of the parts. Mechanisms of degradation caused by temperature cycling and exposure to high temperatures, and the requirements of MIL-PRF-55365 for assessment of the resistance of the parts to soldering heat are discussed.
As integrated circuit miniaturization trends continue, they drive the need for smaller higher input/output (I/O) packages. Hermetically sealed ceramic area array parts are the package of choice by the space community for high reliability space flight electronic hardware. Unfortunately, the coefficient of thermal expansion mismatch between the ceramic area array package and the epoxy glass printed wiring board limits the life of the interconnecting solder joint. This work presents the results of an investigation by Marshall Space Flight Center into a method to increase the life of this second level interconnection by the use of compliant microcoil springs. The design of the spring and its attachment process are presented along with thermal cycling results of microcoil springs (MCS) compared with state-of-the-art ball and column interconnections. Vibration testing has been conducted on MCS and high lead column parts. Radio frequency simulation and measurements have been made and the MCS has been modeled and a stress analysis performed. Thermal cycling and vibration testing have shown MCS interconnects to be significantly more reliable than solder columns. Also, MCS interconnects are less prone to handling damage than solder columns. Future work that includes shock testing, incorporation into a digital signal processor board, and process evaluation of expansion from a 400 I/O device to a device with over 1,100 I/O is identified.