Publications

Abstract not provided.

The effects of heat treatment parameters were examined in complex electrical contact alloys containing Pd-Ag-Cu-Au-Pt. These alloys (Paliney tradename, Deringer-Ney Inc., Bloomfield, CT) are strengthened by precipitation reactions. During processing such as glass-to-metal joining in hermetic connectors, if the cooling rate is too slow, discontinuous precipitation (DP) of lamellar 2nd phases can spoil the strengthening effect. Two different solutionizing temperatures were employed and the effects of cooling rates between 6 C/min and >200 C/min were studied. Novel metallographic techniques were developed to reveal the microstructure of these corrosion resistant alloys and quantitative image analysis (QIA) was used to determine the amount of 2nd phase precipitates. Vickers and Knoop microhardness testing was performed to determine the effects of heat treatment parameters on mechanical properties.

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New Pb-free alloys that are variations of the Sn-Ag-Cu (SAC) ternary system, having reduced Ag content, are being developed to address the poor shock load survivability of current SAC305, SAC396, and SAC405 compositions. However, the thermal mechanical fatigue properties must be determined for the new alloys in order to develop constitutive models for predicting solder joint fatigue. A long-term study was initiated to investigate the time-independent (stress-strain) and time-dependent (creep) deformation properties of the alloy 98.5Sn-1.0Ag-0.5Cu (wt.% SAC105). The compression stress-strain properties, which are reported herein, were obtained for the solder in as-cast and aged conditions. The test temperatures were -25°C, 25°C, 75°C, 125°C, and 160°C and the strain rates were 4.2 × 10 -5 s -1 and 8.3 × 10 -4s -1. The SAC105 performance was compared with that of the 95.5Sn-3.9Ag-0.6Cu (SAC396) solder. Like the SAC396 solder, the SAC105 microstructure exhibited only small microstructural changes after deformation. The stress-strain curves showed work-hardening behavior that diminished with increased temperature to a degree that indicated dynamic recrystallization activity. The aging treatment had a small effect on the stress-strain curves, increasing the degree of work hardening. The yield stresses of SAC105 were significantly less than those of SAC396. The aging treatment caused a small drop in yield stress, as is observed with the SAC396 material. The static modulus values of SAC105 were lower than those of SAC396 and exhibited both temperature and aging treatment dependencies that differed from those of the SAC396 material. These trends clearly show that the stress-strain behavior of Sn-Ag-Cu solders is sensitive to the specific, individual composition. © 2009 U.S. Department of Energy.

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The assembly of the BDYE detector requires the attachment of sixteen silicon (Si) processor dice (eight on the top side; eight on the bottom side) onto a low-temperature, co-fired ceramic (LTCC) substrate using 63Sn-37Pb (wt.%, Sn-Pb) in a double-reflow soldering process (nitrogen). There are 132 solder joints per die. The bond pads were gold-platinum-palladium (71Au-26Pt-3Pd, wt.%) thick film layers fired onto the LTCC in a post-process sequence. The pull strength and failure modes provided the quality metrics for the Sn-Pb solder joints. Pull strengths were measured in both the as-fabricated condition and after exposure to thermal cycling (-55/125 C; 15 min hold times; 20 cycles). Extremely low pull strengths--referred to as the low pull strength phenomenon--were observed intermittently throughout the product build, resulting in added program costs, schedule delays, and a long-term reliability concern for the detector. There was no statistically significant correlation between the low pull strength phenomenon and (1) the LTCC 'sub-floor' lot; (2) grit blasting the LTCC surfaces prior to the post-process steps; (3) the post-process parameters; (4) the conductor pad height (thickness); (5) the dice soldering assembly sequence; or (5) the dice pull test sequence. Formation of an intermetallic compound (IMC)/LTCC interface caused by thick film consumption during either the soldering process or by solid-state IMC formation was not directly responsible for the low-strength phenomenon. Metallographic cross sections of solder joints from dice that exhibited the low pull strength behavior, revealed the presence of a reaction layer resulting from an interaction between Sn from the molten Sn-Pb and the glassy phase at the TKN/LTCC interface. The thick film porosity did not contribute, explicitly, to the occurrence of reaction layer. Rather, the process of printing the very thin conductor pads was too sensitive to minor thixotropic changes to ink, which resulted in inconsistent proportions of metal and glassy phase particles present during the subsequent firing process. The consequences were subtle, intermittent changes to the thick film microstructure that gave rise to the reaction layer and, thus, the low pull strength phenomenon. A mitigation strategy would be the use of physical vapor deposition (PVD) techniques to create thin film bond pads; this is multi-chip module, deposited (MCM-D) technology.

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The 63Sn-37Pb (wt.%, designated Sn-Pb) solder interconnections made to copper (Cu) pads were examined on two printed wiring assemblies (PWAs) that had been in the field for 17 years and subsequently exposed to an accelerated aging test environment. A qualitative assessment of the solder joints indicated that there was excellent solderabiliry of the pins and Cu pads. Void formation was minimal or did not occur at all. Manufacturing defects were limited to minor Cu pad lifting with cracks in the underlying epoxy resin and local areas of Cu barrel separation from the laminate hole wall. Both defects would not have influenced the effects of the accelerated aging environment. A quantitative analysis examined the intermetallic compound (IMC) layer thickness of selected components on the PWAs. The IMC thickness data indicated that the PWAs were exposed to considerably lower, cumulative temperatures inside the product assembly than were present outside as a result of the accelerated aging environment. The quantitative analysis also evaluated the Pb-rich phase particle size in both fillets and the hole region of the PWA solder joints. The Pb-rich phase size confirmed that the temperature environment at the PWA level was significantly less severe than that of the accelerated aging environment. The Pb-rich phase size data indicated that the solder joints were exposed to a limited degree of thermal mechanical fatigue (TMF) that likely originated from the nominal temperature fluctuations coupled with the thermal expansion of the encapsulant as well as large expansion of the circuit board laminate in the z-axis (through-thickness) direction. This study demonstrated the methodology by which, IMC thickness and Pb-rich phase size were used to assess the temperature/time conditions experienced at the Sn-Pb/Cu interconnection level versus the external environment. Copyright © 2006 ASM International®.

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The SA1358-10 and SA2052-4 circular JT Type plug connectors are used on a number of nuclear weapons and Joint Test Assembly (JTA) systems. Prototype units were evaluated for the following specific defects associated with the 95Sn-5Sb (Sn-Sb, wt.%) solder joint used to attach the beryllium-copper (BeCu) spring fingers to the aluminum (Al) connector shell: (1) extended cracking within the fillet; (2) remelting of the solder joint during the follow-on, soldering step that attached the EMR adapter ring to the connector shell (and/or soldering the EMR shell to the adapter ring) that used the lower melting temperature 63Sn-37Pb (Sn-Pb) alloy; and (3) spalling of the Cd (Cr) layer overplating layer from the fillet surface. Several pedigrees of connectors were evaluated, which represented older fielded units as well as those assemblies that were recently constructed at Kansas City Plant. The solder joints were evaluated that were in place on connectors made with the current soldering process as well as an alternative induction soldering process for attaching the EMR adapter ring to the shell. Very similar observations were made, which crossed the different pedigrees of parts and processes. The extent of cracking in the top side fillets varied between the different connector samples and likely the EMR adapter ring to the shell. Very similar observations were made, which crossed the different pedigrees of parts and processes. The extent of cracking in the top side fillets varied between the different connector samples and likely reflected the different extents to which the connector was mated to its counterpart assembly. In all cases, the spring finger solder joints on the SA1358-10 connectors were remelted as a result of the subsequent EMR adapter ring attachment process. Spalling of the Cd (Cr) overplating layer was also observed for these connectors, which was a consequence of the remelting activity. On the other hand, the SA2052-4 connector did not exhibit evidence of remelting of the spring finger solder joint. The Cd (Cr) layer did not show signs of spalling. These results suggested that, due to the size of the SA1358-10 connector, any of the former or current soldering processes used to attach the EMR adapter ring and/or EMR shell to the connector shell, requires a level of heat energy that will always result in the remelting of the spring finger solder joint attached with either the Sn-Ag or the Sn-Sb alloy. Lastly, it was construed that the induction soldering process, which is used to attach the EMR adapter ring onto the shell, was more likely to have caused the remelting event rather than the more localized heat source of the hand soldering iron used to attach the EMR shell to the adapter ring.

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This report is a summary of the work completed in FY01 for science-based characterization of the processes used to fabricate 1) cermet vias in source feedthrus using slurry and paste-filling techniques and 2) cermet powder for dry pressing. Common defects found in cermet vias were characterized based on the ability of subsequent processing techniques (isopressing and firing) to remove the defects. Non-aqueous spray drying and mist granulation techniques were explored as alternative methods of creating CND50, the powder commonly used for dry pressed parts. Compaction and flow characteristics of these techniques were analyzed and compared to standard dry-ball-milled CND50. Due to processing changes, changes in microstructure can occur. A microstructure characterization technique was developed to numerically describe cermet microstructure. Machining and electrical properties of dry pressed parts were also analyzed and related to microstructure using this analytical technique.3 Executive SummaryThis report outlines accomplishments in the science-based understanding of cermet processing up to fiscal year 2002 for Sandia National Laboratories. The three main areas of work are centered on 1) increasing production yields of slurry-filled cermets, 2) evaluating the viability of high-solids-loading pastes for the same cermet components, and 3) optimizing cermet powder used in pressing processes (CND50). An additional development that was created as a result of the effort to fully understand the impacts of alternative processing techniques is the use of analytical methods to relate microstructure to physical properties. Recommendations are suggested at the end of this report. Summaries of these four efforts are as follows:1.Increase Production Yields of Slurry-Filled Cermet Vias Finalized slurry filling criteria were determined based on three designs of experiments where the following factors were analyzed: vacuum time, solids loading, pressure drop across the filter paper, slurry injection rate, via prewetting, slurry injection angle, filter paper prewetting, and slurry mixing time. Many of these factors did not have an influence on defect formation. In order of decreasing importance, critical factors for defect formation by slurry filling are vacuum time (20 sec. optimal), slurry solids loading (20.0 g of cermet with 13.00 g of DGBEA solvent (21.2 vol%)), filling with the pipette in a vertical position, and faster injection rates (%7E765 l/s) as preferable to slower. No further recommendations for improvement to this process can be suggested. All findings of the slurry filling process have been transferred to CeramTec, the supplier. Paste filling methods appear to show more promise of increasing production yields. The types of flaws commonly found in slurry-filled vias were identified and followed throughout the entire source feedthru process. In general, all sizes of cracks healed during isopressing and firing steps. Additionally, small to medium sized voids (less than 1/3 the via diameter) can be healed. Porosity will usually lead to via necking, which may cause the part to be out of specification. Large voids (greater 4 than 1/3 of the diameter) and partial fills are not healed or produce significant necking. 2.Viability of High-Solids-Loading-Cermet Paste for Filling Source Feedthru ViaThe paste-filling process is easy to implement and easier to use. The high solids loading (>40 vol %) reduces the incidence of drying defects, which are seen in slurry filled (%7E23 vol %) vias. Additionally, the way in which the vias are filled (the paste is pushed from entrance to exit, displacing air as the paste front progresses), reduces the chance of entrapped voids, which are common in the slurry filling process. From the fair number of samples already filled, the likelihood of this process being a viable and reliable process is very good. Issues of concern for the paste process, as with any new process, are any problems that may arise in subsequent manufacturing stages of the neutron tube that may be affected by subtle changes in microstructure. Both MC4277 and MC4300-type source feedthrus were paste-filled by hand. X-ray analysis showed a much lower existence of voids in the green parts as compared to slurry-filled parts. The paste shows improvements in shelf life (weeks) as compared to slurry (minutes). This method of introducing the cermet to the via also lends itself very well to an automated filling process where a machine can either drill vias or, with the aid of a vision system, find pre-drilled vias and fill them with paste. The pastes used in this work prove the concept of this automated filling process as MC4277 sources have been filled using such a prototype machine, however, better performing pastes can be developed which are less hazardous (aqueous systems). The paste process was also used to successfully fill MC4300 "dogleg" type sources.3.Optimize CND50 Two methods of creating granulated cermet powder for comparison with dry-ball milled CND50 were explored. The first method, non-aqueous spray drying, was performed at Niro Inc. used a 40/60 (wt %) ethanol/toluene solvent and three binder systems; polyvinyl butyral (B79), ethylcellulose (Ethocel), and hydroxypropylcellulose (Klucel). Due to the nature of small spray-dry systems, an excess amount of fines was present in the granulated powder, which may have contributed to the low angles of repose (68 to 78). This is a moderate increase in 5 flowability as standard dry-ball milled powder possesses an angle of repose of 79-89. Mist granulated powders were produced with a tert-butanol solvent and polyvinyl butyral binder system. The angles of repose were more promising (28). More investigation into the mist granulation method is required. Also, aqueous spray drying may be possible with cermet and should be explored. Compaction of all granulated powders is much closer to a proven pressing powder (Sandi94 - angle of repose 29) which should allow cermet to be pressed to near net shape where die filling is difficult for non-flowing powders.4.Microstructure Characterization An analytical technique was developed to numerically characterize microstructures in terms of molybdenum dispersion, homogeneity, and percolation indices. This technique was applied to dry-ball-milled samples of various ball-milling times (0.5 to 20 hours). Significant change in the microstructure could be seen with milling time. Increased milling time caused agglomeration of molybdenum particles, increasing the percolation index, whereas short milling times promoted higher dispersion indices. This phenomenon is contrary to conventional understanding of mixing. However, conventional ball milling does not usually incorporate granules with binder and separate particles. This discrepancy may explain the odd mixing behavior. It is important to note that the high percolation index possessed by long ball mill times showed lower electrical resistance than low-percolation-index microstructures. However, machinability of high percolation, low-dispersion-index microstructures were poor as compared to microstructures with high dispersion indices and moderate percolation indices. This trade-off between dispersion and percolation (at constant molybdenum levels) suggests that microstructures can be achieved that posses good mechanical and electrical properties. Coincidentally, microstructures that satisfy this condition are produced by the standard dry-ball-milled CND50 (4 hour ball mill time). The performance and sensitivity of the microstructure characterization technique should be evaluated, specifically for electrical conductivity. Processing techniques to decrease the percolation index (lowering molybdenum content, excess ball milling, 6 larger molybdenum particles, etc.) should be employed to determine the point where cermet is not conductive or falls below electrical conduction specifications.7

The purpose of this SAND Report is to document efforts in the extraction and failure analyses of sleeve-style Lightning Arrestor Connectors (LACs). Several MC3080 and MC3079 LACs were recovered from the field and tested as part of the Enhanced Surveillance Campaign. A portion of these LACs failed retesting. Terry Ernest (01733), the LAC Component Engineer, provided eleven MC3080 LACs for evaluation where four of the LACs failed IR/DCW and one failed FRB requirements. The extraction of rutile sleeves from failed LACs was required to determine the source of failure. Rutile sleeves associated with connector function failures were examined for cracks, debris as well as any other anomalies which could have caused the LAC to not function properly. Sleeves that failed FRB or that experienced high FRB exhibited high symmetry, smooth surface, long-flow amicon, and slightly over-sized inside diameter. LACs that failed DCW or IR requirements had rutile sleeves that exhibited breakdown tracks.

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The stainless steel alloy 17-4PH contains a martensitic microstructure and second phase delta ({delta}) ferrite. Strengthening of 17-4PH is attributed to Cu-rich precipitates produced during age hardening treatments at 900-1150 F (H900-H1150). For wrought 17-4PH, the effects of heat treatment and microstructure on mechanical properties are well-documented [for example, Ref. 1]. Fewer studies are available on cast 17-4PH, although it has been a popular casting alloy for high strength applications where moderate corrosion resistance is needed. Microstructural features and defects particular to castings may have adverse effects on properties, especially when the alloy is heat treated to high strength. The objective of this work was to outline the effects of microstructural features specific to castings, such as shrinkage/solidification porosity, on the mechanical behavior of investment cast 17-4PH. Besides heat treatment effects, the results of metallography and SEM studies showed that the largest effect on mechanical properties is from shrinkage/solidification porosity. Figure 1a shows stress-strain curves obtained from samples machined from castings in the H925 condition. The strength levels were fairly similar but the ductility varied significantly. Figure 1b shows an example of porosity on a fracture surface from a room-temperature, quasi-static tensile test. The rounded features represent the surfaces of dendrites which did not fuse or only partially fused together during solidification. Some evidence of local areas of fracture is found on some dendrite surfaces. The shrinkage pores are due to inadequate backfilling of liquid metal and simultaneous solidification shrinkage during casting. A summary of percent elongation results is displayed in Figure 2a. It was found that higher amounts of porosity generally result in lower ductility. Note that the porosity content was measured on the fracture surfaces. The results are qualitatively similar to those found by Gokhale et al. and Surappa et al. in cast A356 Al and by Gokhale et al. for a cast Mg alloys. The quantitative fractography and metallography work by Gokhale et al. illustrated the strong preference for fracture in regions of porosity in cast material. That is, the fracture process is not correlated to the average microstructure in the material but is related to the extremes in microstructure (local regions of high void content). In the present study, image analysis on random cross-sections of several heats indicated an overall porosity content of 0.03%. In contrast, the area % porosity was as high as 16% when measured on fracture surfaces of tensile specimens using stereology techniques. The results confirm that the fracture properties of cast 17-4PH cannot be predicted based on the overall 'average' porosity content in the castings.

A severe fire and explosion occurred at a propane storage yard in Truth or Consequences, N.M., when a truck ran into the pumping and plumbing system beneath a large propane tank. The storage tank emptied when the liquid-phase excess flow valve tore out of the tank. The ensuing fire engulfed several propane delivery trucks, causing one of them to explode. A series of elevated-temperature stress-rupture tears developed along the top of a 9800 L (2600 gal) truck-mounted tank as it was heated by the fire. Unstable fracture then occurred suddenly along the length of the tank and around both end caps, along the girth welds connecting the end caps to the center portion of the tank. The remaining contents of the tank were suddenly released, aerosolized, and combusted, creating a powerful boiling liquid expanding vapor explosion (BLEVE). Based on metallography of the tank pieces, the approximate tank temperature at the onset of the BLEVE was determined. Metallurgical analysis of the ruptured tank also permitted several hypotheses regarding BLEVE mechanisms to be evaluated. Suggestions are made for additional work that could provide improved predictive capabilities regarding BLEVEs and for methods to decrease the susceptibility of propane tanks to BLEVEs.

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Minimizing the likelihood of solder joint embrittlement in connectors is realized by reducing or eliminating retained Au plating and/or Au-Sn intermetallic compound formation from the assemblies. Gold removal is performed most effectively by using a double wicking process. When only a single wicking procedure can be used, a higher soldering temperature improves the process of Au removal from the connector surfaces and to a nominal extent, removal of Au-contaminated solder from the joint. A longer soldering time did not appear to offer any appreciable improvement toward removing the Au-contaminated solder from the joint. Because the wicking procedure was a manual process, it was operator dependent.