Fabrication of Highly Reliable Joint Based on Cu/Ni/Sn Double-Layer Powder for High Temperature Application

Abstract A highly reliable three-dimensional network structure joint was fabricated based on Cu/Ni/Sn powder with double-layer coatings and transient liquid phase bonding (TLPB) technology for high temperature application. The Cu/Ni/Sn joint is characterized by Cu metal particles embedded in the matrix of (Cu,Ni)6Sn5/Ni3Sn4 intermetallic compounds (IMCs), with a low void ratio, and can be reflowed at low temperatures (&amp;lt;260°C), but it can reliably work at a high temperature up to 415°C. Cu/Ni/Sn double-layer powders with different Sn layer and Ni layer thickness were was fabricated and compressed as preform used for TLPB joint bonding. The microstructure and phase composition evolution for Cu/Sn and Cu/Ni/Sn systems during reflow and aging were comparatively studied. Two kinds of interfacial structure designs were made, and corresponding interfacial microscopic morphology was analyzed and compared under once and twice reflow soldering processes. The results indicated that the Sn-coating layer was completely consumed to form (Cu,Ni)6Sn5/Ni3Sn4 IMCs, and the Cu/Ni/Sn joint had a lower void ratio and a higher shear strength than those of Cu/Sn. The mechanism of the Ni-coating layer inhibiting phase transformation was studied. The high reliable three-dimensional network structure joint based on Cu/Ni/Sn double-layer powder was fabricated for high temperature application.</jats:p

Liquid-phase diffusion bonding and the development of a Cu-Ni/Sn composite solder paste for high temperature lead-free electrical connections

After gathering sufficient microstructural evidence that a practical high-temperature lead-free solder could be produced through liquid-phase diffusion bonding (LPDB) of gas-atomized Cu-Ni powder blended with SN100C (Sn-0.7Cu-0.05Ni, wt.%) commercial solder powder, additional research was completed to enable design of a prototype composite solder paste. By reviewing several experimental alternatives to LPDB for use as replacements and improvements to the currently-used high-Pb solders (which will inevitably be banned by the EU’s RoHS directive), the choice to pursue the prototype solder paste research was verified. One key difference between this research and that of others is the method of pre-alloying Cu with Ni before atomization of the metal powder particles. These Ni additions prevent a detrimental unit cell volume change from occurring in typical Sn-Cu solder alloys by suppressing the hexagonal-to-monoclinic isothermal phase transformation upon cooling during reflow to below 186ðC. Rapid diffusion of the Ni and Cu was explored in order to better design the composite paste’s suggested reflow profile. Research to study and maximize the grain boundary diffusion of Ni and its effect on the resulting intermetallic compounds helped determine the final composition of the composite paste for use in industry

Modeling the SAC microstructure evolution under thermal, thermomechanical and electrical constraints

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Kupari-tina mikroliitosten karakterisointimenetelmät

The microelectronics industry constantly aspires to shrink the device features. At the package level, this implies a decrease in the interconnect size leading to small volume interconnections that are commonly called micro-connects. Smaller material volumes may give rise to new reliability challenges, such as open circuits, due to Kirkendall voiding. The root cause(s) for Kirkendall voiding is not yet clear and the methods for characterization are still varied. This thesis reviews techniques to characterize the microstructure and impurities in Cu-Sn micro-connects. The evaluated techniques are Auger Electron Spectroscopy (AES), Electron Energy Loss Spectroscopy (EELS), Energy-Dispersive X-Ray Spectroscopy (EDX), X-Ray Spectroscopy (XPS), Secondary Ion Mass Spectrometry (SIMS), Rutherford Backscattering Spectrometry (RBS), Elastic Recoil Detection Analysis (EELS), Transmission Electron Microscopy (TEM), Focused Ion Beam (FIB), and Scanning Acoustic Microscopy (SAM). From the reviewed techniques, EDX, FIB, SAM, and TEM are used in the experimental section. For the first time, impurities are measured directly inside Kirkendall voids. It was discovered that the Kirkendall voids in annealed Cu-Sn samples contained a significant amount of chlorine and oxygen. The ASTM grain size counting method was applied to FIB-polished samples. It was observed that the grain size did not increase by annealing at 150 ◦C. Furthermore, for the first time, GHz-SAM was used to characterize Kirkendall voids. The technique is promising but it is still affected by the low lateral resolution.Mikroelektroniikkateollisuus pyrkii jatkuvasti pienentämään laitekokoa. Paketointitasolla tämä tarkoittaa sitä, että sirujen välisten liitosten kokoluokka on siirtymässä kohti mikroliitoksia, jotka saattavat aiheuttaa uusia luotettavuusongelmia. Kirkendall-aukot ovat yksi syy kyseisiin luotettavuusongelmiin ja aukkojen alkuperä on vielä tuntematon. Sen lisäksi, mikroliitosten ja Kirkendall aukkojen karakterisointiin käytetään toisistaan poikkeavia menetelmiä eikä sopivista metodeista ole vielä yhteisymmärrystä. Tämä diplomityö tarkastelee kupari-tina mikroliitoksien mikrorakenteen ja epäpuhtauksien analysointiin käytettyjä menetelmiä. Tarkasteltavat menetelmät ovat Auger elektronispektroskopia (AES), epäelastinen elektronisironta (EELS), energiadispersiivinen röntgenspektroskopia (EDX), röntgenfotoelektronispektroskopia (XPS), sekundääri ionimassaspektroskopia (SIMS), Rutherford-takaisinsirontaspektroskopia (RBS), rekyylispektrometria (ERDA), läpäisyelektronimikroskopia (TEM), keskitetty ionisuihku (FIB) ja akustinen mikroskopia (SAM). Esitellyistä menetelmistä kokeellisessa osiossa käytettiin EDX:ää, FIB:ä, SAM:a ja (S)TEM:ä. Tässä diplomityössä on mitattu ensimmäistä kertaa epäpuhtauksia Kirkendall-aukkojen sisältä. Mittauksista saatiin selville, että hehkutettujen kupari-tina -näytteiden Kirkendall-aukot sisälsivät huomattavan määrän happea ja klooria. Raekokoa tarkasteltiin kiillottamalla näytteet FIB:llä ja soveltamalla ASTM:n raekoko -standardia. Työssä huomattiin, että raekoko ei kasvanut, jos näytteitä hehkutettiin 150 ◦C lämpötilassa. Tämä on myös ensimmäinen kerta, kun GHzSAM:a on käytetty Kirkendall-aukkojen tutkimiseen. Tulokset olivat lupaavia, mutta menetelmän alhainen sivuttaissuuntainen resoluutio on vielä rajoittava tekijä

Extreme Environment Reliability of Components for Computing with SAC305 and Alternative High Reliability Solders

The semiconductor and packaging industries have been moving away from the use of Lead (Pb) due to the increasing awareness of the health and safety concerns surrounding its use. For many applications, the industry has moved from eutectic Sn-Pb solder to the near-eutectic Sn-Ag-Cu (SAC) solders, and more applications – including those considered “extreme environment” – are likely to take place in the near future. However, the reliability of electronic assemblies with SAC solder joints has proven hard to predict based on previous experience with SnPb solders. The reliability of electronic solder joints is determined by a variety of factors including bulk solder properties and failure mechanics. Both the composition and microstructure of the solder joint will affect its bulk properties. Although an initial microstructure will be present following assembly – which will involve one or more soldering steps – this structure will continue to evolve over the lifetime of the joint. The microstructure and microstructural evolution of the Sn-Ag-Cu solders differ significantly from that of eutectic SnPb solders. Because of the risks and uncertainties involved, a new body of reliability engineering knowledge must be built of for the Sn-Ag-Cu solders based on application-specific process and service parameters. This experiment considers the thermal cycle reliability of an assortment of different electronic components and evaluates them on a 0.200” (200 mils) thick printed circuit board. Two substrate materials are tested: FR4-06 and Megtron6. Organic Solderability Preservative (OSP) surface finish is used with all test vehicles. The primary solders for package attachment in this experiment are SnPb and SAC305. Two solders designed for high-temperature reliability are also considered, including a Bi-doped SAC material and the six-element alloy Innolot (Sn3.8Ag0.7Cu3Bi1.4Sb0.15Ni). Isothermal storage at high temperature was used to accelerate the aging of the assemblies. Aging Temperatures are 25oC, 50oC, and 75oC. Aging durations are 0-Months (No Aging, baseline), 6-Months, 12-Months, and 24-Months. The test vehicles were then subjected thermal cycles of -40°C to +125°C on a 120-minute thermal profile in a single-zone environmental chamber to assess the solder joint performance. The as-reflowed failure data (No Aging Group) was found to follow specific reliability trends depending on the type and size of the component. The smaller plastic ball grid array (BGA) packages show the following pattern in Characteristic Life value, listed from best to worst: (1) Matched Innolot, (2) [S]SAC305 doped with [P]Innolot, (3) Matched SAC305, and (4) Matched SnPb. However, when considering the effects of isothermal aging on the relative reliability of various packages, the data indicate that even components that show similar initial reliability trends may display differences following aging. Following isothermal aging, several components exhibit higher reliability when paired with SnPb solder than with the SAC solder materials. Significant differences in reliability were seen between equivalent packages mounted to the two substrate materials tested (FR4-06 and Megtron6). For all of the over-molded plastic BGA components, reliability was higher on the standard glass-epoxy material (FR4-06). Performance for these packages on the high-electrical-performance polyphenylene oxide (PPO) blend material (Megtron6) was much worse. The degradation in reliability with aging were also found to be worse on Megtron6 for the Sn-Ag-Cu materials when paired with these components. However, the two Super-BGA components – SBGA 304 and SBGA 600 – dramatically reverse this substrate-based reliability trend. For the smaller plastic BGA packages, Innolot doping (micro-alloying) appears to be an effective strategy for improving characteristic life. However, as component size and pitch increase, this improvement seems to wane (and in some cases reverse itself altogether). This may be attributable to under-doping of the large-component joints. Based on assembly-level reliability, the “paste doping” strategy appears to be a promising approach to improve reliability in high-stress environments, but one that requires significant further study

Dealloying of Al-based alloys and their mechanisms

Metal based anodes, like tin (Sn), are promising candidate anodes for lithium ion batteries (LIBs) due to their higher specific capacities than traditional graphite electrodes. However, their dramatic volume expansion during lithiation and delithiation could lead to pulverization of the material as well as inadequate cycle life. Materials with nano/microporosity hold promise to accommodate the volume change. This thesis focuses on preparing porous metallic materials for batteries through a dealloying approach. Dealloying is a selective dissolution process, during which one or more active components dissolve from a binary or multicomponent alloy, leaving behind a (nano)porous-structured material enriched in the nobler or less active alloy component(s). In this thesis, porous Sn and nanoporous Cu-Sn composites, which can be used as anodes, and bimodal porous Cu, which can be used as current collector, have been fabricated by dealloying immiscible Al-Sn alloys, ternary Al-Cu-Sn alloy and two-phase Al-Cu alloy, respectively. The dealloying mechanisms of these precursor alloys have been systematically investigated by a variety of means including both ex-situ and in-situ synchrotron X-ray diffraction (XRD). The following findings are most notable. 1) Micro-sized porous Sn (anode material) can be fabricated by dealloying of immiscible Al-Sn alloys. 2) Nanoporous Cu-Sn composite structures (anode material) can be fabricated by concurrent dealloying and realloying of a ternary Al-Cu-Sn alloy. 3) Bimodal porous Cu materials (current collector) can be fabricated from annealing-electrochemical dealloying of Al-Cu alloys. 4) The dealloying of Al2Cu (first dealloyed) and AlCu occurred in sequence and resulted in a hierarchical nanoporous structure. 5)The temperature sensitivity of intermetallic formation in the Cu-Sn system was confirmed by synchrotron studies of the Al67Cu18Sn15 alloy subjected to dealloying at different temperatures (55 &amp;deg;C, 70 &amp;deg;C and 90 &amp;deg;C). The following findings are most notable