Indium Corporation (United States)

Low temperature soldering (reflow peak temperature at 200oC and below) has been considered as one of the effective ways to reduce the processing defects caused by warpage of large BGA assembly. Most of the large BGAs are still surfaced with SnAgCu (SAC) and/or modified SAC balls, whose melting temperature ranges from 210oC to 220oC. Although BGA warpage can be reduced under low reflow temperature, the limited solder paste volume, because SnAgCu ball will not melt but dissolve during reflow, may still be insufficient to compensate for the mitigated warpage, possibly resulting in forming the defective joints, including non-contact, head-in-pillow and non-wet-open etc. Adding more paste would reduce the possibility of forming afore-mentioned defective joints but bring in the risk of joint bridging. Increasing the reflow peak temperature may enlarge the component warpage but allow more molten solder volume during soldering, which may compensate for the warpage displacement and help the joint formation as well. Hot-tearing from using low temperature solder is another concern related to the overlap of component warpage inversion temperature and solder solidification temperature. Thus, optimization of both paste volume and reflow temperature, as well as selection of the appropriate solder alloy are critical to succeed. The current article has systematically, in the first time, investigated the joint formation through the selection of (1) solder alloys of different melting temperature (138oC for BiSnAg eutectic, 205oC for DFLT-1 and 196oC for DFLT-2), (2) paste to ball volume ratio (PBVR of 0.13, 0.25 and 0.5), and (3) reflow profiles (low temperature reflow of 185 and 200oC peak, mid temperature reflow of 210 and 220oC, and traditional SAC reflow of 240oC peak) with a 40x40mm2 PBGA928 component (max concave warpage of -11mil at 100oC and max convex warpage of 34mil at 250oC with inversion temperature of 140oC). Defective joints are always seen under low temperature soldering for all three pastes regardless of PBVR. Increasing reflow temperature to above 200oC improved the joint shape and greatly reduced the defects. Under P220 and PBVR of 0.13, all three pastes have achieved the optimal joints without “fatal” defects. DOE analysis had confirmed the reflow peak is the most impact factor to form the optimal joint, followed by PBVR and alloy selection. SAC/BiSnAg joints were dominated by hot-tearing under low temperature reflows, whose wide pasty range, overlapped with the component inversion temperature of 140oC, is attributed to the higher occurrence. SAC/DFLT and SAC/DFLT-2 had much narrower pasty range and always solidified above the inversion temperature, showing much less hot tearing. The investigation of the joint reliability performance is still ongoing and will be presented in Phase II.

Gallium-based liquid metals (LMs) are materials that possess some unique properties. Just like any other metal, they have high thermal conductivity and low interfacial resistance, but they are in a liquid phase at room temperature. In contrast to mercury alloys, gallium alloys are non-toxic. They don’t evaporate and they can’t be inhaled. The viscosity of those alloys is very similar to water, but they are six times as dense as water. All those properties make Gallium-based liquid metals very good candidates for thermal interface material (TIM) in electronics applications. On the other hand, the reaction and incompatibility of those alloys with some metals is one of the challenges for this type of TIM. The other challenge is that those materials are not just thermally, but also electrically conductive and that is not a desirable property for TIM. A suitable barrier that will prevent any leakage of LMs and the best way to apply the appropriate volume of LM in high-volume production (HVP) would be one of the most important things for any application. A key challenge is applying the liquid metal consistently through a traditional dispensing method due to its property and behavior which involves high surface tension. Through advanced dispensing techniques like jetting technology liquid metal can be applied reliably on a flat, uneven surface or in arrays of minuscule confined spaces or cavities. This paper highlights the dispensing quality, weight repeatability from one substrate to another, and valve hardware stability. This paper will also address the challenges faced during dispensing of liquid metal in high volume manufacturing, and how to achieve desired bondline thickness with jet dispensing for higher throughput and process reliability.

ABSTRACT As advanced packaging continues to drive towards heterogeneous integration, new materials need to be developed to address the many challenges faced in the assembly process. This presentation will focus on innovative materials for advanced packaging, particularly for small form factor modules, including novel flux formulations for flip chip attach that are compatible with underfill and molding compound, an adhesive agent for fluxless soldering with a formic acid atmosphere, solders with various temperatures for hierarchical soldering. The technology drives toward increased functionality, better efficiency, and higher power density; advanced packaging silicon chip/die size is now bigger, with densely packed interconnect, tighter pitch, and smaller bumps. Some of the common issues we observed due to these are warpage-induced open joints from the big and thin die, flux residue removal challenges due to the tight spacings, and thermal-induced reliability issues. Novel flux formulations and adhesive agents for wafer/die bonding are needed, and interactions between the flux material and various dielectric materials on the wafer need to be better studied. The shift towards using semiconductor-grade ultra-low residue no-clean flip-chip fluxes, which eliminates the cleaning process, is one of the solutions for overcoming these challenges. Another possible solution, studied in recent years, is using a near-zero residue fluxless adhesive agent to hold the die in place while reflowing in a formic acid reflow atmosphere. The development of new solders is also a critical enabler for heterogeneous integration. If there are different reflow cycles that require a solder hierarchy, alloys with varying melting temperatures may be needed for SIP. In addition, lower melting-temperature solders can provide better warpage control, especially for thin die and substrate, as well as for heat-sensitive sensors, die, or substrate that require low-temperature processing. Alloys with enhanced reliability are also needed for certain SIP modules. In summary, proper selection and application of interconnect materials for various assembly processes is crucial to ensuring high production yield for the assembly process of advanced packaging and SIP application.

ABSTRACT Miniaturization has been a pivotal trend in the electronics industry since Gordon Moore's prediction of doubling microchip transistor counts every two years. While this trend has driven the development of smaller, lighter electronic devices, recent advancements in semiconductor technology are pushing towards larger and more complex integrated circuit chips. This evolution, necessitated by the need for higher performing semiconductors and enhanced manufacturing processes, introduces significant challenges during reflow attachment to printed circuit boards, particularly concerning component warpage and boards that heat unevenly. Component warpage is a growing challenge when it comes to printed circuit board assembly. Warpage can appear in a “smile, frown, M or W“ shape. Warpage complexity typically increases as component dimensions increase. Warpage during reflow can directly lead to defects such as head-in-pillow (HiP), non-wet opens and bridging. One of the best ways to control, or reduce component warpage is by lowering the peak reflow temperature. Checking the moiré shadow diagram for each component can help ensure that the reflow peak temperature is compatible with the component warpage requirement. The warpage issue extends beyond component size alone; modern circuit boards are also growing in complexity. High-performance IC components require thicker PWB copper traces and take up more board real estate, contributing to densely populated boards with varied component types and shapes. During reflow, these boards often exhibit uneven heating, with standard SAC305 profiles showing temperature differentials ranging from 40-60°C. This temperature variance forces a compromise: while ensuring components with substantial thermal mass reach 240°C, smaller components may inadvertently reach temperatures as high as 300°C. A novel mixed solder powder alloy, referred to as DFLT, has emerged as a crucial solution to these manufacturing complexities. Leveraging advanced alloy technology, DFLT mitigates challenges associated with uneven heating due to varying thermal mass across components. This innovative alloy not only widens the reflow process window, but also can effectively reduce warpage during reflow processes. Moreover, its peak reflow temperature can be 20-40°C lower than SAC305, ensuring quality solder joints without compromising component integrity. In summary, DFLT represents a transformative solution for modern electronics manufacturing, adeptly addressing the intricate demands posed by larger, more complex IC components and high-density circuit boards.

Near-eutectic tin-bismuth alloy is a strong contender as the solder of choice for low-temperature electronic assembly applications. This paper will describe the recent significant developments by the iNEMI project team on the understanding of the electromigration behavior of Sn-Bi solder which has a unique physical metallurgy and electromigration behavior quite different for that of Sn-Ag-Cu (SAC) solder and other solders in common use today. The high rate of Bi electromigration and its high homologous temperature sets the near-eutectic Sn-Bi solder apart from the SAC and the Sn-Pb solders. The latter have lower homologous temperatures and have no one element dominating the electromigration phenomenon that can be readily tracked to monitor their electromigration behavior. In Sn-Bi alloys, the rates of Bi electromigration are clearly reflected in the rates of electrical resistance increase making it more convenient to use the fundamental physics-based electromigration equations rather than Black’s somewhat empirical equation that deals with the mean time to failure of solder joints. The high homologous temperature of Sn-Bi alloys makes in-situ study of electromigration in a scanning electron microscope more convenient, resulting in a much deeper understanding of the physical metallurgy and electromigration of the Sn-Bi alloys. For example, recent in-situ studies have shown that the root cause of decreasing solder electrical resistance in the early stage of electromigration, very evident at low temperatures and low current densities, is due to the microstructural coarsening of the alloy. When results are summarized on Arrhenius plots, the prediction of electromigration life under any given field condition becomes straightforward.

Because of their high thermal and electrical conductivity, copper (Cu) sintering pastes were developed to meet the requirements for high-power applications. Compared to mature silver (Ag) sintering pastes, which are also designed for high-power applications, the Cu sintering paste advantages, such as low cost and insusceptibility to Ag migration, are appealing. Through extensive research, another important feature for Cu sintering paste was found: Cu sintering paste sinters well on bare Cu. In other words, no special precious metal plating (e.g. gold (Au) or Ag) is required for die or activate metal brazing (AMB). This eliminates the step of plating with Au or Ag for die or AMB and can potentially reduce costs. In this paper, pressure and pressure-less Cu sinter pastes with high-reliability were developed for high-power applications. The bonding performance of both pastes was determined on the Cu substrate. At 250-270°C, 15-25 MPa, 3-7.5 min, and under a nitrogen (N<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2)</inf> atmosphere, the pressure Cu sintering paste reached 40-70 MPa shear strength. For pressureless Cu sintering paste, a shear strength of >25 MPa was demonstrated under the sintering conditions of 250°C, 30 min, and an N<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> atmosphere. Meanwhile, both pressure and pressureless Cu pastes have passed 1,000 cycles of thermo-cycling tests (TCT) (-65-150°C), which indicates its high-reliability.

ABSTRACT This paper is only available in Chinese.

For System-in-Package (SiP) assembly, solder paste printing remains the preferred solder application method of choice. However, as passive components (008004) and next-generation flip-chip components continue to shrink, smaller stencil openings are needed and these small stencil apertures still need to achieve consistent printing. In addition, as component standoff height continues to decrease for fine-pitch applications, the cleanability of the final SiP build becomes a concern; for many applications which require water-soluble pastes, pure DI water becomes difficult to use when the standoff height lessens. This paper will investigate important parameters from a materials supplier’s perspective for meeting the challenges of fine-feature solder paste printing and package assembly. First, innovations in solder powder technology will be discussed which allow for consistent solder paste (Type 7) printing at 60um apertures. Next, the effect of stencil technology will be discussed. Many stencil manufacturers are approaching a manufacturing limit both with respect to stencil thinness and aperture size. This paper will discuss two common stencil manufacturing designs, laser-cut and electroform, and present findings on the effect these designs have on solder paste print consistency. Lastly, solder paste cleanability will be discussed; the results of a study investigating novel Type-7 solder pastes for use with semi-aqueous cleaning technology will be presented. Important process considerations with respect to stencil and package cleaning will also be discussed.

In this paper, a low-temperature Pb-free soldering solution for high power density power electronics is discussed. To this point, a critical trade-off between cost, complexity, and performance has been required when designing in a thermal management/mechanical attach solution for power modules. Here, a SAC-In alloy solution is presented in a solder preform configuration that can be used with reflow temperatures below that of industry standard lead-free and/or high temp. solders. This alloy is a drop-in replacement relative to existing soldering processes and offers improved thermal performance beyond organic-based thermal management solutions (e.g., thermal grease, silicones, PCM’s) [1]. Further, with a peak reflow temperature below ~215 ° in molded package-to-cooler attach soldering applications, this SAC-In solution prevents delamination within the module assembly. In addition, the reduced processing temperature removes the risk of damage to components and materials in molded package power modules, mitigates warpage and material stress issues during the soldering process, reduces energy consumption, and provides a consistent, repeatable process for high volume manufacturing (HVM). This Bi-free alloy offers mechanical properties and ductility that contribute to robust lifecycle performance comparable to standard Pb-free alloys as shown by thermal shock testing -40°/+125°C.

ABSTRACT Lower temperature soldering has been considered as one of the effective ways to reduce the risk of warpage on PCB assembly. Use of lower-temperature solders, including a BiSnAgCu eutectic and two In-containing, Bi-free solders DFLT and DFLT-2, has been attempted to reflow the 40mmx40mm plastic ball grid array (PBGA928). BiSnAg, under the combinations of reflow profiles P185 and P200 (~185°C and ~200°C peak temperature) and the paste-to-ball (P2B) volume ratio (0.13, 0.25, and 0.5), always formed defective joints. Under the P200 profile with P2B volume ratio of 0.5, SAC305/BiSnAg joints were dominated exclusively by hot-tearing and shrinkage voids despite forming the desired drum shape. Using P220 profile (~220°C peak temperature, right above the liquidus temperature of SAC305) with P2B ratio of 0.13, SAC/BiSnAg had presented the drum shape joint and free of hot-tearing and shrinkage voids. With the constant P2B volume ratio of 0.13, P200 also renders various malformed joints for both DFLT and DFLT-2. The major challenge will be the insufficient molten solder volume under low temperature soldering to accommodate the displacement caused by the warpage. Increasing reflow temperature improved the joint shape and greatly reduced the defects for both DFLT and DFLT-2. Under P220, both SAC/DFLT and SAC/DFLT-2 joints achieved the desired short-and-fat drum shape, comparable to those reflowed under P240. The formation of the optimal joint for all three pastes under the hot profiles is attributed to the sufficient liquid solder volume since both paste and SAC ball fully melt and merge, which compensates the displacement caused by the dynamic warpage. Both SAC/DFLT and SAC/DFLT-2 joints exhibited the limited hot-tearing under the P200 profile while SAC/BiSnAg joints were dominated by those defects. The different metallurgy of DFLT and DFLT-2 did not result in an enlarged pasty range after joining with SAC305 and thus led to the lower defect rate. Investigation of the joint reliability performance is still ongoing.