Query: Die attach

title: A Study of Dispense Needle for Die Attach Voids Mitigation
abstract: The need of data storage devices and new technologies continues to grow with more robust and stable functioning capability. With that, improvement and utilization never stop for a seamless output. One device that is currently on new product introduction is a quad flat no leads (QFN) utilizing a tapeless leadframe technology. The new device used highly conductive glue with metal spacer and has experienced gross glue voids defect parts per million (ppm) Glue voids were measured for cumulative voids criteria, with values higher compared to the specification. The study used analysis of variance (ANOVA) on the dispense needle diameter and revealed the effect levels of needle diameter on cumulative voids reduction. For subsequent works, the configuration could be applied for packages with similar requirement.

title: Microstructural and mechanical evolution of silver sintering die attach for SiC power devices during high temperature applications
abstract: Silver sintering is a promising die attach technology to ensure high thermal reliability. The long term reliability of SiC device sintered by nano Ag paste has been evaluated by the high temperature storage (HTS) process at 350 degC in air and vacuum, respectively. Although the SiC chip and direct bonding copper (DBC) substrate could be bonded firmly by the nano Ag paste after sintering at low temperature, the microstructure and shear strength of sintered die attachment experienced the huge evolution during HTS process. The bondline of die attachment became compact, and some pores grow up and pore distribution became nonuniform after HTS in air. While the densification of bondline was significantly delayed because residual organics inhibited the growth and migration of pores during HTS in vacuum. The shear strength of die attachment first increased then decreased slowly with the increasing of storage time. The fracture surface showed that the Ni(P) layer was oxidized, and the formed NiO layer provided the failure location. The results indicated that the electroless nickel/immersion gold (ENIG) surface of DBC substrate was not the ideal metallization when the sintered die attachment applied at the long term high temperature.

title: Failure Mechanism of Die Attach Solder Joints in IGBT Modules Under Pulse High Current Power Cycling
abstract: Applications under extreme conditions, such as solid circuit breakers and electromagnetic launching systems, are great challenges to semiconductor power devices. The die attach solder joint is as one of the most vulnerable structures and critical to the reliability of insulated gate bipolar transistor (IGBT) modules. In this paper, IGBT modules were cross sectioned and tested under pulse high current power cycling. The failure mechanism of the die attach solder in IGBTs at pulse high current modes was investigated. Evolution of microdefects in the die attach solder during power cycling was characterized and factors for the failure of die attach solder joints was discussed. The results revealed that voids, cracks, and detachment of interface were the major microdefects in the die attach solder layer. A detachment of the Si/Sn Ag Cu (SAC) interface is verified as the major failure mode under pulse high current power cycling. Interface cracks between the Si chip and die attach solder layer were found to initiate first at the solder layer edges and then extended to the center of the solder layer with the increase of power cycles. The detachment of Si/SAC interface was more similar to the brittle fracture. The junction temperature swing and heating rate were the key factors for detachment of the Si/SAC interface.

title: Pressureless die attach by transient liquid phase sintering of Cu nanoparticles and Sn 58Bi particles assisted by polyvinylpyrrolidone dispersant
abstract: Highly reliable bonding materials have attracted tremendous interest due to a growing demand for high temperature electronics. We developed a fluxless and binder free paste comprising Cu nanoparticles (NPs) Sn 58Bi (SnBi) particles, and polyvinylpyrrolidone (PVP) dispersing agent, which enables pressureless, low temperature (190 250 degC) formation of robust joints (over 7 MPa) by means of transient liquid phase sintering (TLPS) Microstructural evolution of the joint was investigated under variations in PVP molecular weight (MW; 10,000, 55,000, 360,000, or 1,300,000) and bonding conditions including temperature and time. In a die shear test, the joint formed with PVP MW 360,000 was the strongest due to its proper particle dispersion and the formation of intermetallic compounds (IMCs) Conditions of excessive PVP MW, bonding temperature, and time impeded the bonding characteristics of the TLPS joint, with formation of voids and increasing brittleness. TLPS bonding with the optimal dispersing agent enabled pressureless die attach without chip damage, demonstrating applicability as a simple, robust interconnection for high temperature electronics.

title: Reliability Evaluation of SiC Power Module With Sintered Ag Die Attach and Stress Relaxation Structure
abstract: Silicon carbide (SiC) power modules with Ag sinter bonding die attach were designed on the basis of thermal stress analysis for reliable high temperature operations. Both the finite element analysis (FEA) simulations and preliminary experiments confirmed that inserting the direct bonded copper (DBC) substrates can effectively reduce the maximum thermal stress in the module. A prototype SiC power module using sintered Ag die attach with a DBC substrate was designed and fabricated. The modules exhibited excellent durability in power cycling between 65 degC and 250 degC up to 20 000 cycles. FEA calculations of cumulative thermal strain and stress distributions adequately predicted the initial cracking position in the specimens after prolonged power cycles, observed by scanning electron microscopy.

title: Thermal Cycling of Sintered Silver (Ag) Joint as Die Attach Material
abstract: Sintered silver is a promising die attach material capable of operating at temperatures of more than 200degC. However, while sintering reliably bonds Ag paste on Ag plated substrate, reliability studies are needed to understand the behavior of this sintered bond on copper and direct bond copper (DBC) substrates. Here, we thermally cycled micron Ag and nano Ag joints created via sintering on Cu, DBC, and Ag plated substrates between 65degC and 150degC in order to understand their evolving microstructures and reliability. Short periods at high temperature did not oxidize the substrate, but the absence of copper oxides did not prevent adhesive failure of the nano Ag joint at the Cu interface. We found that Ag filler size influences pore shapes, pore sizes, and shear strength; the micron Ag joint produced a mixture of irregular and regular spherical pore shapes that reduced bond strength more than the predominantly spherical pores present in the nano Ag joint.

title: Development of high strength and superior thermal shock resistant GaN/DBA die attach structure with Ag sinter joining by thick Ni metallization
abstract: With the advent of wide band gap (WBG) semiconductors like silicon carbide (SiC) and gallium nitride (GaN) the operation temperature of WBG power modules are expected to be over 250 degC. High temperature die attach materials that can withstand these harsh conditions are emerging as one of the main issues of the reliability of power electronic applications. In this study, high strength and superior thermal shock resistant GaN/DBA die attach structures with Ag sinter joining by thick Ni metallization (7 mm) were developed and compared to conventional Pb 5Sn soldering die attach. A thermal shock test was conducted at a temperature range of 50/250 degC, holding 30 min at each extreme temperature up to 500 cycles. Consequently, the shear strength of the Ag sinter joining structure was sustained above the initial strength after 500 cycles, which showed durable sinter necks and non deformed interfaces due to the thick Ni metallization. On the other hand, the Pb 5Sn solder joined structure showed a 50% reduction in shear strength after 500 cycles compared to the initial state. Some disadvantages of these solder joints occurred such as an intermetallic compound and interface degradation. Thus, with the new design and evaluation reported in this study, Ag sinter joining can be expected as an environment friendly replacement die material for the long term reliability of WBG semiconductor module packaging.

title: A Brief Review on High Temperature, Pb Free Die Attach Materials
abstract: High Pb solders have been used as die attach and interconnect materials for decades in discrete power components. Because of the known harmful effects of Pb to human health and the environment, as well as the demand of the wide band gap SiC/GaN power devices serving under high power density and high junction temperature, alternative Pb free solders/materials and solutions have been studied intensively. Pb free alternatives to replace high Pb solders are still in their infancy. The exemption of using high Pb solders has been extended to 2021, although it may be terminated anytime if a new technology or material were to be accepted industry wide. The potential high temperature Pb free materials, including solders (AuSn/AuSi/AuGe, ZnAl and BiAgX(r) Ag sintering material, and transient liquid phase bonding (TLPB) material, are reviewed in this paper with an emphasis on BiAgX(r) solders. The extremely high cost of Au based solders limits their applications, although they have been used in high temperature electronics. Zn Al has a high melting temperature, good mechanical properties, and good thermal performance. However, the high reactivity makes Zn Al solders only available in wire and preform. BiAgX(r) solder (available in paste, wire and preform) has been developed to replace the high Pb solders for use in low power discrete components, with relatively low cost and better reliability than the high lead counterpart. Ag sintering material and TLPB material form joints by atomic diffusion (either solid diffusion for sintering or solid liquid interdiffusion for TLPB) The challenges in mass production and the cost have restricted their success to be widely used in industry. Although it is likely that none of these materials/technologies are ideal to satisfy all the requirements of the variety of high temperature Pb free die attach applications in terms of processing/reliability/cost, each solution has the potential to satisfy a niche within this broader categorization.

title: Effects of Voids on Mechanical and Thermal Properties of the Die Attach Solder Layer Used in High Power LED Chip Scale Packages
abstract: High power light emitting diode (LED) chip scale packages (CSPs) prepared by the flip chip technology have become one of the most promising light sources. The die attach solder layer always plays an important role in heat dissipation, mechanical support, and electronic conductivity. Among different types of solder materials, Sn 3.0Ag 0.5Cu (SAC305) solder alloy shows its great competitiveness on solderability and mechanical properties for the interconnection of high power LED CSPs. However, reliability problems caused by voids in the SAC305 solder limit its wide application in the high power LED chip scale packaging process. Existence of the voids has been considered as one of the major issues causing chip on substrate level reliability problems in microelectronic and optoelectronic devices. In this paper, mechanical and thermal properties of SAC305 solder layers with arbitrary voids used in high power LED CSPs are studied with both finite element simulations and experiments. The results show that void size and void position within the solder layer are the two most critical issues on the shear strength of interconnection and the chip on substrate level thermal distribution in high power LED CSPs.

title: Thermostable Ag die attach structure for high temperature power devices
abstract: This paper explores the possibility of using Ag paste containing silicon carbide particles (SiC p) as a novel high temperature die attachment solution for the design of power devices. The bonding structure used in this research was composed of silicon dies and a direct bonded copper (DBC) substrate. A SiC p/microporous Ag composite structure was prepared by sintering a Ag microflake paste containing 2 wt% sub micron SiC p under mild conditions (250 degC and 0.4 MPa for 30 min) In addition to the Ag paste, the surface metallization of the DBC substrate was also evaluated in this research. Ag metallization layers deposited by electroplating and sputtering were compared, along with samples also containing a titanium (Ti) diffusion barrier layer between Cu and Ag. The results indicated that the SiC p containing Ag sinter paste showed better stability in storage tests than the paste without SiC p at the temperatures such as 150, 250 and 350 degC. Additionally, the Ti diffusion barrier layer played an active role in preventing the oxidation of Cu and inter diffusion between Cu and Ag during use at high temperatures exceeding 250 degC. The joint bonded by SiC p containing Ag paste on DBC substrate with Ti barrier layer exhibited excellent stability up to 1000 h at 150 and 250 degC.