Pedro O. Quintero

Temperature Cycling Reliability of High-Temperature Lead-Free Die-Attach Technologies

The demand for electronics capable of operating at temperatures above the traditional 125°C limit continues to increase. Devices based on wide bandgap semiconductors have been demonstrated to operate at temperatures up to 500°C, but packaging remains the major hurdle to product development. Recent regulations, such as RoHS and WEEE, increase the complexity of the packaging task by prohibiting the use of certain materials, such as lead, in electronic products. Traditionally, lead has been widely used in high-temperature solder attach. In this paper, a series of Pb-free die-attach technologies have been identified as possible alternatives to Pb-based ones for high-temperature applications. This paper describes the fabrication sequence for each system and assesses their long-term reliability using accelerated thermal cycling and physics-of-failure modeling. The reliability of the lead-rich alloy was confirmed during this investigation, while early failures of the silver-filled epoxy demonstrated their inability to survive high temperatures. An empirical damage model was developed for the silver nanoparticle paste based on fatigue-induced failures. Encouraging reliability data have been presented for the gold-tin solid-liquid interdiffusion system where bond quality was demonstrated to be a critical factor in its failure mode and mechanism.

Thermal Stability Characterization of the Au–Sn Bonding for High-Temperature Applications

There is a need for electromechanical devices capable of operating in high-temperature environments (>200°C) for a wide variety of applications. Todays wide-bandgap semiconductor-based power electronics have demonstrated a potential of operating above 400°C, however, they are still limited by packaging. Among the most promising alternatives is the Au-Sn eutectic solder, which has been widely used due to its excellent mechanical and thermal properties. However, the operating temperature of this metallurgical system is still limited to ~250°C owing to its melting temperature of 280°C. Therefore, a high-temperature-resistant system is much needed, but without affecting the current processing temperature of ~325°C, typically exhibited in most high-temperature Pb-free solders. In this paper, we present the development and characterization of a fluxless die-attach soldering process based on gold-enriched solid-liquid interdiffusion (SLID). A low-melting-point material (eutectic Au-Sn) is deposited in the face of a substrate, whereas a high-melting-point material, gold in this instance, is deposited in its mating substrate. Deposition of all materials was performed using a jet vapor deposition (JVD) equipment where thicknesses are controlled to achieve specific compositions in the mixture. Sandwiched coupons are isothermally processed in a vacuum reflow furnace for different reflow times. Post-processed samples confirm the interdiffusion mechanism as evidenced by the formation of sound joints that prove to be thermally stable up to ~490°C after the completion of the SLID process. Differential scanning calorimetry demonstrate the progression of the SLID process by quantifying the remaining low-melting-point constituent as a function of time and temperature, this serving as an indicator of the completion of the soldering process. Mechanical testing reveals a joint with shear strength varying from 39 to 45.5 MPa, demonstrating to be stable even after 500 h of isothermal aging. Moreover, these investigations successfully demonstrate the use of the Au-Sn SLID system and the JVD technology as potential manufacturing processes and as a lead-free die-attach technology.

Silver-Indium Transient Liquid Phase Sintering for High Temperature Die Attachment

The demand for electronics capable of operating at temperatures above the traditional 125°C limit continues to increase. Devices based on wide band gap semiconductors have been demonstrated to operate at temperatures up to 500°C, but packaging remains a major hurdle to product development. Recent regulations, such as RoHS and WEEE, increase the complexity of the packaging task as they prohibit the use of certain materials in electronic products such as lead (Pb), which has traditionally been used in high temperature solder die attach. In this investigation, an Ag-In solder paste is presented as a die attach alternative for high temperature applications. The proposed material has been processed by a transient liquid phase sintering method resulting in an in situ alloying of its main constituents. A shift of the melting point of the system, confirmed by differential scanning calorimetry, provided the basis for a breakthrough in the typical processing temperature rule. The mechanical integrity and reliabilit...

Reliability assessment of high temperature lead-free device attach technologies

The demand for electronics capable of operating at temperatures above the traditional 125degC limit continues to increase. Devices based on wide band gap semiconductors have been demonstrated to operate at temperatures up to 500degC, but packaging them remains major hurdle to product development. Recent regulations, such as RoHS and WEEE, increase the complexity of the packaging task as they prohibit the use of certain materials in electronic products such as lead, which has traditionally been widely used in high temperature solder attach. In this investigation, a series of Pb-free die attach technologies have been identified as possible alternatives to Pb-based ones for high temperature applications. This paper describes the fabrication sequence for each system and assesses their long term reliability using accelerated thermal cycling and physics-of-failure modeling. The reliability of the lead rich alloy was confirmed during this investigation while early failures of the silver filled epoxy demonstrated their inability to survive high temperatures. An empirical damage model was developed for the silver nanoparticle paste based on fatigue induced failures. Encouraging reliability data has been presented for the gold-tin SLID system where bond quality was demonstrated to be a critical factor on its failure mode and mechanism.

High Temperature Lead-Free Attach Reliability

The increasing demand for electronics capable of operating at temperatures above the traditional 125°C limit is driving major research efforts. Wide band gap semiconductors have been demonstrated to operate at temperatures up to 500°C, but packaging is still a major hurdle to product development. Recent regulations, such as RoHS and WEEE, increase the complexity of the packaging task as they prohibit the use of toxic materials in electronic products; lead being a major concern due to its widespread use in solder attach. In this investigation, a series of Pb-free die attach technologies have been identified as possible alternatives to Pb-based materials for high temperature applications. This paper describes the fabrication sequence used to create attachments with these materials and their resultant microstructure. The long term reliability is also determined by accelerated thermal cycling and physics-of-failure modeling.Copyright

Microstructural Stability of Au-Sn SLID Joints for Harsh Environments

Solid liquid inter diffusion (SLID) is an interconnection technique for electronic packaging, particularly beneficial for high power and harsh environments conditions. It consists of the bonding of two materials with different melting points at a low processing temperature to achieve a high melting point interconnection. The materials investigated in this work are a gold-tin bond attaching a SiC diode to an AlN direct-bond-copper (DBC) substrate. Gold (Au) is the high melting point constituent while the eutectic gold-tin (80 wt.% Au-20 wt.%Sn) offers the low melting point (280°C). This work is aimed at the microstructural evaluation of the joints at different bonding and aging conditions in an effort to get the insights of this interconnection technology from a metallurgical perspective. Four different bonding conditions were used: 315°C-5min, 315°C-10min, 340°C-1min and 340°C-5min; from which a base-line as built condition was assessed by means of metallographical analysis. Furthermore, the samples were aged at 250°C from 1000 to 4000 hours in increments of 1000hrs to study and quantify the microstructural stability and intermetallic (IMC) growth at the interface. This aging experiment has been designed to obtain accelerated information on the kinetics of this reaction so that predictive models can be developed for the real application conditions. The samples were diced, polished and analyzed following standard metallographical techniques; both optical and electronic microscopy (SEM-EDS) was employed. The as-built samples, for the four bonding conditions, presented differences in IMC growth with the thickest layers appearing at the harshest processing conditions. After aging the IMC kept growing and the formation of a new IMC layer was discovered and investigated, furthermore, cracks started to show in some of the samples. It was observed that after 4000 hours some of the cracks extended across the whole interface.Copyright

High Temperature Die Attach by Low Temperature Solid-Liquid Interdiffusion

There is a need for electromechanical devices capable of operating in high temperature environments (>200°C) for a wide variety of applications. Today’s wide-bandgap semiconductor based power electronics have demonstrated a potential of operating above 400°C, however they are still limited by packaging. Our group has been conducting research in novel interconnect technologies to develop reliable electronic packaging for high temperature environments. Among the most promising alternative is the Au-Sn eutectic solder (80 wt.% Au - 20 wt.% Sn), which have been widely used due to its excellent mechanical and thermal properties. However, the operating temperature of this metallurgical system is still limited to ∼250°C owing to its melting temperature of 280°C. Therefore, a higher temperature resistant system is much needed, but without affecting the current processing temperature of ∼325°C typically exhibited in most high temperature Pb-Free solders. This paper presents the development and characterization of a fluxless die attach soldering process based on gold enriched solid liquid inter-diffusion (SLID). A low melting point eutectic Au-Sn was deposited in the faces of two substrates, followed by the deposition of a subsequent layer of high melting point material, gold in this instance, in one of the substrates. Deposition of all materials was performed using Jet Vapor Deposition (JVD) equipment where thicknesses were controlled to achieve specific compositions in the mixture. Sandwiched coupons where isothermally processed in a vacuum reflow furnace. Scanning electron microscopy (SEM) was employed to reveal the microstructural evolution of the samples in order to study the interfacial reactions of this fluxless bonding process. EDS analysis was used to identify the intermetallic formation and to characterize the joint in an attempt to study the kinetics of this diffusion couple. Post-processed samples confirmed the inter-diffusion mechanism evidenced by the formation of sound joints between the two substrates. As expected, it was observed that the Au was dissolved into the eutectic Au-Sn as the reflow time and temperature were increased.Copyright

Microstructural Evolution and Thermal Stability Characterization of a High Temperature Die Attach Lead-Free System

There is a need for electromechanical devices capable of operating in high temperature environments (>200°C) for a wide variety of applications. Todays wide-bandgap (WBG) semiconductor based power electronics have demonstrated a potential of operating above 400°C, however they are still limited by packaging. Among the most promising alternative is the Au-Sn eutectic solder, which have been widely used due to its excellent mechanical and thermal properties. However, the operating temperature of this metallurgical system is still limited to ∼250°C owing to its melting temperature of 280°C. Therefore, a higher temperature resistant system is much needed, but without affecting the current processing temperature of ∼325°C typically exhibited in most high temperature Pb-Free solders. This paper presents the development and characterization of a fluxless die attach soldering process based on gold enriched solid liquid inter-diffusion (SLID). A low melting point material (eutectic Au-Sn) was deposited in the fac...

Pressureless Sintering Progression of a Silver Nanoparticle Attach Material Via Micrographic Analysis

Increasing demand for reliable power electronics for harsh environments, together with regulations in packaging materials, have resulted in new efforts in research and development in this area. The need for an attach material that can be processed at a relatively low temperature yet able to withstand the expected high temperature operating conditions is of particular interest. In this work, sintering of silver nanoparticles is examined through microstructural evolution. An image analysis technique was developed and used to analyze SEM micrographs. Results showed a strong correlation between characteristic structural features and the degree of nanoparticle arrangement prior to the sintering stage. The initial ordering of the nanopowder within the paste, and the availability of smaller particles acting as satellites were found to be pivotal for the progression of the sintering as observed from the formation of continuous structures at a larger length scale. The advancement presented in this work is a useful...

Study of the Thermomechanical Inelastic Energy Response of Backward Compatible Solder Joints Made With Sn-3.8Ag-0.7Cu versus Reballed Sn37.0Pb Components

Conflicting results in reliability tests for backward compatible and Pb-free soldered assemblies has motivated RoHS-exempted industries to practice reballing. Reballing is the name given to the process of removing Pb-free solder balls from the copper (Cu) pads of the Ball Grid Array (BGA) components received through the supply chain and replacing them with SnPb solder balls. Recent studies on the subject of reballing have shown the possibility that the removed Pb-free solder ball leaves behind some intermetallic remnants of the Pb-free solder alloy and the Cu from the pads. A modeling approach based on physics of failure (PoF) is presented that quantifies the interactions between different thermal cycles applied to reballed Ball Grid Arrays (BGA) with remnants of the Pb-free solder alloy on the Cu pads. These resulting interactions are compared to backward compatible Sn-3.8 Ag-0.7Cu (SAC) balls soldered with eutectic SnPb paste for the same thermal cycles. For the latter, the risk of having improper mixing during the assembly process is also studied. The approach is formulated at the microscale, incorporating physical mechanisms of the intermetallics created with Cu, and at the macroscale, capturing the creep phenomenon of the bulk solder as dominant failure driver. Simulation results show that the reballed cases have higher inelastic energy density per cycle averaged over damage volume near the copper pads and that the inelastic energy density is higher across the bulk of the improperly mixed backward compatible solder balls when compared to properly mixed backward compatible solder balls. The results of this study permit extrapolation of laboratory results to field life predictions and to explore the design of accelerated re-balled or backward compatible BGA tests that relate better to application-specific usage environments.Copyright