Goran Matijasevic

A new bonding technology using gold and tin multilayer composite structures

A bonding technology which utilizes chromium, gold, and tin and gold deposited directly on the backside of a device die to form a multilayer composite is reported. The substrate accepting the die is coated with chromium and gold layers. The die and the substrate are brought into contact and heated to 310-320 degrees C. Due to the unique feature of the gold-tin alloy system, the tin layer melts first and dissolves the gold layers of the composite to produce a solution mixed with solid, which in turn would dissolve a portion of the gold layer on the substrate to develop a near eutectic bonding. In the composite, since the tin layer is protected by an outer gold layer in the same vacuum cycle, tin oxidation, which is a major cause of difficulty in achieving quality bondings, is reduced. This technology thus eliminates the requirement of preforms, prevents tin oxidation, and provides precise control of the bonding thickness. Results of bonding 4-mm by 4-mm GaAs dice on alumina substrates show that high-quality bondings are obtained as determined by a scanning acoustic microscope (SAM). >

AuSn alloy phase diagram and properties related to its use as a bonding medium

Abstract AuSn eutectic alloy has been successfully used in microelectronic packaging for high reliability applications where a hard solder as well as a low processing temperature are required. A new multilayer bonding technology not only has produced nearly perfect bonding but also has reduced the processing temperature even below the eutectic melting point. Knowledge of the different phases of the alloy and their formation, as well as the interdiffusion that occurs, thus becomes important in studying the bonding principle and the long-term reliability. In this paper, we review a large number of publications on the AuSn system and summarize the important properties. We hope that this summary would further enhance the development of new AuSn bonding methods as a result of an overall understanding of oxidation and diffusion properties.

Au-In bonding below the eutectic temperature

A bonding method using Au-In alloy which requires a low process temperature of 200 degrees C to produce high temperature (454 degrees C) bonds is reported. Multiple layers of Au and In are deposited on semiconductor wafers in one vacuum cycle to reduce In oxidation. The semiconductor dice are then bonded to substrates coated with Au. Above 157 degrees C, the indium layer melts and dissolves the Au layers to form a mixture of liquid and solid. The solid-liquid interdiffusion process continues until the mixture solidifies to form the Au-In bond. A scanning acoustic microscope (SAM) was used to determine the excellent bonding quality before and after thermal shock tests while an energy dispersive X-ray (EDX) was employed to determine the composition of the resulting bonds. The resulting bond has an unbonding temperature greater than 545 degrees C. Due to the low process temperature, the stress on the bonded structure caused by thermal expansion mismatch is reduced. This type of bonding is useful when bonding at a low temperature is followed by a subsequent higher temperature process. >

Void–free Au–Sn eutectic bonding of GaAs dice and its characterization using scanning acoustic microscopy

A new technique to produce perfect bonding between GaAs dice and alumina substrates is reported. Utilizing this technique, void-free bondings have been achieved consistently. The quality of the bonded devices is confirmed by a Scanning Acoustic Microscope (SAM) having a spatial resolution of 25 µm. Thermal cycling between -25° C and 125° C, and thermal shock between -196° C and 135° C, have been used to assess the reliability of the specimens. The SAM was used to study the variation of the bonds in the tests. After the tests, the bonds show no sign of degradation and the GaAs dice did not crack. Shear test has also been performed. All the well bonded specimens passed the shear test. The shear strength correlated very well with the SAM images of the specimens taken before the test.

Void free bonding of large silicon dice using gold-tin alloys

The successful bonding of large (6 mm*10 mm) silicon dice on alumina substrates with Au-Sn eutectic using a particular bonding technique is described. The bonding quality was examined by a scanning acoustic microscope having a resolution of 25 mu m and it was found that nearly perfect bondings have been achieved. Use of Au-Sn alloy rather than Au-Si resulted in not only lower processing temperature but also lower stress on the dice. This is confirmed by simple stress analysis. The die-bonded specimens endured 40 cycles of thermal shock between -196 degrees C (liquid nitrogen) and +160 degrees C (boiling cyclohexanol) without cracking or bond degradation despite the significant mismatch of thermal expansion coefficients between silicon and alumina. Storage tests at -196 and 250 degrees C also do not induce cracking or bond degradation. Pull test results indicate that the bondings are stronger than the silicon dice themselves. >

Transient liquid phase sintering conductive adhesives as solder replacements

New transient liquid phase sintering conductive adhesives, which have an interpenetrating polymer/metal network, have been developed to mitigate some of the deficiencies of standard particle-filled conductive adhesives. The metal network is formed in situ by a process known as transient liquid phase sintering (TLPS) and is mutually reinforcing with the polymer network. Bulk as well as interface electrical connections are metallurgically alloyed providing stable electrical and thermal conduction. These new conductive adhesive compositions are compatible with bare copper as well as alloy finishes. The TLPS conductive adhesives utilize conventional surface mount technology dispensing and processing equipment. In testing this type of adhesive for surface mount component attach, boards were subjected to 7-foot drop tests. All components, including J-lead, gull wing and leadless, survived this test, as would be expected of typical solder joints; most traditional conductive adhesives did not. Electrical conductivity results also indicate values closer to those of traditional solder alloys. Furthermore, reliability testing including humidity followed by air-to-air thermal shock (-55/spl deg/C to +125/spl deg/C) has demonstrated that this type of adhesive performs substantially better than standard, passive filler loaded conductive adhesives.

Highly reliable die attachment on polished GaAs surfaces using gold-tin eutectic alloy

GaAs test dice with polished surfaces were successfully bonded on alumina substrates using Au-Sn alloy. The bonding process was carried out in H/sub 2/ or N/sub 2/ atmosphere with applied static pressure, but without the use of scrubbing. The quality of the bonds was examined by a scanning acoustic microscope (SAM) having a spatial resolution of 25 mu m. After 100 cycles of thermal shock between -196 degrees C to +160 degrees C, perfectly bonded devices remained perfect, as confirmed by the SAM images. No void was induced, and the dice did not crack. The results of a shear test indicate that the strength of the bondings is greater than that of the GaAs dice. >

Extremely reliable bonding of large silicon dice using gold-tin alloy

Large silicon dice have been bonded on alumina substrates with Au-Sn eutectic using a novel bonding technique. The bonding quality was examined by a scanning acoustic microscope (SAM) having a resolution of 20 mu m. Nearly perfect bondings have been achieved. Au-Sn alloy was chosen rather than the commonly used Au-Si alloy because the lower eutectic point of the Au-Sn system would result not only in lower processing temperature but also lower stress on the dice. The die-bonded specimens endured 40 cycles of thermal shock between -196 degrees C and +160 degrees C without cracking or bond degradation despite the significant mismatch of thermal expansion coefficients between silicon and alumina. Storage tests at -196 degrees C and +250 degrees C also do not induce cracking or bond degradation. Pull-test results indicate that the bondings are stronger than the silicon dice themselves.<<ETX>>

A fluxless oxidation-free bonding technology

A fluxless oxidation-free bonding technology is reported. The technology uses the direct deposition of multilayer composite in high vacuum to prevent oxidation. The outer layer of the composite is either gold or copper which protects the inner layers from oxidation when the composite is later exposed to atmosphere. As a result of oxidation prevention neither flux nor scrubbing action is required in the bonding process. Two processes based on Pb-Sn-Au and Sn-Cu have been developed for a processing temperature of 250/spl deg/C to prove the working concept. GaAs dies have been well bonded on glass and alumina substrates as shown by scanning acoustic microscope. SEM and EDX studies verify the bonding principle, reveal the bonding mechanism and identify the intermetallic compounds in the joints. Apart from fluxless feature, other advantages include good control of joint thickness, good control of composition, and direct deposition of bonding media on wafers or substrates.<<ETX>>

Materials selection issues for high operating temperature (HOT) electronic packaging

Currently, there are two major drivers for high operating temperature (HOT) electronics in the 135-200/spl deg/C range. Products with significant heat generation such as power electronics or small portable electronics without space for cooling mechanisms provide a large market segment in the low end of this temperature range. The upper end of the temperature regime is represented by products that are placed into a HOT environment such as distributed sensors and control systems. Use of polymers for packaging in these applications is typically hampered by the low thermal conductivity and thermal degradation resistance of polymeric materials used in low-cost laminate PWB technology. Also, HOT applications generally require exposure to rigorous thermal cycling for power up and down and environmental exposure. Classes of polymeric materials appropriate for the various aspects of electronic packaging at high operating temperatures and -55/spl deg/C-225/spl deg/C cycling are discussed. Also, a new electronic packaging technology which uses some elements of laminate, except that circuits are directly deposited on insulated metal substrates, is presented. This packaging has several-fold better thermal conductivity than conventional ceramic or polyimide glued-to-heat-sink HOT packaging, is more mechanically robust than ceramic, is lightweight and compact and can be deposited on 3D surfaces to minimize space requirements. Multilayer circuits with dimensions as small as 50 /spl mu/m lines and spaces and 75 /spl mu/m vias are possible, as well as large dimension circuits for power applications. This approach is also more cost effective than conventional HOT packaging.