40 μm Ag/Au Flip-Chip Joints by Solid-State Bonding at 200°C

2013 ◽  
Vol 10 (3) ◽  
pp. 120-127
Author(s):  
Wen P. Lin ◽  
Chu-Hsuan Sha ◽  
Chin C. Lee
Keyword(s):  
Solid State ◽  
Flip Chip ◽  
Coefficient Of Thermal Expansion ◽  
Bonding Interface ◽  
Breaking Force ◽  
Sem Images ◽  
Cross Sectional ◽  
Cu Substrate ◽  
Solid State Bonding ◽  
State Process

In this research, 40 μm silver/gold (Ag/Au) composite flip-chip interconnect joints between silicon (Si) chips and copper (Cu) substrates were demonstrated. The bonding was achieved by a solid-state process at a low temperature of 200°C for 5 min with the pressure applied at 250–400 psi (1.7–2.7 MPa), corresponding to 0.22–0.35 g of force per joint. To begin with, an array of 50 × 50 30 μm Ag/10 μm Au columns with 40 μm in diameter and 100 μm in pitch was fabricated by photolitho-graphic and electroplating processes on silicon (Si) chips which were first coated with chromium (Cr) and Au films. The columns on the chip were then bonded to a Cu substrate by solid-state bonding. Cross-sectional scanning electron microscopy (SEM) images show that the exposed Ag/Au columns were well bonded to the Cu substrate. No joint breakage was observed despite the large coefficient of thermal expansion (CTE) mismatch between Si and Cu. A pull test was conducted. The breaking force and fracture strength are 6.5–7.3 kg and 2,940–3,310 psi (20.2–22.8 MPa), respectively. The breaking force is 2.5× of the criterion specified in MIL-STD-883E. Fracture modes were examined. Three modes were classified by fracture interfaces as Si-glue, Si/Cr/Au/Ag, and Au-Cu bonding interface. Of all joints evaluated, 27% of them break on the Au-Cu substrate bonding interface. Accordingly, the bonding interface is least likely to break among interfaces of the joint structure.

Silver Flip-Chip Technology by Solid-State Bonding

2010 ◽  
Vol 132 (3) ◽  
Author(s):  
Pin J. Wang ◽  
Chin C. Lee
Keyword(s):  
Solid State ◽  
Flip Chip ◽  
New Technology ◽  
Bonding Process ◽  
Cu Substrate ◽  
Solid State Bonding ◽  
Chip Technology ◽  
Cu Substrates ◽  
One Step ◽  
Lead Free Solders

Silver flip-chip joints between silicon (Si) chips and copper (Cu) substrates were fabricated using a solid-state bonding process without any solder and without flux. The bonding process was performed at 250°C, compatible with typical reflow temperature for lead-free solders. During the bonding process, there was no molten phase involved. The Ag joints fabricated consisted of only pure Ag without any intermetallic compound (IMC). Thus, reliability issues associated with IMCs and IMC growth do not exist anymore. Silver has the highest electrical conductivity and highest thermal conductivity among all metals. It is also quite ductile and able to deform to release stresses caused by thermal expansion mismatch. Flip-chip joints of high aspect ratio can be accomplished because the joints stay in a solid state during the bonding process. It looks like that silver is the ultimate joining material for flip-chip as well as through-Si-via interconnect technologies. In this study, the solid-state bonding process was first developed using a pure Ag foil to bond a Si chip to a Cu substrate in one step. The bonding strength on two interfaces, Si/Ag and Ag/Cu, passes the MIL-STD-883G Method 2019.7. To demonstrate Ag flip-chip interconnects, Si chips were electroplated with Ag bumps, followed by the solid-state bonding process on Cu substrates. The flip-chip bumps are well bonded to the Cu substrate. It would take some time for this new technology to be probably accepted and utilized in production. On the other hand, the preliminary results in this study show that Ag flip-chip joints can indeed be fabricated at 250°C.

40 μm Copper–Silver Composite Flip-Chip Interconnect Technology Using Solid-State Bonding

2012 ◽  
Vol 134 (2) ◽  
Author(s):  
Chu-Hsuan Sha ◽  
Wen P. Lin ◽  
Chin C. Lee
Keyword(s):  
Solid State ◽  
Flip Chip ◽  
Thermal Expansion Mismatch ◽  
Conduction Electrons ◽  
Solid State Bonding ◽  
Cu Substrates ◽  
Atomic Distance ◽  
Free Solder ◽  
Interconnect Technology ◽  
Silver Composite

Copper–silver (Cu–Ag) composite flip-chip interconnect between silicon (Si) chips and Cu substrates is demonstrated. Array of Cu–Ag columns, each 28 μm in height and 40 μm in diameter, is electroplated on 2-in. Si wafers coated with chromium (Cr)/gold (Au) dual layer. The Si wafers are diced into 6 mm × 6 mm chips, each containing 50 × 50 Cu–Ag columns. The Si chip with Cu–Ag columns is bonded to Cu substrates at 260 °C in 80 mTorr vacuum. A bonding force of only 1.8 kg is applied, corresponding to 0.71 g per Cu–Ag column. During bonding, Ag atoms in Cu–Ag columns deform and their surfaces conform to and mate with the surface of Cu substrate. Solid-state bonding incurs when Ag atoms in Cu–Ag columns and Cu atoms in Cu substrates are brought within atomic distance so that they share conduction electrons. The Cu–Ag columns are indeed bonded to the Cu. No molten phase is involved in the bonding. The joint consists of 60% Cu section and 40% Ag section. The ductile Ag is able to accommodate the thermal expansion mismatch between Si and Cu. The Cu–Ag joints do not contain any intermetallic compound (IMC). This interconnect technology avoids all reliability issues associated with IMC growth in conventional soldering processes. Compared to tin-based lead-free solder joints, Cu–Ag composite joints have superior electrical and thermal properties.

A Study on High Temperature Oxidation of Titanium Alloys in Solid State Bonding Process

2007 ◽  
Vol 544-545 ◽  
pp. 183-186 ◽  
Author(s):  
Ho Sung Lee ◽  
Jong Hoon Yoon ◽  
Yeong Moo Yi
Keyword(s):  
Solid State ◽  
High Temperature Oxidation ◽  
Oxidation Behavior ◽  
Surface Oxidation ◽  
Bonding Process ◽  
Bonding Interface ◽  
Temperature Oxidation ◽  
Solid State Bonding ◽  
Bonding Condition ◽  
State Diffusion

The surface oxidation behavior was investigated over a range of solid state bonding condition of the Ti-6Al-4V ELI alloy. Since the oxides at the bonding interface may prevent the materials from complete bonding, it is important to understand the oxidation behavior at solid state bonding condition. The activation energy of oxidation of Ti-6Al-4V ELI is estimated to be 318 KJ/mol in an environment of solid state bonding process. For Ti-6Al-4V ELI alloy, strucutral integrity of bonding interface without oxides have been obtained at 850°C applying pressure of 3MPa for 1 hour. Solid state diffusion bonding of Ti-15V-3Cr-3Sn-3Al alloy was also obtained under a pressure of 6MPa for 3 hours at 925°C.

Electromigration Reliability of 96.5Sn–3Ag–0.5Cu Flip-Chip Solder Joints With Au/Ni/Cu or Cu Substrate Pad Metallization

2009 ◽  
Vol 131 (2) ◽  
Author(s):  
Yi-Shao Lai ◽  
Ying-Ta Chiu ◽  
Chiu-Wen Lee
Keyword(s):  
Flip Chip ◽  
Solder Joints ◽  
Average Current Density ◽  
Cross Sectional ◽  
Cu Substrate ◽  
Cu Metallization ◽  
Substrate Side ◽  
Current Stressing ◽  
Average Current ◽  
A Current

Designed experiments were conducted in this paper to study the effect of Au/Ni/Cu or Cu substrate pad metallization on the electromigration reliability of 96.5Sn–3Ag–0.5Cu flip-chip solder joints with Ti/Ni(V)/Cu under bump metallurgy (UBM) under a current stressing condition with an average current density of around 5 kA/cm2 at an ambient temperature of 150°C. Cross-sectional observations on current-stressed solder joints indicate that although Cu metallization results in severe voiding compared with Au/Ni/Cu metallization on the substrate side of the solder joint, the dominant failure has been identified as UBM consumption, and test vehicles with Cu metallization exhibit better electromigration reliability than those with Au/Ni/Cu metallization. The stronger durability against current stressing for test vehicles with Cu metallization may attribute to the lower UBM consumption rate due to the continuous Cu diffusion toward UBM as a result of the concentration gradient. The consumption of UBM is faster for test vehicles with Au/Ni/Cu metallization because Cu diffusion from the substrate pad is retarded by the Ni barrier.

Low Temperature Solid State Gold Bonding of Si Chips to Alumina Substrates

2011 ◽  
Vol 133 (2) ◽  
Author(s):  
Chu-Hsuan Sha ◽  
Chin C. Lee
Keyword(s):  
Solid State ◽  
Static Pressure ◽  
Coefficient Of Thermal Expansion ◽  
Bonding Temperature ◽  
Bond Line ◽  
Bonding Process ◽  
Pure Gold ◽  
Solid State Bonding ◽  
Reflow Temperature ◽  
Atomic Distance

Pure gold (Au) is used as a bonding medium to bond silicon (Si) chips to alumina substrates. The bonding process is performed at 260 °C with only 150 psi (1.0 MPa) static pressure applied. This is a solid-state bonding without any molten phase involved. The Au layer plated on alumina is ductile enough to deform for its surface to mate with the thin Au layer coated on Si. Au atoms on both sides of the bond line are brought within atomic distance and bonding is achieved. The ductile Au joint also accommodates the significant mismatch in coefficient of thermal expansion (CTE) between Si and alumina. Scanning electron microscope (SEM) evaluations show that nearly perfect joints are achieved and no voids are observed. Five samples are shear tested. They all pass the MIL-STD-883G standard. This bonding technique can be applied to bonding any two objects that can be coated with smooth Au layers. The 260 °C bonding temperature is compatible with typical reflow temperature of Sn3.5Ag solders used in electronic industries.

40 μm Silver Flip-Chip Interconnect Technology With Solid-State Bonding

2011 ◽  
Vol 133 (3) ◽  
Author(s):  
Chu-Hsuan Sha ◽  
Chin C. Lee
Keyword(s):  
Flip Chip ◽  
Bonding Process ◽  
Thermal Expansion Mismatch ◽  
Pure Silver ◽  
Bonding Pressure ◽  
Cu Substrate ◽  
Solid State Bonding ◽  
Cu Substrates ◽  
Lead Free Solders ◽  
Interconnect Technology

Formation of pure silver (Ag) flip-chip interconnect of silicon (Si) chips on copper (Cu) substrates is reported. Arrays of Ag columns, each 36 μm in height and 40 μm in diameter, are fabricated on 2-in. Si wafers which are first coated with chromium (Cr)/gold (Au) dual layers. The Si wafers are diced into 6 mm × 6 mm chips, each having 50 × 50 Ag columns. The Si chip with Ag columns is directly bonded to Cu substrate at 260 °C in 80 mTorr vacuum to inhibit oxidation. The static bonding pressure is as low as 680 psi (4.69 MPa), corresponding to a load of 0.021 oz (0.60 g) per column. During bonding, the Ag columns deform and conform to the Cu substrate. They are well bonded to the Cu. No molten phase is involved in the bonding process. The joints consist of pure Ag only. The ductile Ag joints are able to accommodate the thermal expansion mismatch between Si and Cu. It is well known that in nearly all soldering processes used in electronic industries, intermetallic compound (IMC) formation is essential to make a solder joint. In the pure Ag interconnect, no IMCs exist. Thus, reliability issues associated with IMCs are eliminated. Compared to tin-based lead-free solders, pure Ag joints have superior electrical and thermal properties.

Solid State Bonding of Silver Foils to Metalized Alumina Substrates at 260°C

2011 ◽  
Vol 133 (4) ◽  
Author(s):  
Chu-Hsuan Sha ◽  
Pin J. Wang ◽  
Wen P. Lin ◽  
Chin C. Lee
Keyword(s):  
Shear Strength ◽  
Solid State ◽  
Shear Test ◽  
Static Pressure ◽  
Bonding Temperature ◽  
Bonding Process ◽  
Sem Images ◽  
Solid State Bonding ◽  
Reflow Temperature ◽  
Atomic Distance

Silver (Ag) foils are bonded to alumina substrates by a low temperature solid state bonding process. The alumina substrate is premetalized with 40 nm titanium tungsten (TiW) and 2.54 μm gold (Au). The bonding temperature is just 260 °C, compatible with the peak reflow temperature of lead-free (Pb-free) solders used in electronic industries. The Ag foil is quite soft and ductile. It can deform to mate with the Au surface on alumina. Thus, only 1000 psi of static pressure is needed to bring Ag atoms and Au atoms within atomic distance on the interface. Ag has superior physical properties. It has the highest electrical and thermal conductivities among the metals. Scanning electron microscope (SEM) images show that the Ag foil is well bonded to the Au layer on alumina. A standard shear test is performed to determine the shear strength of the bonding. The shear strength of five samples tested far exceeds the strength requirement of MIL-STD-883 G standard.

Resistance Microwelding of Au/Ni-Plated Phosphor Bronze Sheet to Insulated Cu Wire

2011 ◽  
Vol 189-193 ◽  
pp. 3596-3600
Author(s):  
Bing Hua Mo ◽  
Zhong Ning Guo ◽  
Yuan Bo Li ◽  
Guan Wang
Keyword(s):  
Electron Microscopy ◽  
Scanning Electron Microscopy ◽  
Solid State ◽  
Optical Microscopy ◽  
Melting Temperature ◽  
Welding Current ◽  
Breaking Force ◽  
X Ray ◽  
Solid State Bonding ◽  
Scanning Electron

Resistance microwelding of Au/Ni-plated phosphor bronze sheet to insulated Cu wire have been investigated using mechanical testing, optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results show that mechanisms of joint formation involve solid-state bonding with Au and Cu atoms diffusion at low welding current, a combination of brazing and solid-state bonding at high welding current. Due to the hindrance of Ni plating which has a higher melting temperature and hardness, the joint breaking force is smaller than that without plating.

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