10μm Ag flip-chip by solid-state bonding at 250°C

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

Journal of Electronic Packaging ◽

10.1115/1.4006705 ◽

2012 ◽

Vol 134 (2) ◽

Cited By ~ 1

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.

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40-$\mu{\rm m}$ Cu/Au Flip-Chip Joints Made by 200$^{\circ}{\rm C}$ Solid-State Bonding Process

IEEE Transactions on Components Packaging and Manufacturing Technology ◽

10.1109/tcpmt.2012.2199991 ◽

2013 ◽

Vol 3 (1) ◽

pp. 126-132 ◽

Cited By ~ 3

Author(s):

Wen P. Lin ◽

Chu-Hsuan Sha ◽

Chin C. Lee

Keyword(s):

Solid State ◽

Flip Chip ◽

Bonding Process ◽

Solid State Bonding

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

Journal of Microelectronics and Electronic Packaging ◽

10.4071/imaps.379 ◽

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.

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Silver Flip-Chip Technology by Solid-State Bonding

Journal of Electronic Packaging ◽

10.1115/1.4002297 ◽

2010 ◽

Vol 132 (3) ◽

Cited By ~ 8

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.

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