extended to 3D in a few cases. Some systems may require exotic heat dissipation apparatus as shown in Fig. 58.
1.3.2.2 Cost
Although 3D-packaging is the lowest cost 3D integration category, there is still a push to further lower the cost in the consumer product applications market. In some analysis, it has been shown [86] that 3D-packaging can be even more cost- effective than traditional approach. In cost analysis, the higher cost of assem- bling 3d-packages can be countered by the following cost advantages:
1. Fewer discrete components have to be assembled on a board.
2. A reduction in overall packaging cost i.e. in wire-bonded die-stacks.
3. Cost savings due to the reduction in area of printed wiring board assemblies.
The second category is 3D IC-stacking, in which individual wafers are fabri- cated and then integrated in 3D either at the wafer-level or the chip-level. In this category, each IC wafer design is first fabricated independently. Then the wafers or ICs are bonded and electrical interconnection between ICs is made using thru- Si vias. There are many different types of 3D IC-stacking technologies, the major difference between them being how they are bonded. Three main bonding tech- nologies used are direct oxide bonding, metal-to-metal bonding, and adhesive bonding. The other enabling technologies needed for 3D IC-stacking are wafer/
IC thinning, fabrication of thru-Si vias and precision alignment. Many major companies around the world like IBM, Intel, Toshiba and Infineon are pursuing 3D IC-stacking.
The last category, 3D-packaging, is an extension of single-chip packages into 3D. These technologies are matured and are currently used in many consumer products like smart cell phones, cameras, MP3 players and laptop computers.
There are also many versions of this category; the two main technologies being wire-bonded die-stack and BGA-stack.
3D IC-stacking technologies have many unresolved issues, some of which are thermal management, low module yields, inadequate infrastructure, and higher cost. 3D-packaging technologies, being already in use in many applications, have relatively fewer unresolved issues, two of which are thermal management and cost. 3D integration will play a major role to enable many future products in consumer, medical, defense, and security applications.
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Advanced Bonding/Joining Techniques
Chin C. Lee, Pin J. Wang, and Jong S. Kim
Abstract In this chapter, three advanced bonding/joining techniques, adhesive bonding, direct bonding, and lead-free soldering, are presented. For each technique, we first review the bonding principles and applications in electronic industries, followed by novel bonding materials and processes.
For adhesive bonding, four popular adhesives, epoxy resins, silicon resins, polymides, and acrylics, are reviewed. Two new adhesives, liquid crystal poly- mer (LCP) and SU8, are covered too. LCP has the properties of both polymers and liquid crystals. It, thus, can be bonded to silicon, metal, and glass, and used as flexible circuit board. SU 8, an epoxy-based negative type photoresist, has been applied to zero-level-packaging technology for low-cost wafer-level MEMS packaging.
For direct bonding, three popular methods, anodic bonding, diffusion bond- ing, and surface-activated bonding, are discussed. Anodic bonding process has extensive applications in silicon-glass bonding and glass-glass bonding. Diffu- sion bonding process forms chemical bonds by inter-diffusion of two different atoms over the bond line. Surface-activated bonding is valuable in bonding objects with large difference in coefficients of thermal expansion because of low process temperature, usually room temperature. A novel Ag-to-Cu direct bond- ing technique at bonding temperature of 2508C is reported.
In lead-free soldering, fundamental soldering principle is presented. To eliminate the use of fluxes, oxidation-free fluxless soldering technology has been developed. It has been applied to developing numerous soldering processes based on systems such as Sn-Au, Sn-Cu, Sn-Ag, In-Au, In-Cu, and In-Ag. Two fluxless processes are reported. One is bonding between Si/Cr/Au/Sn/Ag and Si/Cr/Au. The other is between Si/Cr/Au/Ag and Cu/Ag/In/Ag. In either process, high bonding quality is achieved without using any flux. Fluxless process has also been demonstrated in flip-chip configuration using Sn-rich solder joints.
C.C. Lee (*)
Electrical Engineering and Computer Science, University of California, Irvine, 2226 Engineering Gateway Building, Irvine, CA 92697-2660
e-mail: [email protected]
D. Lu, C.P. Wong (eds.),Materials for Advanced Packaging,
DOI 10.1007/978-0-387-78219-5_2,ÓSpringer ScienceþBusiness Media, LLC 2009 51
Keywords BondingSolderingFluxless solderingDirect bondingAnodic bondingAdhesives Epoxies