Chapter 22 Materials Selection and Design Considerations

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22.20 Tape Automated Bonding • W127 Table 22.7 Property Comparisons of Four Classes of Polymers Used for IC Package Encapsulation Epoxy Resins Silicones Polyurethanes Polysulphides Dielectric strength Good Good Good Good Elastic modulus High Low Wide range Low Tensile strength High Low Wide range — Precursor viscosity Low Low Low High Adhesion to package Excellent Poor Good Good (to ceramics) Moisture diffusion rate High High Low Very low Source: From C. R. M. Grovenor, Microelectronic Materials. Copyright © 1989 by Institute of Physics Publishing, Bristol, England. has undesirable electrical consequences. A comparison of the important encapsulation characteristics of four different polymer types is given in Table 22.7. 22.20 TAPE AUTOMATED BONDING Another packaging design, tape automated bonding (or TAB), a variation of the leadframe discussed above, has found widespread use by virtue of its low cost. The tape-bonded package consists of a thin and flexible polyimide polymer backing film substrate; onto this substrate surface is patterned an array of copper “finger” highconductivity conduction paths similar in configuration to the contact leads for the conventional leadframe. A schematic diagram of a tape-bonded film leadframe is shown in Figure 22.34. Mechanical support for the assembly is provided by the polyimide film, onto which the die is bonded using an adhesive. Polyimide strip widths are typically 35 mm (1.38 in.), and sprocket holes are incorporated along opposing edges to facilitate movement and positioning of the TAB leadframes. Literally thousands of these individual units, attached end to end, are spooled onto reels in preparation for automated processing. Figure 22.34 Schematic diagram of a complete tapebonded (TAB) leadframe. [From Electronic Materials Handbook, Vol. 1, Packaging, C. A. Dostal (Editor), ASM International, 1989, p. 233.] Polymer (polyimide) film Sprocket drive hole IC Copper finger inner lead (bonded to IC) Outer lead Window in polymer film — to expedite outer lead bonding
W128 • Chapter 22 / Materials Selection and Design Considerations Figure 22.35 Schematic diagrams showing (a) the cross section of an encapsulated TAB leadframe package, and (b) how bonding between the IC chip and a copper finger is achieved with a solder bump. [Adapted from Electronic Materials Handbook, Vol. 1, Packaging, C. A. Dostal (Editor), ASM International, 1989, pp. 233, 234.] Solder joint Passivation layer Polymer film Encapsulation Solder bump Die attachment Solder bump IC chip IC chip Leadframe plate or substrate (b) (a) Contact pad Gold layer Copper finger Polymer film Leadframe finger The copper fingers are extremely narrow and positioned close together. Separation distances of the inner contact leads are on the order of 50 mm, which is much smaller than is possible for the stamped leadframe. Furthermore, each die chip contact pad is microjoined directly to one of these copper fingers, which eliminates the need for any connecting wires. The copper fingers are very thin, so that, for this direct bonding to be achieved, the chip pad bonding sites must be raised above the metallized coating. This is accomplished using “solder bumps,” which are normally layers of gold (or gold-plated copper) approximately 25 mm thick. Schematic representations illustrating this attachment design are presented in Figure 22.35. The finger contacts are bonded to these raised bumps by soldering using a thermal-compression bonding tool. This tape-bonding design is fully automated in that all of the hundred or so microjoints can be made in a single step, a feature not possible with leadframes that require multiple wire-bonding operations. The packaging operation for the TAB leadframe is completed, as with the stamped leadframe, by encapsulation of the assembly (i.e., tape leadframe and its attached chip) within a fluid polymeric material that subsequently cures to form a protective shield. Protruding from this package are the copper finger conducting paths to which external electrical connections are made. Furthermore, excess heat generated by the chip must be dissipated along these copper fingers since the polymer tape backing does not provide an effective thermal conduction path because of its low thermal conductivity. The ultimate design goal of the IC package is to allow for the proper electrical operation of the packaged device. As frequencies and computing speeds creep ever higher, the mechanical and electrical design considerations of the package design must become more and more integrated. The overall electrical performance of the package is as important to the end user as the overall reliability.
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Chapter 22 Materials Selection and Design Considerations

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