by Dr. Ken Gilleo
“Substrates for Flip Chips,” excerpted from Dr. Ken Gilleo’s latest book on flip chip and related technologies, summarizes the choices and tradeoffs in flip chip substrate materials
7.7 Substrates for Flip Chips
The first flip chip FC substrates were ceramic, and they are still preferred from the viewpoint of performance and ease of assembly. Flatness and low thermal expansion coefficient (CTE) are two preferred attributes for FC substrates that are inherent in most ceramics. During the 1990s, organic substrates became popular for many FCs, especially the smaller, lower-lead-count variety. However, the move to organic substrate introduced the problem of significant thermomechanical mismatch because of the relatively high CTE of organic materials. However, underfilling adequately reduces joint stress, and reliability can be regained. Underfilling, while allowing popular, low cost substrates to be used, increases processing time and adds cost. Nonetheless, underfill allows virtually all substrates to be used. The preferred organic substrates include standard FR4 circuits, higher grades of epoxy such as bismaleimide triazine (BT) and flexible circuit substrates. Large, high-performance chips, such as CPUs, continue to favor ceramic materials.
IBM first used ceramic circuits and chip carriers for FCs. Ceramic offers two important advantages for FCs. First and most important, the CTE is low enough so that there is only a small thermomechanical mismatch between chip and substrate. This means that underfill is not required for thermomechanical enhancement, at least for smaller die. The second feature of ceramic is high-temperature stability as well as superior thermal conductivity. Ceramic can handle 300oC or higher, a valuable feature for a chip carrier. Some materials conduct heat better than aluminum and are therefore ideal for heat dissipation.
Although ceramic has become less popular in recent years because of higher cost, it has been used to build FC ball-grid arrays (BGAs). AMD, for example, has used ceramic BGAs for the K6 and K7 microprocessors. The high-temperature performance of ceramic allows higher-melting solders to be used for chip attach so melting will not occur when the package is soldered to the printed wiring board (PWB). A ceramic substrate is also much less prone to warping than organic laminate and is a good material for FC-BGAs. The factor that will continue to reduce usage is cost, especially for high density. The PWB industry continues to advance methods, such as microvias, that improve the cost/density picture, and this will likely increase the cost differential.
7.7.2 Organic, Rigid
Organic laminate has become the preferred substrate for both FCOB (flip-chip-on-board) and FCIP (flip-chip-in-package). In fact, second-generation FC is really centered on organic substrate to lower cost. The advent of the BGA in the early 1990’s, especially the plastic BGA (PBGA), pushed the demand for better laminates. New materials were introduced, and more are under evaluation aimed at providing improved mechanical stability and higher heat ranges. BT resin has become one of the most popular advanced laminates and is now used widely for BGAs. The same high-performance BGA substrates also provide FCs with desirable properties. Smaller chips used on cellular phones, pagers, and other consumer products do not necessarily require higher performance than offered by FR4. It is the FCIP products that need better substrates, and these have been developed for wire bonded BGAs.
7.7.3 Organic, Flexible, High Temperature
Flexible circuit materials have long been used for chip carriers. TAB (tape-automated-bonding), called tape carrier package (TCP) by Intel, has been used to handle high-density chips with very good results. The polyimide class of substrate, such as Dupont’s Kaptonâ , is noted for extraordinary high-temperature performance. Some of these materials have a zero strength factor at 800oC, a region usually confined to ceramics. The flex materials are also thin and compliant, making them well suited for use as chip carriers and FC assembly substrates. A number of companies, including IBM, 3M, and Amkor, have developed products that can be classed as flex-based packages. Both wire-bonded and FC assembles have been made. Thermal compression bonding to flex is also possible, and Motorola has developed technology here.
Flexible circuitry is an enabling technology for many industries such as disk drives. The flex allows the read/write head to move over the disk. Faster and higher-density drives have been made possible by using lighter components placed closer to the head. Today, the FC is the optimal solution for high-density, magnetoresistive disk drive technology. The FC is assembled to the polyimide drive circuit using reflow soldering. Stud bumps of various materials have also been used for flex. Underfill must be used even though the thin, compliant flex produces lower thermomechanical stress than hard board. Although it is possible to make flex substrate with a low CTE value approaching that of the chip, a value of around 18 ppm/oC is used in order to match the copper conductors. This means that underfill is required.
Disk drive makers, most notably Seagate, have been able to apply underfill without a fillet step. Underfill is applied from one side, and the assembly moves to the curing oven. Underfill with lower filler and hence lower viscosity can be used, and this produces self-filleting. The high volume, low cost requirements of the disk drive industry suggest that this segment of electronics will continue to lead productivity and cost reduction. Expect to see the implementation of preapplied flux/underfill here.
7.7.4 Organic, Flexible, Temperature-Limited
There is a large segment of the flexible circuit industry that uses very low cost polyester substrate, such as Dupont’s Mylarâ . The polyesters have a temperature-processing limit of about 150oC, so one probably can rule out solder assembly even though low- temperature alloys exist. This is the area where conductive adhesives excel. Both anisotropic (ACA) and isotropic ((ICA) types can be used. Two basic types of conductors are used with polyester film, and they are traditional subtractive copper and additive polymer thick film (PTF). Bare copper can be used with adhesives but not without junction stability concerns. A much better interface is gold plate over nickel plating on the copper. All adhesives form a stable electrical junction with gold-plated conductors.
PTF silver ink makes a very good interface with conductive adhesives, as would be expected because of the similarities. PTF ink has a silver-rich surface that mates well with the silver matrix of the ICAs. Millions of such junctions have been made without any issues using surface-mount devices (SMDs). ACAs also form good connections with PTF ink because the conductive spheres tend to embed into the ink surface. The oxide that forms on silver is of no real consequence since it is electrically conductive. Radiofrequency identification (RFID) products are now being made with PTF inks and FCs bonded with adhesives, and they will be covered later under “Applications.”
FOR MORE INFORMATION
This material was excerpted from “Chapter 7. FLIP CHIP TECHNOLOGY” by Dr. Ken Gilleo
in AREA ARRAY PACKAGING HANDBOOK — MANUFACTURING AND ASSEMBLY
K. Gilleo, Editor
Copyright 2002, The McGraw-Hill Companies, Inc, New York, NY