WHAT IS FLIP CHIP?
Flip chip microelectronic assembly is the direct electrical connection of face-down (hence, “flipped”) electronic components onto substrates, circuit boards, or carriers, by means of conductive bumps on the chip bond pads. In contrast, wire bonding, the older technology which flip chip is replacing, uses face-up chips with a wire connection to each pad.
Flip chip components are predominantly semiconductor devices; however, components such as passive filters, detector arrays, and MEMs devices are also beginning to be used in flip chip form. Flip chip is also called Direct Chip Attach (DCA), a more descriptive term, since the chip is directly attached to the substrate, board, or carrier by the conductive bumps.
IBM introduced flip chip interconnection in the early sixties for their mainframe computers, and has continued to use flip chip since then. Delco Electronics developed flip chip for automotive applications in the seventies. Delphi Delco currently places over 300,000 flip chip die per day into automotive electronics. Most electronic watches, and a growing percentage of cellular phones, pagers, and high speed microprocessors are assembled with flip chip.
Worldwide flip chip consumption is over 600,000 units per year, with a projected annual growth rate of nearly 50% per year. Semiconductor manufacturers currently bump for flip chip assembly about 3% of wafers produced, and are expected to be bumping 10% within a few years.
WHY USE FLIP CHIP?
The boom in flip chip packaging results both from flip chip’s advantages in size, performance, flexibility, reliability, and cost over other packaging methods and from the widening availability of flip chip materials, equipment, and services.
Eliminating packages and bond wires reduces the required board area by up to 95%, and requires far less height. Weight can be less than 5% of packaged device weight. Flip chip is the simplest minimal package, smaller than Chip Scale Packages (CSP’s) because it is chip size.
Flip chip offers the highest speed electrical performance of any assembly method. Eliminating bond wires reduces the delaying inductance and capacitance of the connection by a factor of 10, and shortens the path by a factor of 25 to 100. The result is high speed off-chip interconnection.
Greatest I/O Flexibility
Flip chip gives the greatest input/output connection flexibility. Wire bond connections are limited to the perimeter of the die, driving die sizes up as the number of connections increases. Flip chip connections can use the whole area of the die, accommodating many more connections on a smaller die. Area connections also allow 3-D stacking of die and other components.
Flip chip is mechanically the most rugged interconnection method. Flip chips, when completed with an adhesive “underfill,” are solid little blocks of cured epoxy. They have survived the laboratory equivalents of rocket liftoff and of artillery firing, as well as millions of cumulative total hours of actual use in computers and under automobile hoods
Flip chip can be the lowest cost interconnection for high volume automated production, with costs below $0.01 per connection. This explains flip chip’s longevity in the cost-conscious automotive world, pervasiveness in low cost consumer watches, and growing popularity in smart cards, RF-ID cards, cellular telephones, and other cost-dominated applications.
HOW IS FLIP CHIP MADE?
There are three stages in making flip chip assemblies: bumping the die or wafer, attaching the bumped die to the board or substrate, and, in most cases, filling the remaining space under the die with an electrically non-conductive material. The conductive bump, the attachment materials, and the processes used differentiate the various kinds of flip chip assemblies. The following sections describe the most common bumping and attaching methods. The cost, performance, and space constraints of the application determine which method best suits it.
The bump serves several functions in the flip chip assembly. Electrically, the bump provides the conductive path from chip to substrate. The bump also provides a thermally conductive path to carry heat from the chip to the substrate. In addition, the bump provides part of the mechanical mounting of the die to the substrate. Finally, the bump provides a spacer, preventing electrical contact between the chip and substrate conductors, and acting as a short lead to relieve mechanical strain between board and substrate.
Solder Bump Flip Chip
The solder bumping process first requires that an under bump metallization (UBM) be placed on the chip bond pads, by sputtering, plating, or other means, to replace the insulating aluminum oxide layer and to define and limit the solder-wetted area. Solder is deposited over the UBM by evaporation, electroplating, screen printing solder paste, or needle-depositing.
After solder bumping, the wafer is sawn into bumped die. The bumped die are placed on the substrate pads, and the assembly is heated to make a solder connection. Solder bumped die and wafers, and assembly services are available from several suppliers.
Plated Bump Flip Chip
Plated bump flip chip uses wet chemical processes to remove the aluminum oxide and plate conductive metal bumps onto the wafer bond pads. Plated nickel-gold bumps are formed on the semiconductor wafer by electroless nickel plating of the aluminum bond pads of the chips. After plating the desired thickness of nickel, an immersion gold layer is added for protection, and the wafer is sawn into bumped die. Attachment generally is by solder or adhesive, which may be applied to the bumps or the substrate bond pads by various techniques. Plated bump die, and assembly services, are available from several suppliers.
Stud Bump Flip Chip
The gold stud bump flip chip process bumps die by a modified standard wire bonding technique. This technique makes a gold ball for wire bonding by melting the end of a gold wire to form a sphere. The gold ball is attached to the chip bond pad as the first part of a wire bond. To form gold bumps instead of wire bonds, wire bonders are modified to break off the wire after attaching the ball to the chip bond pad. The gold ball, or “stud bump” remaining on the bond pad provides a permanent connection through the aluminum oxide to the underlying metal.
The gold stud bump process is unique in being readily applied to individual single die or to wafers. Gold stud bump flip chips may be attached to the substrate bond pads with adhesive or by thermosonic gold-to-gold connection. Die bumping and assembly services are available from several suppliers.
Adhesive Bump Flip Chip
The adhesive bump flip chip process stencils conductive adhesive to form bumps on an under-bump metal. The cured adhesive acts as bumps. Attachment is by an additional layer of conductive adhesive. Adhesive bumping and assembly is available from licensed suppliers.
Flip Chip Underfill
As described above, one function of the bump is to provide a space between the chip and the board. In the final stage of assembly, this under-chip space is usually filled with a non-conductive “underfill” adhesive joining the entire surface of the chip to the substrate.
The underfill protects the bumps from moisture or other environmental hazards, and provides additional mechanical strength to the assembly. However, its most important purpose is to compensate for any thermal expansion difference between the chip and the substrate. Underfill mechanically “locks together” chip and substrate so that differences in thermal expansion do not break or damage the electrical connection of the bumps.
Underfill may be needle-dispensed along the edges of each chip. It is drawn into the under-chip space by capillary action, and heat-cured to form a permanent bond. Newer methods for underfill distribution are discussed in our technology update article, “Underfill Update.”
Flip chip assembly has significant advantages over other microelectronic packaging. Several varieties of flip chip assembly, including solder bump, plated bump, gold stud bump, and adhesive bump suit flip chip to a wide range of applications.
FOR FURTHER INFORMATION
Book, “Flip Chip Technologies,” John H. Lau, Editor
McGraw-Hill, NY, 1995. ISBN 0-07-036609-8
An excellent survey of the field as of 1995, including many of the basic processes.
Article, “Underfill Update,” Dr. Ken Gilleo. A summary of some of the latest developments in underfill.
Article, “Bump, Dip, Flip: Single Chip”, George Riley, Proceedings 1997 Surface Mount International, September 1987, pp. 535-541. Full text available from SMTA. A more detailed look at gold bump die-based flip chip.