Seminar on Flip Chip

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    Slide 3

    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 ofconductive bumps on the chip bond pads. In contrast, wire bonding, the older technology whichflip chip is replacing, uses face-up chips with a wire connection to each pad.

    Slide 4

    Introduction

    To attach the flip chip into a circuit, the chip is inverted to bring the solder dots down ontoconnectors on the underlying electronics orcircuit board. The solder is then re-melted to producean electrical connection, typically using an ultrasonic or alternatively reflow solderprocess. Thisalso leaves a small space between the chip's circuitry and the underlying mounting. In most casesan electrically-insulating adhesive is then "underfilled" to provide a stronger mechanicalconnection, provide a heat bridge, and to ensure the solder joints are not stressed due todifferential heating of the chip and the rest of the system

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    Slide 4

    History

    IBM introduced flip chip interconnection in the early sixties for their mainframe computers, andhas continued to use flip chip since then. Delco Electronics developed flip chip for automotiveapplications in the seventies. Delphi Delco currently places over 300,000 flip chip die per dayinto automotive electronics. Most electronic watches, and a growing percentage of cellularphones, 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 growthrate of nearly 50% per year. Semiconductor manufacturers currently bump for flip chip assemblyabout 3% of wafers produced, and are expected to be bumping 10% within a few years.

    Slide 5 n 6

    Solder bump

    The solder bump flip chip process may be considered as four sequential steps:

    y preparing the wafer for solder bumping,

    y forming or placing the solder bumps,

    y attaching the bumped die to the board, substrate, or carrier, and

    y completing the assembly with an adhesive underfill.

    This preparation may include cleaning, removing insulating oxides, and providing a padmetallurgy that will protect the IC while making a good mechanical and electrical connectionto the solder bump and the board.

    Solder bumps may be formed or placed on the wafer in many ways, including evaporation,electroplating, printing, jetting, stud bumping, and direct placement. As discussed below, theresults of these methods may differ in bump size and spacing ("pitch"), solder componentsand composition, cost, manufacturing time, equipment required, assembly temperature

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    Solder bump bonding (SBB) uses solder wire in a modified wire bonder to place a ball of solderdirectly onto the bond pad. The scrubbing action of the wire bonder causes the solder ball tobond to the bond pad. The solder wire is broken off above the bump, leaving the bump on thepad, where it can be reflowed.

    Solder bump bonding is a serial process, producing bumps one by one at rates up to about 8 persecond. It has advantages in allowing closer spacing than printed bumps.

    Placing the bumped die may be by fine-pitch surface-mount equipment, or by high-accuracy flipchip placement equipment. In either case, the die must be aligned with the bond pads on theboard before placement. Fluxes may be conventional or no-clean fluxes, with differingapplication and cleaning requirements. Soldering may be in a belt furnace or by hot gas or otherlocal means.

    One function of the solder bump is to provide a space between the chip and the board. In the laststage 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 providesadditional mechanical strength to the assembly. However, its most important purpose,particularly with solder bumps connections on large die or to organic substrates, is to compensatefor thermal expansion differences 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 has been found to increase the fatiguelife of solder bumps by at least an order of magnitude.

    CONCLUSIONS

    Solder bump flip chip, the oldest flip chip assembly method, has evolved over severalgenerations into a variety of species which may be distinguished by their under-bump metallurgyand solder placement methods. Each has a differing set of strengths and limitations which suitsthem for differing applications.

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    Slide 7 n 8

    Stud bump

    The gold stud bump flip chip assembly process creates conductive gold bumps on the die bond

    pads, and connects the die to the circuit board or substrate with adhesive or ultrasonic assembly.These kind of chips does not require wafer processing; individual die can be stud bumped aseasily as they can be wire bonded.

    Gold stud bumps are placed on the die bond pads through a modification of the "ball bonding"process used in conventional wire bonding. In ball bonding, the tip of the gold bond wire ismelted to form a sphere. The wire bonding tool presses this sphere against the aluminum bond

    pad, applying mechanical force, heat, and ultrasonic energy to create a metallic connection. Thewire bonding tool next extends the gold wire to the connection pad onthe board, substrate, orlead frame, and makes a "stitch" bond to that pad, finishing by breaking off the bond wire tobegin another cycle.

    ASSEMBLY

    Gold stud bumped die may be attached by conductive or non-conductive adhesives, or byultrasonic assembly without adhesive. Conductive adhesive may be isotropic, conducting in alldirections, or anisotropic, conducting in a preferred direction only.

    Non-Conductive Adhesive AssemblyNon-conductive adhesive assembly is in some ways similar to anisotropic adhesive assembly. Anon-conductive adhesive is dispensed or stenciled at the die location on the substrate. Thebumped die is pressed against the substrate pads with enough force give compressive dispersionof the adhesive, allowing no adhesive to remain between the stud bump and substrate pad matingsurfaces. This pressure is maintained while the assembly temperature is elevated for sufficienttime to at least partially cure the adhesive. The chip is mechanically bonded to the substrate bythe cured adhesive, with metal to metal contact between the bumps and substrate pads. Noseparate underfill adhesive is required.

    Ultrasonic AssemblyA non-adhesive assembly for gold stud bumped die results from pressing the bumped chip ontogold substrate pads, applying heat, pressure, and sonic energy sufficient to form gold to goldmetallic bonds as in thermosonic wire bonding. Depending on the die size and the applicationtemperature requirements, assemblies with these gold to gold connections may not requireundefilling.

    BUMPING

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    ADVANTAGES AND LIMITATIONS

    Gold stud bump flip chip offers several advantages. The bumping equipment, a wire bonder ordedicated stud bumper, is widely available and well characterized. Since stud bumps are formedby wire bonders, they can be placed anywhere a wire bond might be placed. They can easily

    achieve pitches of less than 100 microns and be placed on pads of less than 75 microns.

    Because stud bumping is a serial process, the bumping time required increases with the numberof bumps. However, high speed equipment now can place as many as 12 bumps per second. Studbump assemblies demand more precise die placement equipment and are less tolerant ofplacement errors than self-aligning solder assemblies.

    Slide 10 n 11

    ACF flip chip

    ACF Assembly

    The assembly process is simple, lending itself to a high level of automation and reliability.

    Step 1: Prepare the substrate. The material can beceramic, build-up, polyimide, or any other standardboard material. Its only requirement is that it be goldplated and clean.

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    higher, more and more IC designers and users will find that fine-pitch, small-package, high-reliability ACF is their best choice.

    Slide 14

    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) aretwo 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-countvariety. However, the move to organic substrate introduced the problem of significantthermomechanical mismatch because of the relatively high CTE of organic materials. However,underfilling adequately reduces joint stress, and reliability can be regained. Underfilling, whileallowing popular, low cost substrates to be used, increases processing time and adds cost.Nonetheless, underfill allows virtually all substrates to be used.

    Slide 16

    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 availabilityof flip chip materials, equipment, and services.

    Smallest Size

    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 simplestminimal package, smaller than Chip Scale Packages (CSPs) because it is chip size.

    Highest Performance

    Flip chip offers the highest speed electrical performance of any assembly method. Eliminatingbond 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.

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    Greatest I/O Flexibility

    Flip chip gives the greatest input/output connection flexibility. Wire bond connections arelimited 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.

    Most Rugged

    Flip chip is mechanically the most rugged interconnection method. Flip chips, when completedwith an adhesive "underfill," are solid little blocks of cured epoxy. They have survived thelaboratory equivalents of rocket liftoff and of artillery firing, as well as millions of cumulativetotal hours of actual use in computers and under automobile hoods

    Lowest Cost

    Flip chip can be the lowest cost interconnection for high volume automated production, withcosts below $0.01 per connection. This explains flip chips longevity in the cost-consciousautomotive world, pervasiveness in low cost consumer watches, and growing popularity in smartcards, RF-ID cards, cellular telephones, and other cost-dominated applications.

    Slide 17

    Applications

    y SAW Filters

    A mainstream high volume application of gold stud bump flip chip is for surface acoustic wave(SAW) filters, used in cell phones and other RF applications. SAW filter assembly benefits fromthe form factor advantages of flip chip to yield smaller, thinner, and lighter units than olderpackaging approaches. However, proper functioning of SAW filters requires that the surface ofthe active element remains free from physical contact with extraneous materials.

    y Hearing Aids

    Hearing aids are another form-factor driven application for gold stud bump flip chip assembly.Here, both size and weight must be minimized.

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    y RFID Tags

    Low-cost radio-frequency identification (RFID) tags are potentially an "intelligent replacement"for bar codes.

    y Biomedical Devices

    Advanced biomedical devices such as DNA analyzers, which operate in wet-chemicalenvironments, are another successful application of gold stud bump interconnection.

    y Detector Arrays

    Pixel detector arrays use gold stud bumps for direct connection of each of several hundred sensorelements to its own channel of first-stage signal processing. These arrays, used in high-energy x-ray and particle detectors for astrophysics, accelerators, x-ray machines, and similar applications,keep energy-absorbing lead solder bumps out of the detector by gold stud bumping .

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