Flip Chip Bonding Using Sony Anisotropic Conductive Film (ACF) FP1526Y

Transcription

1 Flip Chip Bonding Using Sony Anisotropic Conductive Film (ACF) FP1526Y Purpose: Author: Rekha S. Pai (07/29/03) To use ACF as an interconnection method for attaching dice to substrates. Direct electrical interconnections were formed by bonding the face down (flipped) dies onto substrates or carriers, by means of ACF laminated stud bumps on the chip bond pads. ACF FP1526Y (Sony Chemicals) was used as the attachment material for the flip chips and the chips were bumped on the Kulicke and Soffa (K&S) 4524D wire bonder. Precautions: Do not touch the electrode or the wire during bonding or when manually sparking the Negative Electric Flame Off (NEFO) on the wire bonder. The system produces a spark between the electrode and the wire, which can cause an electrical shock. The shock is not potentially life threatening. The usage of tweezers and clean gloves is strongly recommended. Flip Chip Theory: Flip chip packaging has advantages over other packaging methods in terms of size, reliability, performance, flexibility and cost. There are three stages in making flip chips: bumping the die, attaching the die to the substrate and filling the space remaining under the die in order to provide mechanical and thermal stability and avoid warping. Flip Chip assemblies are classified by the type of bumping, method of attachment and processes used. In this case, the wire bonder forms gold bumps and ACF is the method of attachment.

2 Fig.1. Structure of the ACF particle ACF works by trapping conductive particles between the bonding pads of the chip and the substrate. FP1526Y ACF consists of 5 um polymer particles electroplated with Nickel/Gold and covered by an insulating layer suspended in an epoxy (Fig.1). The insulating layer ensures that there is no shorting through contact with the neighboring particles. FP1526Y is about 30 um thick. However, the conductive particles (Particle density ~ 3million/mm 3 ) are present only in about 15 um and the rest of the ACF consists of only the resin (Fig.2). Fig.2. FP1526Y Structure Gold stud bumps are placed on the chip bonding pads through a modification of the ball bonding procedure used in conventional wire bonding. In conventional ball bonding, the gold wire is melted to form a sphere forming an interconnection with the chip bonding pad by the application of pressure, temperature and ultrasonic energy. Next the capillary extends the gold wire to the substrate bonding pad and forms a bond before breaking off the wire. In stud bumping the capillary forms a sphere to form a permanent connection to the chip bonding pad and then breaks

3 off the wire. Later the stud is flattened or "coined" by applying pressure in order to give uniform bump heights while pressing any remaining wire ("tail") into the bump. ACF is then laminated onto the chip ensuring that there are no voids around the stud bumps. The chip is flipped and aligned with the substrate optically. The chip and the substrate are lightly tacked together in a pre-bonding step with low pressure and temperature. Finally during bonding the insulation on the particles trapped between the die and the substrate is removed in the Z-axis direction due to the pressure being applied in that direction. This enables the deformed particles to form an electrical connection from the die to the substrate. The epoxy cures due to the high temperature locking the particles in their compressed state forming a reliable electrical path between the die and the substrate. Preparations: FP1526Y ACF (ITO America Corporation, Website: Arizona Telephone: ) must be stored in the freezer (-20 C to +5 C). Allow ACF to rise to room temperature (~30mins) before use. Do not remove ACF from the Ziploc bag before room temperature is attained in order to prevent condensation forming. Procedure: 1. Ash the dice using the March RIE (Reactive Ion Etch SOP) to assure that the bonding pads are free from organic contamination. Typical parameters for the ashing are: Gas Flow = 20sccm Oxygen; Pressure = 400mtorr; Power = 300 watts (RF); Time = 30secs 2. Bump the die using the 4524 Digital Wire bonder (Kulicke and Soffa) shown in Fig.3. Typical parameters for wire bonding (will vary depending on chip and workholder settings) are: Search = 0.47; Power = 2.19; Tail = 4.9; Force = 5.0; Ball = 5.2; Temperature = 150 C

4 Fig.3. Kulicke and Soffa 4524 Digital Wire Bonder 3. Place a clean microscope glass slide on top of the chip and "coin" (flatten) the bumps by putting a weight of 10gms/bump on the glass slide for 2mins. Check that all the bumps have been coined under the microscope. 4. Cut a piece of ACF so that it covers the appropriate bonding pads on the die. Remove the transparent PET cover film from the 3 layer ACF. Flip the ACF and place it on the chip with the base film facing up (Fig.4). 5. Attach a piece of adhesive scotch tape to the base film so that the base film edge is completely covered. Place the chip on the heated chuck of the wire bonder (Temperature = 60 C). The ACF is heated up to 60 C to ensure full contact and adhesion to the dice.

5 Fig.4. Illustration of tacking the ACF to the dice 6. Laminate the ACF to the chip by rubbing the release liner surface slowly with a credit card. Peel the edge of the scotch tape quickly to remove the liner. Heating the ACF for long periods of time (~10secs) while laminating should be avoided since the epoxy in the ACF starts curing after 80 C. Make sure that there are no voids in the lamination. 7. Align the ACF laminated flipped chip with the appropriate bonding pads of the substrate with the help of an optical microscope. 8. Heat the flip chip assembly for 4 seconds (Temperature =60 C) with a pressure of 500gf/cm 2. Pressure is applied by putting standardized weights on top of the assembly and this procedure is accomplished on the wire bonder chuck. 9. Final bonding is done by placing heated 50g/bump weight on top of the flip chip assembly at 180 C for 15secs. This procedure is also done on the wire bonder chuck. A table of recommended bonding conditions for FP4411 is given below: Temperature Time Pressure Pre-bonding 60 C 4 seconds 50 gf/cm^2 Bonding 180 C 15 seconds 50 g/bump Table 1. Recommended bonding conditions for ACF FP1526 Please note that the ACF bonding characteristics are affected by the substrate used, chip size and metal pattern. This is only a recommended starting point for the user to determine optimum conditions for the application on hand.

6 10. An illustration of the procedure is given below (Fig.5). Fig.5. Illustration of Flip Chip Processing 11. Verify the resistances between the chip and the substrate by using a multimeter. If flip chip assembly has been successful the resistance measured must be low (a few ohms at the most) 12. If resistance measurement is high or resistance is increasing it means that the epoxy has not already set i.e., bonding is not complete, repeat the final bonding step for a few more seconds and verify the resistance with the multimeter.