A set of processes has been developed and demonstrated to interconnect flip chips with an electrically conductive adhesive material to laminates. Paste deposition uses a photolithography process to define room temperature stable thermoplastic conductive adhesive bumps that are 0.2 mm in diameter and 0.1 mm high. Photobumping is done at wafer level, and dicing yields chips that are ready for attachment to a carrier. Chip bonding process development defined a process window and identified an optimal process point. Repeatable tensile bond strengths between 10 and 14 MPa can be achieved. Fracture mode typically occurs near an interface but in the joint material. Bonding temperature, pressure, and pressure on cool-down (to 120/spl deg/C) were identified as key process variables. The optimum bonding process point is applying one MPa to the chip, while heating to 235/spl deg/C. Pressure is maintained for 30 seconds at temperature and until cooled to 70/spl deg/C. These optimum bond parameters resulted in bond lines of 0.05/spl plusmn/0.005 mm. The harshest stress test is deep thermal cycling for both blanket and stitched chip designs. The interconnect performance on the blanket chip is comparable to soldered flip chip on laminate. The interconnect performance on the stitch chip is less robust. It is believed that reaction between the photobumping stripper and the polyimide passivation results in a weak interface between the adhesive bump and card metallurgy. Results from stress testing demonstrate the design feasibility of electrically conductive adhesive interconnects for flip chip attach to laminates.

The importance of flip chip technology is beginning to grow as the use of such technology is seen to be more and more advantageous. The search for alternatives to lead-based solder has also led to the study of conductive adhesives as a possible replacement for solder interconnect technology. Under a grant from DARPA, the IBM and Universal Instruments Corporations have sought to create a flip chip package using conductive adhesive interconnects. This paper presents the preliminary results of mechanical testing designed to determine the static failure envelope of the adhesive. A difference in fracture mode was observed between the tensile and compressive samples indicating that a change in failure mechanism occurred. Further work is being conducted in order to isolate the specific failure mechanisms involved.

Electrically conductive adhesives (ECAs) have been proposed as an alternative to solder in the surface mount (SMT) and flip chip attach (FCA) applications. This paper describes the development of a transient heat transfer model of a chip bonding process using the ECA bumps. The chip is heated using a top thermode directly contacting the chip and the card is heated from the back side (Z=0) using a heater. A detailed three-dimensional heat transfer model to account for the conduction, heat storage and convection and radiation from the card is developed using the finite volume technique. The spatial and temporal temperature distributions are studied through initial ramp-up, dwell and cool-down processes. It is seen that the bump temperatures are dominated and controlled by the heating process near the chip as opposed to heating the back side of the card. The numerical model is verified via actual measurements and the agreement is within 15 percent

The accomplishments are broken down into three different categories: materials development (polymer metal composite with increased bond strength), process development (reliability testing, bonding optimization evaluation, testing of bulk conductive adhesives, paste deposition process development), and equipment development (laboratory bonding equipment, cost estimation for flip chip attach methods).

The most common failure mechanisms for electrical contacts are classified, defined, and discussed. Time- and temperature-dependent mechanisms include metallurgical diffusion, surface migration, evaporation, decomposition, outgassing, stress relaxation, and creep. Concentration gradients and mechanical loads also affect the rate of these mechanisms. Other mechanisms are induced from a mismatch in coefficient of thermal expansion and are triggered by thermal cycling. Corrosion mechanisms result from corrosive gases and particulates. Wear mechanisms result from motion at interfaces. Mechanical failures occur from structural defects and foreign particulate contamination. Various parameters influence these failure mechanisms. The ultimate single effect is a degradation of the contact interface resulting in high contact resistance, an open, or an intermittent open. It is concluded that, with sound understanding, these failure mechanisms and their undesirable effects can be avoided by appropriate connector design or selection, implementation, and quality control.< >