Electroless Bumped Bare Dice on Flexible Substrates

Abstract

Modern semiconductor technologies have advanced to the level of sophistication where the benefits of the high functional and power density, high speed, low defect rate and low wafer processing cost can seldom be fully utilised at the final equipment or even at the single packaged semiconductor component level due to the limitations of wire bonds and lead frame fan‐outs. This paper suggests a new assembly method where low‐cost contact bumps are deposited on semiconductor wafers and then the dice are reflow soldered or gang bonded to the substrate. The bumps are electroless nickel deposited and coated with a protective layer of gold. As the nickel bumps are non‐collapsible, they are better suited to Extra High Density Interconnections (EHDI) than the more usual solder bumps. The amount of solder must be accurately dispensed either on the die bumps or on the substrate bonding pads using various methods. Essential to the high volume assembly is fast pick‐and‐place operation and simultaneous soldering of all components in a reflow furnace. In certain applications bonding of the bumped device (one die at a time) can be done using reflow or thermocompression gang bonding by applying a heated thermode to the backside of the die. In this case, the bonding energy will be transferred through the die to the bumps. Tentative solder joint strength and reliability aspects are discussed. Further process and design improvements are suggested.