Reconfiguration and Redistribution of Die Bonding Pads

bonding pad topography both at the die (wafer) level and at the substrate level using polymer dielectrics. For example, a bonding pad can be reconfigured, changing its size and shape, by spin coating a photocurable BCB layer over the entire surface, exposing to uv through a mask having the image of the desired geometry, and developing it (Fig. 4.6). Dielectrics such as BCB or polyimides may also be used to redistribute peripheral bonding pads on a die to form an area array of pads to permit solder bumping for flip-chip bonding or ball grid array packages (Fig. 4.7). Likewise, internal pads on a die can be redistributed to one side to permit wire bonding of vertically stacked dice. In these processes, a dielectric coating is spun or otherwise deposited onto the single chips, or preferably onto the dice at the wafer stage, vias are formed over the original pads, the entire surface is then metallized, and finally, through a photoresist and etching process, conductor lines are formed that route and rearrange the pads to new positions on the die (Fig. 4.. Photoimageable dielectric coatings are desirable because fewer steps are involved compared to using photoresists, but image resolution, uv exposure, and focus must all be taken into account for best results.[12] Dielectric coatings must have a high degree of planarization, have good electrical properties, be able to withstand temperatures as high as 250°C that may be used in solder attach and solder reflow processes, and have low moisture absorption. Polyimides have a long history of use, but the BCB Cyclotenes∗ have several properties such as lower dielectric constants, lower moisture absorptions, and a high degree of planarization that gives them an advantage over polyimides for the new generation of high-speed, high-frequency devices.

Photoimageable polyimides and BCB resins produce excellent via and line definitions when used as plating resists in the fabrication of solder bumps or gold bumps on semiconductor wafers for flip chip devices. Both dielectrics are being used as stress buffer coatings and as interlayer dielectrics.

a soft bake to a hard bake, a weight loss occurs that is reflected in a film thickness reduction of 40–50%, yet pattern definition is not significantly affected by this. A final bake temperature of 350°C for 30 minutes is considered a minimum for most polyimides whether they are photocured or not. However, temperatures as high as 400°C for 1 hour may be required for some applications to assure complete imidization and removal of the water formed during imidization.