A comparison study of different wafer backside coating technologies

Abstract

The market demand for handheld electronic products that are smaller, thinner, and with more functionality has steadily increased in recent years. To increase the functionality of new electronic devices, package designers commonly integrate multiple dice into the same package by placing the semiconductor chips side-by-side and share the same die attach paddle. Although this is the simplest and most practical approach, the trade-off is footprint. A package with a large footprint may not fit the customer's printed circuit board. An alternative approach is to stack the dice on top of each other. Stacking the dice in a pyramid configuration offers the advantage of a smaller package footprint. However, the dice need to be thinned down so that the resulting stack-up will still fit inside the package thickness, with room to spare for the loop height of the wires. But for the die attach process, conventional dispense paste may not be the best choice for thin dice because dispense paste materials are prone to inconsistent dispense volume (e.g., insufficient, excessive, voids, etc.), inconsistent bondline thickness and die tilt. Furthermore, resin bleed from the dispense paste can contaminate the wirebond pads that are nearby and cause non-stick-on-pad (NSOP). As such, using traditional die attach paste for stacked die application can significantly affect the assembly yield. An alternative to conventional dispense pastes materials are Wafer Backside Coating Materials.Carsem (M) Sdn Bhd has been using screen printing technology to apply a thin layer of adhesive (conductive or non-conductive) onto the back of silicon wafers for several years now, and the screen printing process is considered mature and well established. This process has made it possible to attach a die onto the leadfingers of a Chip-On-Lead (COL) package. This approach also enables die stacking. For instance, during the assembly of accelerometer devices where the MEMS die is stacked onto the ASIC die, or vice versa. But as new products are being developed using thinner dice (<100um), it is becoming more and more difficult to screen print die attach adhesive on such thin and fragile wafers. Hence, Carsem proceeded to explore alternative Wafer Backside Coating technologies which can address the current limitations of the screen printing technology. This paper compares the three different process technologies for wafer backside coating, namely: (1) Screen Print Technology, (2) Dicing Die Attach Film (DDAF) Technology, and (3) Spin Coating Technology. The advantages and disadvantages of each technology shall be discussed.