The back-end process: Step 4 - Die attach
Today's challenges

BY PETER BÜHLMANN AND DOMENICO TRUNCELLITO

The Most Common Processes

There are four major die attach processes used in semiconductor packaging.

Eutectic Die Attach: The eutectic die attach process is a well-established bonding technique, having been used in the early days of semiconductors for the first transistors and integrated cicuits (IC). It is still widely used for small signal products - so called "jelly beans" - manufactured by the millions every day. Currently, eutectic die attach has regained importance in the field of packaging optoelectronic components and high-power communication devices. Under high temperature, the silicon-gold combination forms the eutectic bond. A scrub motion during the bond process increases the strength and quality of the intermetallic connection between the chip and substrate. For large chips, additional gold in ribbon form is used. Because of the high temperature of the process (up to 450°C), a protective forming gas atmosphere (a nitrogen/ hydrogen combination) is needed to prevent oxidation of the lead frame.

Soft Solder Die Attach: This process uses a solder material to bond the die to the lead frame. The solder is introduced as a wire preform and melted onto the hot lead frame surface as a liquid solder dot. A chip is placed on the hot solder and as soon as the solder cools down, a solid connection is established. These solders are typically lead- and tin-based alloys. A controlled temperature profile is required to define the liquidus/ solidus transition. Again, a protective forming gas atmosphere is required to prevent oxidation of the lead frame. Soft solder applications are typically used in automotive and high-power devices.

Flip Chip Bonding: A process similar to soft solder die attach is used in flip chip bonding. Here, the chip is flipped before being attached, and solder bumps between the chip and substrate serve as both an electrical and mechanical interconnection. Drivers for this process are high I/O counts and increased electrical performance requirements for high-speed, high-frequency applications.

Tape and Other Die Attach Processes: For some special applications, such as large memory devices, different tapes are used as the adhesive (epoxy, polyimides and thermoset/thermoplastic materials). The latest trend is to apply the adhesive directly to the backside of a wafer instead of using tape as an adhesive carrier. Wafer backside coating provides clear advantages for thin die and stacked die applications, because the adhesive is spread evenly on the bottom of the chip and it is always in the right position.

Epoxy Die Attach: Epoxy die attach is the most commonly used process, and the term generally encompasses the use of other adhesives, such as polyimide- or silicone-based materials. The adhesive is dispensed in paste form on the bond pad of the lead frame or substrate before die placement. Typical applications using epoxy cover a wide range of devices, from simple transistors to high power processors, memory and ASICs.

Today's packaging technologies create challenges for epoxy die attach processes. The key criteria for success (such as productivity and material cost) are not as crucial in advanced packaging. The challenges in this segment lie within the process and its execution.