Friday, July 21, 2006

Epoxy Die Bonding

The materials needed for die bonding are a diced wafer mounted on a wafer frame (using a tacky tape), lead frames or substrates to place the die on, an adhesive (epoxy or solder), bonding tools to pick up the die and place it on the substrate, and of course, a die bonder (Figure 1).

The main components of a die bonder are the wafer handling section, the lead frame handler or work holder, and the pick-and-place module. The die bonding process includes several fully automated steps as detailed below; some of these steps are performed simultaneously (Figures 2-4).

1. A stack of lead frames or substrates is placed into the lead frame loading module of the die bonder. The robotic lead frame loader picks up a lead frame from the stack and places it onto the lead frame handler or indexer.

2. Moveable clamps transport the lead frame from the input position to the first process position, the dispenser (either epoxy or solder, depending on the process).

3. At the dispense position, each bond pad on the lead frame is aligned using optics and precision sensors. An adhesive is dispensed in a pattern and volume appropriate for the chip size.

4. Lead frames and substrates contain multiple bond positions; adhesive is dispensed on each position sequentially.

5. Once the dispensing has been completed on the entire lead frame or substrate, it is transported along the indexer to the second process position, which is bonding.

6. While the lead frame or substrate is processed through dispensing, a diced wafer on a wafer frame is presented to the system and a chip is optically aligned and prepared to be picked for placement on the next lead frame or substrate.

7. The bond head picks a die from the wafer using a vacuum pick-up tool that grabs the die from the top, while a push-up mechanism from below the wafer gently releases the die from the tape.

8. Once picked and held on the bond head, the die is transferred to the bonding position where it is placed on the bond pad with the pre-dispensed adhesive.

9. At the same time the die is picked from the wafer, the bond pad is accurately aligned using optics and precision sensors.

10. Bonding time and bonding force results in a strong bond according to the specified process requirement.

11. Each bond pad on the lead frame or substrate goes through this process before the lead frame is unloaded into an output magazine.

12. Once a magazine is filled with lead frames or substrates, it is transported from the die bonder to a curing station.

There are also options available that enable optical dispense and bond quality checks. Because a number of steps are completed simultaneously, a very fast throughput is possible. A good rule of thumb in estimating throughput is one die attach per second. This results in an average of 3,600 uph. Typical throughput ranges from a high 7,000 uph for a small-outline integrated circuit (SOIC) device to a low of 2,200 uph for a singulated ball grid array (BGA) using epoxy.
What is Die Attach.

In the package assembly environment, die attach is a critical process step. In simple terms, die attach is placing a die (which is presented on a wafer frame) onto a metal or organic substrate using either an epoxy, solder or eutectic process to hold the die to the substrate. The most commonly used type of die attach is epoxy, possibly used by 70 to 80 percent of manufacturers today. However, as the entire scope of packaging changes to meet increasing demands for higher accuracy, greater speed, the ability to handle small die, the ability to handle a variety of substrate types and the need to address technology challenges (such as 300 mm wafers), die attach is advancing.