A packaging process, which is a back-end process for manufacturing semiconductors, proceeds in the order of back grinding, dicing, die bonding, wire bonding, and molding. The order of these processes can change or be closely linked to each other or merged, according to the change in packaging technology. In the previous session, we took a look at the dicing process which divides a wafer into individual chips. Today, we will have a look at die bonding, one of the packaging technologies for bonding a chip separated from a wafer with a package substrate (lead frame or PCB) after the dicing process.
1. What is Bonding?
Figure 1. Type of Bonding
In the semiconductor process, “bonding” means attaching a wafer chip to a substrate. Bonding can be divided into two types, which are conventional and advanced methods. The conventional method includes die bonding (or die attach) and wire bonding, while the advanced method includes flip chip bonding developed by IBM in the late 60s. Flip chip bonding is a method combining die bonding and wire bonding, and is a method of connecting a chip and a substrate by forming bumps on the chip pad.
Just as an engine is mounted on a vehicle to supply power, die bonding electrically connects a chip and the outside, by bonding a semiconductor chip onto a lead frame or a printed circuit board (PCB). After die bonding, the chip should endure the physical pressure generated after packaging and should be able to dissipate the heat generated during the operation of the chip. When necessary, it must maintain constant electrical conduction or realize a high level of insulation. Therefore, bonding methods are becoming more important as chips continue to become smaller and smaller.
2. Procedure of Die Bonding
Figure 2. Comparison between die bonding and flip chip bonding
For die bonding, the first thing to do is to dispense an adhesive on the package substrate. Then, a chip is placed on it, with the top side facing up. On the contrary, in the case of flip chip bonding, which is a more advanced method, small bumps called solder balls are attached to the pad of the chip. Then, the chip is placed onto the substrate, with the top side facing down. In both methods, the assembled unit passes through a tunnel called temperature reflow that can adjust temperature over time to melt the adhesive or solder balls. Then, it is cooled to fix the chip (or bumps) to the substrate.
3. Pick & Place of Chips
Figure 3. Pick & place of a chip
Individually removing hundreds of chips attached to the dicing tape is called “picking up”. When good chips are picked up from a wafer with a plunger, placing them on the surface of the package substrate is called “placing”. These two tasks called “Pick & Place” are performed on a die bonder1. After die-bonding all the good chips, unremoved faulty chips remain on the dicing tape, which are all discarded while the frame is recycled. In this process, good chips are sorted by entering the wafer test result (Go / No Go) in the Mapping Table2.
4. Chip Ejection
Figure 4. Chip ejection: Enlarged form with the force applied in three directions
Each chip which has completed the dicing process is individually separated and weakly attached to the dicing tape. At this time, it is not that easy to pick up the chips placed horizontally on the dicing tape one by one. This is because it doesn’t come off easily even when pulled up by using a vacuum, and if pulled out forcibly, it can cause physical damage to the chip.
For this reason, as a method to easily pick up chips, “ejection” is performed in which an ejector3 is used to apply physical force to the target chip to create a slight step difference from other chips. The bottom of the chip is ejected and the chip is pulled up by using a vacuum with a plunger from above. At the same time, the bottom of the dicing tape is pulled with a vacuum to level the wafer.
5. Die Bonding Using Epoxy for Adhesion
When bonding die, an alloy is made by using gold or silver (or nickel), especially for large-sized hermetic packages. It can also be connected by using a solder or a paste containing metal (Power Tr), or polymers (polyimide) are also used for die bonding. Among polymer materials, silver-containing paste-type or liquid-type epoxy is relatively easy to use and is frequently used.
When performing die bonding by using epoxy, a very small amount of epoxy is precisely applied to the substrate by dispensing it. After placing a chip on it, the epoxy is hardened at 150 to 250°C through reflow or curing in order to bond the die and the substrate together. At this time, if the thickness of the applied epoxy is not constant, the warpage that causes bending or distortion can occur due to the difference in the coefficient of thermal expansion. For this reason, while it is more advantageous when the amount of epoxy is less, the warpage occurs in any form as long as epoxy is used.
This is why a more advanced bonding method using a die attach film (DAF) is preferred in recent days. While DAF has some disadvantages of being expensive and difficult to handle, it is easy to apply a certain amount, simplifying the process so that its use is gradually increasing.
6. Die Bonding Using Die Attach Film (DAF)
Figure 6. Die bonding using die attach film (DAF)
DAF is a film that is attached to the bottom of a die. ▶ Refer to <Singulation, the Moment When a Wafer is Separated into Multiple Semiconductor Chips> With DAF, the thickness can be adjusted to be very thin and constant than when using polymeric materials. It is widely used not only for chip-to-substrate bonding, but also for chip-to-chip bonding to create a multi-chip package (MCP). In other words, the DAF closely bonded to the chip waits for the dicing to be completed and then performs its own function in the die bonding process.
Looking at the structure of the diced chip, the DAF located at the bottom of the chip is holding the chip, and the dicing tape is pulling the DAF under it with weak adhesion. To perform die bonding in this structure, after removing the chip and DAF from the dicing tape at once, the die should be placed on the substrate without using epoxy. Since the dispensing procedure can be skipped in this process, the pros and cons of epoxy are not seen and the pros and cons of DAF newly appear instead.
When using DAFs, some air can penetrate films, causing problems such as deformation of the film. In particular, high precision is required for the equipment that handles DAFs. Nevertheless, using DAFs is the preferred method because it can reduce defect rates and enhance productivity as it simplifies the process and increases uniformity in thickness.
The direction of performing die bonding varies greatly depending on which type of substrate (lead frame or PCB) is based on. PCB-based substrates are used frequently since long ago, as it is possible to mass-produce packages in small sizes. Accordingly, the temperature profile for baking adhesives is also evolving along with the diversification of the bonding technology. Some representative bonding methods include compressing with heat or bonding by using ultrasonic waves. As packages continue to evolve to ultra-thin types with an increased degree of integration, packaging technologies are also diversifying. In the next session, we will cover wire bonding, which is one of these packaging technologies.
1Die bonder: A device used for die bonding
2Mapping Table: A software that sets the standards of good and faulty chips.
3Ejector: A pin that lifts the chip from under the dicing tape
ByJong-moon Jin
Teacher at Chungbuk Semiconductor High School
