Die Bonding: A Bridge to the Future

As a back-end process in semiconductor manufacturing, the packaging process includes back grinding, dicing, die bonding, wire bonding, and molding. The sequence of these processes can be adjusted, combined with each other, or combined according to changes in packaging technology. In the previous issue, we introduced the dicing process of cutting wafers into individual chips. Today, we're going to talk about die bonding, which is a packaging process in which chips cut from wafers are bonded to a package substrate (leadframe or printed circuit board) after the dicing process.

<!--[if !supportLists]-->1.<!--[endif]-->What is Bonding? / Figure 1. Bonding type

In the semiconductor process, "bonding" means fixing a wafer chip to a substrate. The bonding process can be divided into two categories: traditional methods and advanced methods. The traditional method uses Die Bonding (or Die Attach) and Wire Bonding, while the advanced method uses Flip Chip Bonding, which IBM developed in the late 60s. Flip chip bonding technology combines die bonding with wire bonding to connect the chip to the substrate by forming bumps on the die pads.

Just as an engine powers a car, chip bonding technology connects a chip to the outside world by attaching a semiconductor chip to a lead frame or printed circuit board (PCB). Once the die is bonded, make sure that the die can withstand the physical stress that occurs after packaging and is able to dissipate the heat generated during the operation of the die. If necessary, constant electrical conductivity must be maintained or a high level of insulation must be achieved. As a result, as chip sizes continue to shrink, bonding technology becomes more and more important.

<!--[if !supportLists]-->1、   <!--[endif]-->Die Bonding Steps: Comparison between Die Bonding and Flip Chip Bonding

In the die bonding process, an adhesive is first applied to the package substrate and then the chip is placed on the substrate with the top side facing up. In contrast, flip chip bonding is a more advanced technique in which a small bump called a "solder ball" is attached to the die pad and the die is placed on the substrate with the top side down of the chip. In both methods, the assembled unit goes through a channel called Temperature Reflow, which adjusts the temperature over time to melt the adhesive or solder balls. Then, after cooling, the chip (or bump) is fixed to the substrate.

1. Chip Pick & Place

The process of removing hundreds of chips attached to the cutting tape one by one is known as "picking". The process of using a plunger to pick up a good chip from a wafer and place it on the surface of a package substrate is known as "placement". Together, these two tasks are called "pick and place" and are performed on Die Bonder 1. After die bonding to all good dies, the unqualified chips that have not been removed are left on the cutting tape and are all discarded when the frame is recycled. In this process, the good chips are classified by entering the wafer test results (pass/fail) in Mapping Table 2.

Ejection ProcessChip Ejection Process: Enlarged view of force applied in three directions

After the dicing process, the chip is split into individual modules and gently attached to the dicing tape. At this point, it is not easy to pick up the chips one by one that are placed horizontally on the cutting tape, as it is difficult to pick up the chips easily even with a vacuum, and if they are forcibly pulled, they can cause physical damage to the chips.

To do this, the "Ejection process" can be used, in which a physical force is applied to the target chip by an ejector that causes it to form a slight step with the other chips, making it easy to pick up the chip. After ejecting the bottom of the chip, the chip can be pulled out from above using a vacuum picker with a plunger. At the same time, use a vacuum picker to pull up the bottom of the dicing tape to keep the wafer flat. 3Ejector: A ejector used to jack up a chip from under the cutting tape.

<!--[if !supportLists]-->1.<!--[endif]-->Die bonding process using epoxy resin for bonding When performing die bonding, alloys made of gold or silver (or nickel) can be used, especially for large hermetic packages. Joining can also be done by using solder or a paste containing metal (Power Tr), or by using a polymer (Polyimide) for die bonding. Among polymer materials, epoxy resins containing silver paste or liquid are relatively easy to use and are used frequently.

When using epoxy for die bonding, a very small amount of epoxy can be precisely placed on the substrate. After the chip is placed on the substrate, the epoxy resin is hardened at a temperature of 150°C to 250°C by reflow or curing, which bonds the chip to the substrate. In this case, if the thickness of the epoxy resin used is not constant, warpage due to differences in the coefficient of expansion may occur, which may lead to bending or deformation. So, while it is advantageous to use a small amount of epoxy resin, there are different forms of warpage as long as epoxy resin is used.

Because of this, an advanced bonding method using die attach film (DAF) has become the preferred method in recent years. Although DAF has the disadvantage of being expensive and difficult to handle, it is easy to grasp the amount of usage and simplifies the process, so the usage rate is gradually increasing.

<!--[if !supportLists]-->1.<!--[endif]-->Die bonding process using wafer bonded film (DAF) / Figure 6. Die bonding process using wafer bonded film (DAF).

DAF is a thin film that adheres to the bottom of a grain. ▶ Please refer to: Dicing, splitting a wafer into multiple semiconductor chips. Compared to polymer materials, DAF enables the thickness to be adjusted to a very small and constant degree. DAF is not only used for chip-to-substrate bonding, but also widely used for chip-to-chip bonding, resulting in multi-die packaging (MCP). In other words, the DAF that is tightly bonded to the chip waits for the dicing process to complete and then plays its part in the die bonding process.

From the structure of the dicing chip, the DAF located at the bottom of the chip supports the chip, while the cutting tape pulls the DAF located below it with a weak adhesive force. In this structure, die bonding is performed by placing the die on the substrate immediately after removing the chip and DAF from the dicing tape, and epoxy resin cannot be used. Since the dispensing process can be skipped during this process, the pros and cons of epoxy resins are ignored and replaced by the pros and cons of DAF.

When DAF is used, some air penetrates the film, causing problems such as film deformation. As a result, the precision requirements for the equipment that processes DAFs are particularly high. Still, DAF is the preferred method due to its ability to simplify the process and improve thickness uniformity, resulting in lower defect rates and higher productivity.

Depending on the type of substrate (lead frame or printed circuit board) used to place the chip, the direction in which the die bonding is performed varies greatly. PCB-based substrates have long been widely used because they can be used in small-format mass production packages. Correspondingly, as bonding technologies become more diverse, the temperature profile used to dry adhesives is constantly changing. Some of the representative bonding methods include heat bonding and ultrasonic bonding. With the continuous improvement of integration technology, the packaging process continues to develop in the direction of ultra-thin, and the packaging technology has also become diversified.

Die bonding technology has different development directions under different substrate types. For PCB-based substrates, they have a long history and a wide range of applications for small-format mass production packaging. With the advancement of technology, the diversified development of bonding technology has also led to the continuous change of the temperature profile of the drying adhesive. Bonding methods such as heat bonding and ultrasonic bonding are representative of this diverse development.

In the whole process of packaging process development, with the continuous improvement of integration technology, packaging is developing in the direction of ultra-thin, which makes packaging technology more diversified. Die bonding processes, whether traditional methods using materials such as epoxy resins or emerging methods using wafer bonding films, are constantly evolving and optimizing to accommodate the trend of shrinking semiconductor chips and increasing packaging requirements. From the steps of die bonding to the comparison to flip chip bonding, from the use of different materials to the differences in operation under different substrate types, each step demonstrates the importance of this process in the back-end process of semiconductor manufacturing and the need for its continuous development.