Die Bonding: How to Achieve "Seamless Connection" Between Chips?

As a post-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, 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 that allows chips cut from wafers to be bonded to a package substrate (leadframe or printed circuit board) after the dicing process.

<!--[if !supportLists]-->1.<!--[endif]-->What is Bonding? In the semiconductor process, "bonding" refers to the attachment of a wafer chip to a substrate. The bonding process can be divided into two types: traditional and advanced. The traditional method uses die bonding (or die attach) and wire bonding, while the advanced method uses Flip Chip Bonding, which was developed by IBM in the late 60s. Flip chip bonding technology combines die bonding and 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 enables an electrical connection between a chip and 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 chip can withstand the physical stress that occurs after packaging and that it is able to dissipate the heat generated by the die as it operates. If necessary, constant electrical conductivity must be maintained or a high level of insulation must be achieved. So, as chips get smaller and smaller, bonding technology becomes more and more important.

<!--[if !supportLists]-->1.<!--[endif]-->Die Bonding Steps In the die bonding process, an adhesive is first applied to the package substrate. Then, place the chip top side up on the substrate. In contrast, flip chip bonding is a more advanced technique in which small bumps called "solder balls" are first attached to the die pads. Second, place the chip top side down on the substrate. In both methods, the assembled unit passes 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.

<!--[if !supportLists]-->2.<!--[endif]-->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 done on a die bonder. After die bonding of all good dies, the non-conforming 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 the mapping table.

<!--[if !supportLists]-->3.<!--[endif]-->Ejection 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. Because it is difficult to pick up the chip easily even with a vacuum, if it is forcibly pulled out, it will cause physical damage to the chip.

For this purpose, the "Ejection process" can be used, in which a physical force is applied to the target chip by an ejector that creates a slight height difference with 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.

<!--[if !supportLists]-->1.<!--[endif]-->Die bonding process using epoxy resin for bonding Alloys made of gold or silver (or nickel) can be used for die bonding, 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, it will warpage due to the difference in expansion coefficient, which may cause 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) DAF is a thin film that adheres to the bottom of a die. Compared to polymer materials, the thickness can be adjusted to a very small and constant degree with DAF. 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 die and DAF from the dicing tape, and without the use of epoxy resin. Since the dispensing process can be skipped in 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. For a long time, PCB-based substrates have been widely used because they can be used in small-scale 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. In the next issue, we'll look at another packaging technology – wire bonding.

Chip bonding, an important semiconductor packaging process, involves a number of different technologies and operations. In terms of bonding types, there are two types: traditional die bonding and wire bonding, as well as advanced flip chip bonding, each of which has different ways of connecting chips and substrates.

The steps of die bonding involve placing adhesive on the substrate, placing the chip, and in the case of flip chip bonding, attaching small bumps, and then passing through a temperature reflow channel to melt the adhesive or solder balls and then cool them for fixation. Chip picking and placement is an important task performed on the die bonder, picking up good chips from the wafer by plunger and placing them on the substrate, and sorting the good chips according to the wafer test results, while the bad chips are discarded when the frame is recycled. The chip ejection process solves the problem of easy damage to pick up chips from dicing tape by applying physical force to create a height difference between the chips for easy pickup.

When it comes to adhesive materials for die bonding, there are a variety of options. Die bonding with epoxy resin is easy to use, but there is a problem with warpage due to the inconstant thickness of the epoxy resin. Although wafer bonded film (DAF) is expensive and difficult to handle, it can be adjusted to a very small and constant thickness, which can be used not only for chip-to-substrate bonding, but also for chip-to-chip bonding to form multi-wafer packaging, and can simplify the process, improve thickness uniformity, reduce defect rate and increase productivity, but some air may penetrate the film during use and cause deformation, which requires high equipment accuracy. In addition, different types of substrates perform die bonding in large orientations, and the temperature profile of drying adhesives is constantly changing as bonding technology evolves, including methods such as heat bonding and ultrasonic bonding, and packaging processes are becoming ultra-thin and diverse with the development of integration technology.