This tutorial is a comprehensive review of underfill processing that appeared in the June, 2006 issue of empfasis, a publication of the National Electronics Manufacturing Center for Excellence.
With today’s advancements in component packaging, dispensing has become a more critical part of the electronics manufacturing line. Consequently, the increasing popularity of such processes as flip chip underfilling has led to complications in the industry, primarily because it has added a process step involving strict attention to accuracy and effectiveness to the assembly line. Although the process of underfilling flip chips has become a manufacturing variable, many of the leading dispense equipment manufactures have developed precision machines that have greatly improved underfilling flip chips while maintaining productivity throughout the assembly line. Improvements in various types of underfills have also minimized many of the common process control issues.
Why It Is Necessary to Underfill Flip Chips
The thermal expansion mismatch, also known as the CTE (Coefficient of Thermal Expansion), between the flip chip device and PCB (Printed Circuit Board) is absorbed by the underfill protecting the small bumped solder joint. Underfill material also protects the chip from moisture, ionic contaminants, radiation, and hostile operating environments such as thermal and mechanical conditions, shock, and vibration.
An Overview of the Underfill Process
The flip chip is conveyed in-line or hand loaded into the dispensing equipment.
The die is then heated to temperature or is heated during dispensing to provide good underfill flow.
The flip chip is located either mechanically by fixture or by an automated vision alignment system.
The fluid is dispensed on one or more sides of the flip chip, sometimes in multiple dispense passes.
The underfill fluid flows via capillary action under the flip chip.
Depending on the pattern chosen, a fillet pass may be required to provide an even fillet around the perimeter.
The underfill is cured in a reflow or microwave oven at the underfill manufacture’s recommended temperature
Heating the Flip Chip to Underfill
Packages are typically heated prior to the dispensing of underfill adhesives. This allows the fluid to reach the recommended temperature just before it reaches the substrate. The underfill fluid is then heat induced using capillary action to draw the fluid under the die. The temperature at which this occurs typically is between 600° C and 900° C. Heating sometimes occurs at the dispense station, creating a gate in the assembly process because of the time it takes for the die to heat up. Generally, there are three types of underfill methods: contact heating, infrared heating, and convection heating.
When substrates are brought into contact with a heated platen or surface and then brought up to temperature by conduction, it is contact heating. The platen is controlled in a closed-loop method that causes the substrate temperature to be controlled passively. Temperature can be held evenly with this method, but it is time consuming to ramp up. Another disadvantage to this system is that it will only work on a single-sided substrate.
Convective heating uses hot air blown on parts, causing them to heat up. This can be done in either an open or closed-loop system. While this is an effective method, it can produce unnecessary heat inside a dispensing machine, reducing the pot life of some underfills.
Infrared heating is the system of choice because it is the most consistent and easily controlled form of heating a flip chip. This system uses bulbs that radiate heat to the substrate, making it easier to accurately measure the no-contact temperature of the board itself. Infrared heating allows for better control of the temperature ramp rate without exceeding the recommended underfill set point temperature. Unlike contact heating and convective heating, infrared heating can provide more concentration of heat in a particular area with the use of moveable IR units. The only disadvantage to a system like this is that it may not work with certain fiducial image recognition systems because of its flashing bulbs.
Vision and Mechanical Alignment
Typically, vision alignment occurs when two fiducial marks on opposite corners are optically recognized (shape and size), aligning the programmed dispense pattern to the flip chip. This is done automatically, based on the location of the fiducial marks programmed. Other methods include semi-automatic systems where a camera is presented over the fiducial mark and the operator fine tunes it to the location with a cross hair generator. Then the operator acknowledges back to the system that the alignment mark has been found. If fiducials are not present, other points, like the corner of a substrate or flip chip, are used.
Mechanical alignment is usually done through the use of a custom fixture for the substrate or flip chip device. Tooling pins that are pre-drilled in the substrate during the bare board fabrication are also sometimes used.
Choosing the right dispensing method is imperative to properly underfilling a flip chip. The following are the three most commonly used dispensing systems in the industry, with a brief description of how each system works and their underfilling advantages and disadvantages.
Time and pressure
Time and pressure is a controlled, pressurized system with a nozzle valve that is used on such applications as chip bonding, conductive adhesion, and solder pasting. It is rarely used for underfilling because of its consistency and material handling.
An auger pump works using a pump with a lead screw rotating in a body adding energy to the adhesive path within the body. This is done by turning on and off the screw’s electric motor, which pumps measured amounts of adhesive through the body. This system is primarily used for underfilling and encapsulating because of its accuracy and flexibility. However, it is not a good system to use for high production volumes due to its inconsistency at higher speeds.
Positive Displacement Pump
Also known as a piston pump, a positive displacement system operates by the movement of a piston in a closed chamber. Much like the time and pressure pump, the piston pump works best with materials such as chip bonding fluid, conductive adhesions, solder pastes, and underfill. This system proves to have better accuracy with underfill at higher speeds in a production environment. The disadvantages are the complexity of cleaning and its sensitivity to air bubbles in the fluid.
To briefly sum it up, choosing the right system is based on the quantity of product. For example, a prototype environment will likely use an auger pump where the production or OEM facility will likely use a positive displacement pump.
Types of Underfills
There are many different types of underfill used in the industry today. The four most commonly used are snap cure, low profile, high performance, and reworkable.
Snap cure underfills are typically used in a production environment because of their fast flow rate and quick cure time schedule. It’s also high in reliability and can fill a gap as small as 3 mils. The only disadvantage is that it may not be as reliable as high performance underfills, depending on the end use of the flip chip.
This type of underfill is much like the snap cure, except it is better suited for flip chips that have smaller gaps to be filled.
High performance underfills are typically used in a prototype environment because of their slower flow rate and longer cure time schedule. The advantage of using this type of underfill is its high reliability factor. It can absorb much more of the coefficient of thermal expansion than snap and low profile types of underfill, allowing experimentation to take place on the flip chip where the CTE mismatch is not known.
Similar to the snap cure and low profile underfills, the reworkable underfill has a fast flow rate and a quick cure time schedule, but at higher temperatures. Its obvious advantage is that it is reworkable, whereas the other underfills are not. The disadvantage is that it may be more expensive to use in terms of underfill material costs and the actual rework labor and specialized equipment involved.
There are three commonly used types of dispensing patterns: single side, L-shaped, and modified U-pattern.
The simplest of the three, the single side pattern dispenses underfill along one side of a flip chip. If the fillet size (size of the angle between the top of the chip and surface of the board) is not important, the entire amount of adhesive can be dispensed along the longest edge of the die in one shot. If a smaller fillet is desired, a multiple straight-line pass with a smaller needle is used. This pattern provides the slowest flow out, which allows the least trapped air. However, the single side pattern does make it difficult to control the fillet size along the three opposite sides of the flip chip.
The L-pattern allows more fluid to be dispensed in a single pass without expanding the fillet size. This pattern also enhances the speed of underfill because more sides of the die are used to initiate the fluid flow while providing better fillet control on the adjacent sides compared to that of the single side pattern. However, the L-pattern may create air pockets (voids) under the die if one flow front becomes ragged and meets the second flow front in a way to trap an air bubble.
The modified U-pattern optimizes fluid flow under the die, making it the quickest dispensing pattern with the most consistent control of fillet size around the perimeter of the flip chip. However, the chance of air pockets becoming trapped is increased for the same reason as with the L-pattern.
To get reliable parts from the underfill process, the right amount of underfill material must be dispensed in the right pattern in a well-timed sequence. The amount of material dispensed is geometrically computed by adding the volume under the chip to the volume of the fillet, then subtracting the volume of the interconnect bumps. As illustrated in Figure 2-1, software is available that will automatically calculate the amount of material required based on the size of the flip chip and desired fillet width.
Underfill Dispensing Tolerances
The tolerance on the amount of material dispensed depends on the physical tolerances of the die and fillet. The minimum amount of underfill dispensed to ensure that the part has been underfilled properly is when the dispense gap is at its maximum and the fillet width is at its minimum. The maximum amount of underfill material that can be dispensed is when the gap is at its minimum tolerance and the fillet is at its maximum width. (See Table 2-1.) In essence, as the die size increases, the volume under the die increases faster than the volume in the fillet. This makes the fillet the reservoir that compensates for variation of volume under the die.
The following conclusions can be made about dispensing tolerances:
The fillet is the reservoir that absorbs the changes in the dispense gap from part to part.
Larger dies have tighter tolerances by percentage be-cause the volume under the chip is relatively larger than the fillet.
The more accurately the dispensing tolerance can be held, the tighter the fillet tolerance can be held.
Choosing the right needle
The needle is also a critical element in dispensing. Due to the importance of accuracy and volume, the type of needle being utilized is a critical factor. There are a variety of needle shapes and sizes. Needles are constructed of either plastic or metal and have straight, bent, or tapered tips. The size of the needle is identified in wire gages 14-30. To determine what type of needle to use, you need to know what kind of material you are dispensing and how much, in terms of volume, you need to dispense. For example, in dot dispensing, the size of your dot should be the same diameter as your needle tip. This also applies to underfilling, but in terms of lines rather than dots.
Curing the Underfill
Curing the underfill has classically been performed in some type of convective and/or radiant oven situated in-line after the fluid dispenser. However, microwave-curing ovens have been implemented in the line because of the improvement in reduced fillet residue that is sometimes left behind. The curing schedule for the underfill depends on the type it is (snap cure, reworkable, low profile, or high performance) and the manufacturer’s recommendations.
Increasing Flip Chip Processing Throughput
The following is a list of suggestions that can help increase throughput and may reduce process defects during flip chip underfilling:
- Heat packages prior to the dispensing step.
- Add a post-dispense station to complete the fillet around the perimeter of the chip and/or to dispense more fluid under the die if required. This will prevent a bottleneck where the original underfilling step took place.
- Use a dispenser that has automated set-up procedures and can automatically calibrate the volume required to underfill the flip chip.
- Use a dispenser with automatic fiducial recognition software.
- Choose a pump or valve with a fast flow rate to help reduce dispensing time.
- Use easy to clean pumps or valves, preferably ones that can be cleaned off-line to reduce down time. Having more than one pump or valve ready to change will also keep your line from shutting down.
- Use multiple dispensers to do more than one flip chip in a single pass.
- Manipulate the statistical process control data from your dispenser so the quality of your product can be monitored while maintaining a consistent throughput.