With today's advancements in component packaging, dispensing has become more of a critical part of the electronics manufacturing process. For example, increasing popularity of such processes as flip chip underfilling has led to complications in the industry. It has become an additional process step in the assembly line that involves strict attention to accuracy and effectiveness. Although the process of underfilling flip chips has become a manufacturing variable, many of the leading dispensing equipment manufacturers have developed precision machines that have greatly improved underfilling processes, while maintaining productivity throughout the assembly line. Improvements in various types of underfills that are now offered have also minimized many of the common process control issues.

This month's Tech Tips, which is the first part of a two part series, focuses on key variables and requirements for flip chip underfilling as well as the necessary hardware, dispensing parameters, heating styles, types of underfills, and cost effective throughput recommendations.

1. 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 under the flip chip via capillary action . Depending on the pattern chosen, a fillet pass may be required to provide an even fillet around the perimeter of the flip chip. The die is then cured at the underfill manufacturer's recommended temperature.

2. 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 specified temperature just before it reaches the substrate. The underfill fluid is then heat-induced using what is known as capillary action to draw the fluid under the die. The temperature at which this occurs typically is between 60 - 90 0 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 heating methods: contact heating, infrared heating, and convection heating.

Contact Heating : When substrates are brought into contact with a heated platen or surface and then brought up to temperature by conduction, it is considered to be contact heating. The platen control is used in a closed-loop method causing the substrate temperature to be controlled passively. Temperature can be held evenly with this method, however, it is time consuming to ramp up. This system will only work on a single sided substrate.

Convective Heating: 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: 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 also allows for better ramp rate control of the temperature by heating a substrate in a minimum amount of time without exceeding the maximum temperature. Unlike that of contact heating and convective heating, infrared heating can also 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. Infrared heating is normally the system of choice because it is the most consistent and can be easily controlled.

3. Vision and Mechanical Alignment
Vision: Typically, vision alignment occurs when two fiducial marks on opposite corners are optically recognized (shape and size) aligning the programmed dispense pattern with the flip chip. This is usually done automatically based on the location of the programmed fiducial marks. If fiducials are not present, the corner of a substrate or the flip chip itself may be used.

Mechanical: Mechanical alignment is usually done through the use of a custom fixture for the substrate or flip chip device. Tooling pins that are predrilled in the substrate during the bare board fabrication process are also sometimes used.

4. Dispensing Methods
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 along with 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 ever used for underfilling because of its lack of consistency and material handling.

Auger Pump: An auger system 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 pumping 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 adhesives, 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 up, choosing the right system is based on the quantity and type 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. Next month, Tech Tips will review types of underfills, dispensing patterns, dispensing accuracy, choosing the right needle, curing the underfill, and increasing flip chip processing throughput.