Feature Article - Underfills, Part 1 - By Christopher Henderson

In this section we will discuss underfills and their use in the electronics assembly process. Underfills are a class of polymers used to help enhance the connection between the die and the substrate. With the advent of solder bumping and BGA packages came the need for underfills. First, we will discuss why underfills are needed. Next, we will discuss the underfill process. Finally, we will discuss the reliability of underfill materials.

BGA underfills are now commonly used for chip-on-board as well as BGA and Chip Scale Package mounting. The underfill helps to distribute the stress evenly across the die, making the solder joints more reliable. It can also provide mechanical strength to a portable system, such as a cell phone, PDA, or other portable device. Because the underfill holds the component rigidly in place on the board, it can also improve the electrical conductivity of the solder joints. Finally, underfills can be used to protect the surface of the die.

The electronics industry increasingly uses flip chips and chip-scale packages in electronic assemblies for portable devices. An early decision engineers must make is the choice of the underfill material. Underfills without filler particles flow more rapidly, but have higher coefficients of thermal expansion. Underfill with filler particles flow more slowly, but have lower coefficients of thermal expansion and better shock-absorbing properties. Several considerations are involved in the design of a board using flip chips or chip-scale packages that will be underfilled. One significant consideration is component density. Components that are placed close to a component that is being underfilled may either draw fluid underfill away from the target component, or assist keeping the fluid underfill in place. Fluid underfill that flows to adjacent components generally causes no harm to the adjacent components, but it subtracts underfill from the precisely measured quantity delivered to the target component. Underfill that flows to adjacent components may also make rework far more difficult.

This diagram shows the basic ball grid array underfill process. First, one begins by dispensing flux on the surface of the substrate. This will help ensure the solder makes adequate contact to the solder pads on the board or substrate. Next, the chip is placed over the appropriate pads. Alignment is critical to help ensure a proper solder joint is formed. Next, the assembly is heated to reflow the solder. This can be done using hot air, infrared heating, vapor phase, or other methods. Next, the underfill is dispensed. There are two different methods for achieving a proper underfill. One is to dispense the epoxy at one end of the chip and allow capillary action to pull the epoxy under the entire part. The other method is to put epoxy on the chip before mounting it to the board or substrate. Many manufacturers are turning to the latter method, since the throughput is higher and the cost is approximately 60% less than the conventional capillary action method. After the underfill is dispensed, one can deposit fillets on the side to provide additional mechanical strength and help reduce stress. Finally, the epoxy is cured, resulting in an assembly that is mechanically strong.

One of the main reasons the electronics industry uses underfills is to improve the reliability of the solder joint. The underfill helps to distribute stress and hold the solder bump in compression to maintain a more permanent connection to both the board and the chip. Engineers use a standardized test from JEDEC known as the drop test to demonstrate reliability. The test involves placing the board assembly in various orientations on a base plate mounted to a drop table. A motor raises the drop table to specified heights depending on the service condition, and releases the drop table onto a rigid base, creating a large mechanical shock. The data in the chart above shows how an underfill will improve the reliability by a factor of 2 to 10.