Underfill

The underfill encapsulation provides the following benefits to a flip chip assembly.

It provides mechanical protection by enhancing the mechanical bonding between the dies or die and distributes any mechanical stress throughout the assembly protecting the conductive balls/bumps.
It provides thermal protection by improving heat transfer from the dies to the substrate and provides better heat distribution relieving the conductive bumps/balls of thermal stress.
It also relieves mechanical stress from the bumps/balls caused by thermal expansion mismatch.
Overall it improves the function, reliability and durability of the flip chip assemblies.

Underfill Work Flow

Material Choice

The choice of encapsulant determines the characteristics of the underfill encapsulation which are tuned to the specific application requirements for the package.

Deposition Volume Calculation

The required volume of an underfill encapsulant can be estimated by calculating the volume of the gap between the dies or die the substrate, the volume of a sealing depositions and subtracting the total volume of the conductive bumps from the sum of two.

Underfill Deposition

The chosen encapsulant underfill is applied to a flip chip assembly by depositing it along the edge of the die. The encapsulant flows through the gap in the assembly by capillary action. In some cases, the encapsulant may require to be heated to a certain temperature before it flows through the gap. The encapsulant is commonly deposited around the die in an ‘I’ or an ‘L’ pattern. The ‘I’ pattern consists of depositing the encapsulant along one edge of the die and the ‘L’ pattern along two edges of the die. The underfill is deposited in multiple passes to ensure that the height of each pass is lower than the die; this prevents the encapsulant from spreading on the top surface of the die. It is important to ensure that the underfill encapsulation does not have any voids or air bubbles to achieve maximum protection. In most cases the air bubbles are introduced by the substrates at higher temperatures because of improper baking or under cured surface depositions.

Sealing Deposition

Commonly after the gap is filled with the encapsulant, additional deposition is made around the die to seal it; this improves mechanical protection of the flip chip assembly. In some cases, the sealing encapsulant may be different to the underfill encapsulant, they may also have different curing properties such as the underfill may be thermally cured and the sealing encapsulant may be UV cured.

Curing

The underfill is cured using the appropriate curing conditions.

Dispensing at our Facility

At the Advanced Packaging Facility we can precisely and accurately deposit the underfill encapsulations for flip chip assembly. The facility has a robotic 3 axis jetting system with an integrated vision system which is ideally suited to the underfill applications.

At the facility we carefully consider the underfill requirements in terms of target applications of the flip chip assembly, the expected sources of failure and the required characteristics. The encapsulants are selected to match the specifications of the flip chip application. The jetting system allows us the flexibility to use either a one material underfill encapsulation or two material underfill encapsulation.