Underfill is the process of depositing an encapsulant in the gap between two dies or a die and the substrate to improve the reliability and durability of the flipchip assembly making both the physical contact and electrical interconnections more reliable.

The process of flipchip bonding consists of forming conductive bumps/balls on the die bond pads and flipping the die to attach it to another silicon die or a substrate. The flipchip process creates a gap between the two dies or the die and the substrate. The gap is similar in height to the bumps/balls. Any mechanical shock, drop and vibration to the assembly would be experienced directly by the conductive bumps/balls which can either damage or break them off completely.

The underfill encapsulation provides the following benefits to a flipchip 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 flipchip assemblies.

The choice of encapsulant determines the characteristics of the underfill encapsulation which are tuned to the specific application requirements for the package. Some encapsulants provide higher mechanical protection and some encapsulants better thermal conduction properties. 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. The chosen encapsulant underfill is applied to a flipchip assembly by dispensing it along the edge of the die. The dispensed 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 dispensed around the die in an ‘I’ or an ‘L’ pattern. The ‘I’ pattern consists of dispensing the encapsulant along one edge of the die and the ‘L’ pattern along two edges of the die. The underfill is dispensed 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. 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 flipchip 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.

Dispensing at our Facility

At the Advanced Packaging Facility we can precisely and accurately deposit the underfill encapsulations for flipchip 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 flipchip assembly, the expected sources of failure and the required characteristics. The encapsulants are selected to match the specifications of the flipchip assembly based on its application. The jetting system allows us the flexibility to use either a one material underfill encapsulation or two material underfill encapsulation.

Dr. Zeeshan Ahmed

4236, Building 59, University of Southampton
Highfield Campus, Southampton
SO17 1BJ
P: 02380593234