Flip Chip Underfill

The space between the chip and the substrate is typically filled with a non-conductive adhesive called the underfill material. The underfill material is dispensed around the edge of the die and capillary action pulls the material under the die. The under fill material provides additional robustness to the joint and helps to compensate for any CTE mis-match between the die and substrate. The under fill also protects the die from moisture and other environmental conditions. However, as previously mentioned the inclusion of an underfill material may impact the high frequency electrical performance.

Space Grade Underfill 

In the space environment, materials face harsh conditions such as extreme thermal cycling, high vacuum, atomic oxygen, UV radiation, and ionizing radiation (electrons, protons). Exposure of polymers such as an epoxy underfill to the space environment can result in degradation of the material and its properties. Surface erosion and modification of material properties (chemical, mechanical, electrical, thermal) can take place. The properties and chemistry of an underfill are tailored for good adhesion and the proper mechanical properties. Modification of these properties by the environment may lead to undesirable effects such as loss of adhesion or change in elastic modulus and CTE.

The main conditions that could affect underfill material are the high vacuum and ionizing conditions (electron and protons), other space conditions are important but in this case are shielded by other elements of integrated circuits.

The vacuum inside satellites is typically 10-6 to 10-7 atm. The vacuum can induce outgassing of volatile components from underfill. The volatile components are mainly low-molecular weight fragments, additives, and absorbed gases. Besides contamination of sensitive optics and sensor surfaces, the outgassing can also result in the degradation of polymeric materials, especially at an elevated temperature.

The principal kinds of high-energy radiation are galactic cosmic rays, the geomagnetically trapped radiation at radiation belts (Van Allen radiation), and particles from solar flares (solar origin cosmic rays). Overall, the major constituents of ionizing radiation are high-energy electrons (up to several MeV), protons (up to several hundreds of MeV), alpha particles, heavy ions and high-energy photons. The galactic cosmic radiation consists mainly of very energetic penetrating protons and ions. Their intensity, however, is very small (less than 20 rads/year). The Van Allen radiation is more intense. It consists mainly of fast electrons (several tens of keV) and protons (tens or hundreds of MeV). The fast electrons can easily be eliminated by shielding. The proton dosage inside the shielding, though, is still about 100 rads/hr, which is high enough to damage semiconductor devices. The intensity and energy of the proton and alpha particle emission associated with solar flares can be very great, even under shielding (25 rads/hr, several hundreds of MeV). These energetic particles can degrade polymeric materials, electronic components, and solar cells by atomic displacement, ionization, and photon excitations.

There are several papers about influence of radiation in the underfill materials used in printed circuit board encapsulation; the question is as to whether or not there is any likelihood radiation may induce arcing of the underfill. Printed circuit board encapsulation can sometimes charge to a point where arcing occurs. This is due to the dielectric breakdown of the encapsulation. In order to minimize the risk of arcing, such dielectrics are recommended to have a minimum conductivity. If underfill shares the same concerns, it will potentially damage the adjacent metallization on the device. Any material that will hold onto the implanted charge from electrons or protons can present a danger unless that charge bleeds away fast enough to avoid dielectric discharges.

Underfill general background

Underfill adhesion is an important factor affecting the reliability of flip chip assemblies. The underfill carries most of the CTE mismatch stress instead of the Au –Au joints. This can only be realized if the underfill maintains good adhesion to the other materials, including Au, die passivation, substrate and die edges. Good adhesion also needs to be maintained even in the presence of contaminants, such as flux residue. This is another potential advantage in our proposed flux free attachment process.

A underfill also needs to be able to maintain adhesion after exposure to adverse environmental conditions such as thermal cycling and/or moisture exposure. The elastic modulus of the underfill should be sufficiently large to prevent stress concentrations in the bumps without exerting excessive stress on the die. The adhesion and elastic modulus parameters will need to be specially selected based on the result of any thermo-mechanical modelling.

The underfill must completely fill the gap between the die and substrate without voids, bubbles or occlusions, as this would represent a reliability and electrical performance impact. In order to achieve this, selecting the appropriate underfill material and underfill dispense pattern will be critical. A number of different types of underfills are available depending on the specific application.