WHAT is UNDERFILL AND WHY is it USEFUL?
Underfill is thermoset epoxies traditionally used in flip chip applications to reduce thermal stresses solder bumps experience due to coefficient of thermal expansion mismatch between a die and the organic substrate. Today, underfills are available in a variety of formulations and are widely used for board level reliability of ball grid array components by reducing thermal and mechanical loads under harsh use environments. Careful consideration to the underfill material properties and intended use environments must be made to assess the relative reliability improvements underfills offer.
APPLICATION OF UNDERFILLS
The image in Figure 1-1 is a contemporary flip chip package utilizing a thick copper lid for heat sinking. Underfill is a critical component that protects against solder bumps from thermal stresses and package warpage during assembly and operation as well as against cracking of the die and in the low-k layers. Underfills act as a structural member between the die and substrate and provides load sharing between that reduces the stresses solder joints experience.
In order to reduce the thermal stresses on the solder bumps, which range in height from 50 to 100 microns, rigid underfill is required with low coefficient of thermal expansion. Optimal selection of flip-chip underfills CTE is in the range of 20 to 30 ppm/°C that are highly filled with silica particles and contain high glass transition temperatures.
The image in Figure 1-2 is a traditional over-molded flip chip package in which the board level solder joints are underfilled.
An important thing to note about the board level underfill diagram (Figure 1-2) is the size of the solder joints that is significantly larger than in flip chip packages. The board level solder interconnects can vary in height from 100 microns to 500 microns, which gives it a much higher volume for underfill material compared to the flip chip package displayed in Figure 1-1. The addition of underfill to the 2nd level solder interconnects have shown to increase or decrease the fatigue life depending on the load environment and underfill properties. Reworkable underfills that show significant improvement under drop and shock loading can degrade fatigue life under thermal cycling if thermal load environments exceed the underfills glass transition temperature. The CTE and modulus of underfills before and after the glass transition temperature will the influence on the fatigue of solder joints.
Conventional underfill materials are thermoset epoxies that are highly constituted of silicon oxide particles, which provide stiffening. The particles reduce the coefficient of thermal expansion and increase the elastic modulus. Conventional underfill materials come in one- and two-part formulations, and deciding whether a one- or two-part is appropriate depends on:
- Curing temperature and time requirements
- Viscosity limitation (how well can underfills flow underneath a package?)
- Chemical resistance (would underfills be subjected to a corrosive environment?)
It is important to consider the different use cases for reworkable versus non-reworkable underfill materials. Reworkable underfill materials tend to not have any (or have very few) filler particles by volume. Non-reworkable underfill materials, on the other hand, offer high filler particles to reduce CTE and increase stiffness.
The go-to application method for underfills is Capillary underfill, and it can be arranged in three ways:
- Full Underfill
- Edge Bonding
- Corner Stake
Capillary underfill is applied using a syringe, either manually or with an injecting device. One major benefit of the Capillary method is that it allows manufacturers to customize inputs to ensure consistent flow and a fully covered package that is void-free.
A second underfill application process, which is becoming more popular, is the No Flow underfill method. No Flow underfill is designed to be cured during the solder reflow process – meaning the soldering and the underfill cure is done simultaneously. No Flow underfills typically do not have filler particles to avoid constricting the soldering of the joints to the pads.
UNDERFILL IN SHERLOCK
While underfilling is a decades long technique, the materials implemented are continuously evolving in response to the growing needs of the electronics industry. New materials require extensive time and resources to fully characterize the mechanical properties and its influence on key components on an assembly. To alleviate the need for some testing, simulation is used as a complimentary effort to qualify material changes. Sherlock Automated Design Analysis™ software distinguishes itself as the only simulation tool used in the electronics industry to predict the success of underfill materials. Sherlock accomplishes this using a combination of FEA and Reliability Physics to quantify the life of a product stressed by its field and test environments.
To learn more about DfR Solutions' services and Sherlock Automated Design Analysis™, request a free trial today!