Indeed, devices with modules containing flip chips could only survive a few temperature cycles without this crucial step. Flip chips are soldered. They have hundreds, if not thousands of micro-connections between them and their substrate. During temperature cycles (ON / OFF), the silicon chip expands and contracts at a different rate than the substrate, inducing a stress which is concentrated in the solder joints.
The level of expansion or contraction of each material depends on its length and its coefficient of thermal expansion (CTE), which is the ratio between the change in length per degree of temperature and the initial length expressed in ppm / ° C. Even small differences in CTE between the silicon chip (2.6 to 3 ppm / ° C) and ceramic (~ 7 ppm / ° C) or organic (~ 14 ppm / ° C) substrates induce sufficient stress to break the solder joints, primarily in the outermost solder-bump interconnects. And that’s where underfill comes in. To prevent damage to the solder bumps, packaging engineers began using epoxy-based glue to fill the gap between the chip and the substrate. Once cured, the epoxy mechanically couples the chip with the substrate, forcing them to move together and distribute the stress over the entire surface.
The capillary effect allows the epoxy to get into the tiny space under the chip. It takes a lot of skill to master the underfill process. The choice of epoxy, the type of surface of the chip, the substrate itself, the size of the chip, the flip chip pattern, the spacing of the interconnect are some examples of factors to consider.