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.