Reliability of ACA joined thinned chips on rigid substrates under humid conditions

Thinned chips are an interesting option for reducing the thickness of an electronics package. In addition to the reduced size, thinned chips are flexible and can dissipate more heat than thicker ones. Joining of the thinned chips can be done using several different techniques. Of these, anisotropic conductive adhesives (ACA) are an interesting option as they have several advantages such as low bonding temperature and capability for high density. The reliability of ACA flip chip joints under thermal cycling conditions has been found to increase when thinned chips are used. However, the effect of humidity has not been fully explored. The effect of humidity on reliability raises specific issues since it causes various types of stress to form in the structure. In this study the reliability of thinned chips under humid conditions was compared to that of thicker chips. Five test lots were assembled with thinned and thick chips using an ACA film and three different FR-4 substrates. The substrates had the same sizes, but their thicknesses and structures varied. A constant humidity test was used to study the reliability of the joints. With one of the substrates the reliability of the thinned chips was noticeably inferior to that of the thicker chips. In order to compare the stresses in the joints during testing, finite element models (FEM) were used. Greater stresses were seen in the thinned chips during humidity testing when compared to those seen in the thicker ones.