1/f noise as a diagnostic tool to investigate the quality of anisotropic conductive adhesive (ACA) bonds for ball grid arrays (BGA's)

Reliability assessment of conductive adhesive bonds by humidity and temperature tests is time consuming and does not provide enough information about the details of the onset of degradation. There is a need for measurements that can give an early warning about the contact quality with more quantitative details about its degradation. Here 1/f noise of such contact arrays is investigated before and after temperature and humidity stress tests with different durations. 1/f Noise stems from conductance fluctuations. We investigated the constriction resistance 1/f noise S-R of the contact constriction resistance Re before accelerated aging and during aging at temperatures of 85degreesC and 110degreesC with an 85% relative humidity. Noise analysis on non-degraded samples is also a fast diagnostic tool for reliability characterization. From the analysis based on a noise model for a multispot contact, the onset of constriction resistance increase goes hand in hand with a strong increase in resistance noise. This is understood and can be characterized by a reduction in the electrical contact area A(e). The degree of contact degradation is expressed in a reduction of the real electrical contact area A(e) of a multispot contact and is quantitatively given by the D-factor.