1/f noise as a diagnostic tool to investigate the quality of isotropic conductive adhesive bonds

Reliability assessment of conductive adhesive bonds by thermo-cycling up to 830 cycles is time consuming, and does not give much information about the details of the onset of degradation. There is a need for faster tests givi ng more details about degradation. In this paper, low frequency noise of su ch contacts is investigated, 1/f Noise stems from conductance fluctuations. The observed voltage noise is enhanced due to current crowding in the elec trical contacts on a microscopic scale. In this research contact bonds were made and compared of isotropic conductive adhesives from three suppliers. The 1/f noise of the contact resistance can be interpreted in terms of a mu ltispot contact behavior. We investigated the relative noise C versus conta ct resistance R in two ways: 1) after an increasing number of thermo-cycles; 2) after increasing mechanical stress, The results often show an increase in relative noise of three orders of mag nitude for poor quality polymer bonds, A maximum increase of one order of m agnitude is observed for the best quality conductive adhesive. The contact resistance increases by a factor 1.7 and not more than 1.14 for the poor an d best quality bonds, respectively. From the analysis based on a noise mode l for multispot contact, the onset of delamination can be characterized as a reduction in electrical contact area A(e). The relative noise is proporti onal to A(e)(-5/2). The surprising result is that samples submitted to a me chanical stress show pictures similar to thermocycled samples. Thermo-cycli ng with less than 200 cycles leads to less noise, an increase in electrical contact area, and hence a contact improvement. This behavior is understood . Noise analysis under mechanical stress on nondegraded or slightly cycled bonds is a fast diagnostic tool for reliability characterization. The degre e of delamination is expressed quantitatively by the D-factor, D = A(emax)/ A(emin) congruent to (C-max/C-min)(2/5).