Reliability Concerns of ICA Interconnects

In order to get a good adhesive joint, the adhesive must wet the bonding surface. A necessary condition for this is that the adhesive has lower surface tension than the bonding surface. Epoxy and polyimides are major polymers used as base matrices of ICAs. These materials have lower surface tension than Sn, Pb, Cu, Au, and Pd. Therefore, a good adhesive joint is expected when bonding on Sn, Sn-Pb, Cu, Ag-Pd and Au surfaces. 

As water molecules can easily penetrate through the adhesive and oxidize/hydrate the bonding surfaces, ICA joints with different metallizations have different reliability performances in high humidity environments. Several investigations (Ref 10-12) showed that joints with noble Au and Ag-Pd metallizations had much lower resistance increases, compared with those with non-noble Sn-Pb and Cu metallizations. With transmission electron microscopy (TEM) and electron spectroscopy for chemical analysis (ESCA), the detailed failure mechanisms were investigated by Liu et al. (Ref 10). 

TEM studies on an adhesive joint with copper metallization show the existence of an oxide layer on the Cu pad. The thickness is approximately 100 nm after 1000 h 85 C (185 T) and 85% RH humidity test. The rings in the diffraction pattern obtained from the oxide layer indicate that the layer consists of fine crystalline grains. The radii of the rings correspond to the spacing of the crystallographic planes of CU2O. It is therefore concluded that the formed oxide is CU2O, which is a poor conductor. This helps to explain why the joint resistance increased ter the humidity test. 

Contrary to non-noble metallization, the electrical resistance of ICA joints mounting a gold-plated QFP80 component on an electroless Au-plated FR4 board is quite stable in the 85 C (185 °F) and 85% RH environment. No significant increase can be observed up to 2000 h. Hence, it can be concluded that a noble metallization such as Au or Ag-Pd is preferable for normal ICAs. However, by adding corrosion inhibitors, some superior ICAs for pre-tinned metallization have been developed (Ref 13). 

There is no doubt that proper curing is very important for joint reliability. It was found that a minimum curing degree is required to provide a certain level of mechanical and electrical performance of adhesive joints, especially with nonnoble metallizations (Ref 14). Once this is achieved, increasing curing times does not result in significant improvement. 

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