| Objectives | Background | Approach |
Use experimental techniques and PoF simulation to assess the
constitutive and thermomechanical durability performance of a
selected anisotropic conductive adhesive (ACA).
New interconnect materials are always necessary as a result of evolving packaging technologies and increasing performance and environmental demands on electronic systems. In particular, conductive adhesives have gained popularity as a potential replacement for solder interconnects. The interest in using conductive adhesives instead of solder, comes partly from the fact that interconnects can be formed at low temperatures, and partly from the interest in eliminating lead from electronics.
A large research community is centered on characterization of conductive adhesives, to meet the needs of the electronics community. Most of the research effort to date has focused on manufacturability issues. While there has been some amount of environmental testing also, there is a lack of systematic quantitative models of the failure mechanisms, to quantify durability under different stress profiles.
In this study, we will focus specifically on thermal cycling loads because most electronic systems see such exposure throughout the life cycle. Experimental results cited in the literature indicate that the interconnect resistance increases with thermal cycling. However, the exact mechanism has not been established, although there are several speculations.
A suitable ACA system will be identified with the help of member companies. Rate-insensitive constitutive properties (stress-strain curves) for the epoxy binder and the filler particles will be determined through monotonic displacement-controlled tests, run across a matrix of appropriate strain rates and temperatures. Micro-moire interferometric tests will be used to characterize the deformation and strain fields for this purpose. Creep properties will be determined through monotonic constant-load tests; here the test matrix variables are load level and temperature.
After constitutive characterization is complete, the cyclic durability of the conductive adhesive will be measured from thermal cycling tests of polymer flip chip samples. Mico-moire interferometric methods will be used to characterize in detail, the thermomechanical deformation fields in the interconnect. Detailed failure analysis will be conducted in order to identify the root-cause failure mechanism. We expect that the reason for the resistance increase is a combination of surface oxidation, interfacial debonding, and loss of contact pressure. Similar results are also reported in the literature and these will be collected through systematic literature searches.
Mechanical models of the flip chip test structure will be constructed, using the finite element method. The filler particles and the bond pads and the epoxy binder will be explicitly modeled and the measured material properties will be used in the simulation. The model will be subjected to thermal cycling, so that the evolution of the observed failure mechanism can be modeled. This model will be used to assess acceleration factors and in-service durability for different life- cycle environments.