When air becomes an electrical pathway

Summary form only given. The ongoing decrease in the feature size of new printed wiring assemblies has made it more difficult to test these boards with conventional methods. We have developed a noncontact electrical test method (NCT) to replace the conventional method of potentially damaging physical contact with the desired lead. The NCT method utilizes a small volume of highly-ionized air, commonly referred to as a plasma, as a bi-directional conductive pathway. The pathway is capable of supplying or measuring current with negligible distortion. We have explored laboratory plasma properties which enhance the possibility of developing a laser NCT plasma method for wafer die features. Generating an NCT plasma is a two step process. The first step, a "multiphoton process" which occurs very quickly, governs the ignition of a plasma, We have determined the photon fluxes required for plasma generation, important because it measures the threshold of ionization of different gas volumes more accurately than the accepted irradiance calculation. Using studies of the plasma lifetime we have calculated the rate of growth of the plasma volume or probe size. The system properties necessary to generate smaller, denser plasmas have been calculated as well.