Understanding Cleaning of Vias Fabricated Using Micro Mechanical Punching in Liquid Crystal Polymer (LCP) Substrate

Abstract

The objective of the research is to understand the effect of chemical treatment's etching times on via wall roughness; and the direct current (DC) and reverse pulse plating (RPP) of vias fabricated using micro mechanical punching process on liquid crystal polymer (LCP) substrate. One major drawback of micro mechanical punching process is the formation of LCP and copper burrs. Sequential wet chemical etching and oxygen plasma cleaning techniques were developed as an effective tool for the removal of these burrs. The wet chemical process required a sequential oxidation, etching, and acid neutralization treatment for the effective removal of LCP and copper burrs. These treatments have an effect on the surface roughness of the through via wall. Under this research time dependent experimental matrices were designed to observe the effect of the wet chemical etching on the via wall surface roughness. Sequential oxidation, etching, and acid neutralization treatments were done, where only one treatment time was varied for 1, 5, and 10 minutes keeping the other treatment times constant at 5 minutes. After examining three different experimental matrices for via wall roughness using a scanning electron microscope (SEM), it was observed that the longer treatment time using chemical etchant will make the surface rougher. In addition, it was also observed that the oxidation treatment time of greater than 5 minutes produced the same roughness irrespective of treatment time variation. The neutralization treatment time does not have any effect on the roughness of the LCP via wall. Two different approaches named - DC and reverse pulse plating (RPP) were used to plate the vias. It was found that the RPP does not result in good via filling, because it produced high current concentration at the edges and consequently over plating at the edge compared to the inside of the via. In stark contrast, DC plating delivers completely plated vias after 4.5 hours, which is a 7 μm/hr plating rate.