Effect of tool electrode roughness on the geometric characteristics of through-holes formed by ECDM

• Published in: Precision engineering Vol. 60; pp. 437 - 447
• Main Authors: Arab, Julfekar, Kannojia, Harindra Kumar, Dixit, Pradeep
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01-11-2019
• Subjects: 3D interconnects; Electrochemical discharge machining; Electrochemical finishing; Surface roughness; Through-glass vias (TGV); Wire-EDM; Through-glass vias (TGV); Electrochemical finishing; Surface roughness; 3D interconnects; Electrochemical discharge machining; Wire-EDM
• ISSN: 0141-6359; 1873-2372
• DOI: 10.1016/j.precisioneng.2019.09.008

Experimental investigation into the effect of tool electrode roughness on the geometric characteristics of through-holes formed in glass substrate by electrochemical discharge machining (ECDM) is reported. Single tip tool electrodes having different surface roughnesses were fabricated by the wire-electric discharge machining (wire-EDM). These tools were then used to produce through-holes in a 400 μm thick glass substrate by ECDM. The surface roughness of the used tools was further improved by a cost-effective electrochemical finishing (ECF) process to re-use the tools for through-hole machining. ECF also helped in removing the debris deposited on the tool during the ECDM. Through-hole geometric characteristics were analyzed by measuring the overcut and heat-affected zone (HAZ) width for through-holes fabricated by tools having different surface roughness. Experimental results showed that the discharges generated were inconsistent and aggressive for tool electrodes having higher surface roughness, which resulted in higher overcut and higher surface damages. The radial overcut, and the HAZ width were reduced significantly when the ECF finished tool electrodes having a lower surface roughness were used. Improvement in the geometric characteristics of through-holes was observed due to thin hydrogen gas film formation around the tool electrodes during the ECDM. Current-time behavior showed that the mean discharge current was reduced when tool electrodes having reduced surface roughness were used. The bottom-up electrodeposition technique was then utilized to fill the through-holes with electroplated copper. Through-holes fabricated by optimized process parameters were filled with copper, thus fabricating through-glass vias (TGVs) which has promising applications in radio-frequency MEMS and electronic packaging.
•Effect of tool surface roughness on the geometric characteristics of through-hole formed in glass by the ECDM is reported.•Tool electrodes having varying roughnesses were made by combining the wire-EDM and electrochemical finishing.•Lower roughness results in smaller gas film thickness which results in smaller discharge current and stable discharges.•Through-holes with lower HAZ and overcut were obtained with smooth tool electrodes.•Fabrication of Copper-filled through-glass vias required in RF MEMS applications was demonstrated.