Electroplated copper filling of through holes on varying substrate thickness

摘要

This paper discusses a through-hole copper filling process for application to high density interconnect constructions and IC substrates. The process consists of two acid copper plating cycles. The first cycle uses periodic pulse reverse electroplating to form a bridge in the middle of the hole, followed by direct current electroplating to fill the resultant vias formed during the bridge cycle. This process can provide defect-free filled holes with total plated copper on the surface below 25 μm, with dimples less than 5 μm for boards with core thickness of 0.2 mm and 0.25 mm. This process was designed to be deployed in vertical continuous plating equipment, (VCP), thus reducing capital equipment costs as compared to horizontal conveyorized electroplaters. The chemical components, copper, acid, and additive, for the periodic pulse reverse plating cycle, are optimized via experimental conditions selected from DOE (design of experiments) software. Critical parameters are identified and the impact on cavity formation during the bridging step is quantified. The additive and copper concentrations play key roles in reducing defects during bridge formation and on the resultant via formation. A high performance via-filling process is used to fill the formed vias, with less than 5 um dimple depth, while depositing approximately 12 microns on the surface. The thin surface copper enables fine line resolution without the need for planarization or grinding. The mechanical properties of the plated deposit meet or exceed all IPC standards. This process is applicable to both laser-drilled X shape through holes and mechanically-drilled straight holes. Laser-drilled through holes are bridged faster than mechanically drilled holes. However, mechanically drilled holes show a lower tendency for drilling induced defects, especially at a hole diameter of 0.1mm. This process has shown capability to fill through-holes in thicker cores of 0.4 mm to 0.8 mm, where further invest- gation continues.