Single Step Metallization Process for the Filling of Through Holes with Copper

Abstract

As designers continue to increase the functionality of electronic devices, printed circuit board substrate fabricators have been forced to develop new fabrication techniques to accommodate the miniaturization and densification of circuitry and the unique requirements that come along with it. This has resulted in the need for new design features utilized in the printed circuit board such as extremely fine lines and spaces with critical trace profiles and coplanarity for controlled signal impedance, the use of filled blind microvias and through holes for vertical signal routing to maintain short signal distances for maximized clock speeds, and thermal vias for the management of the heat concerns associated with more tightly packed, heat generating electrical components. These new design features have required the development of novel metallization chemistries capable of meeting the stringent needs of these new substrate architectures in terms of capability, versatility, and reliability. This paper will discuss a copper plating process capable of filling through holes with solid copper, in one step, for applications such as core layer through hole filling with minimal surface copper build up for HDI technologies and thermal management of heat sensitive electronics. The advantages of the single step filling process benefit both the technology and the fabricator. In the copper through hole filling for core layer applications, the ability to fill a variety of aspect ratio holes, void free and flat, with reduced surface copper, provides improved fine line resolution for HDI and IC substrates. For thermal management of closely packed, heat generating sources, the presence of a solid copper structure offers tremendously enhanced thermal conductance versus conventional technologies to draw heat from the operating devices and dissipate it throughout the printed circuit board. From a processing standpoint, the single step copper filling technology is capable of filling various through holes in a fraction of the time compared to conventional copper plating technologies, greatly increasing potential throughput. In certain applications, the copper filling technology can eliminate complete processes, greatly simplifying the manufacturing process. This paper will discuss in detail, the various aspects of the single step copper filling chemistry and the advantages this technology can provide versus conventional systems. Presented will be the current capabilities and limitations of the process as well as design guidelines that can affect the process.