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Addressing changing via fill requirements for fine-pitch HDI PCBs

John Gebhardt
PCB Marketing Manager, Interconnect Technologies Responsible for the PCB Market Segment, including Making Holes Conductive, Electroplating, and Final Finishes

Today’s high density interconnect (HDI) printed circuit boards (PCBs) increase the functionality of a circuit board while utilizing less area, driven by the miniaturization of components and IC substrate packages. This enables the use of an increasing number of advanced features in today’s mobile and high-performance devices. As the pin count of substrate packages increases, and/or the pitch count decreases, HDI structures provide a solution to the very complex routing requirements of these devices. Dow Electronic Solutions continues to innovate with enhanced Cu via fill chemistries that are central to the ability to build HDI boards.

Today, enhanced via-fill (EVF) must repeatably produce void-free filled laser micro-vias while plating the through-holes with sufficient copper to provide industry-accepted reliability. The fine lines and features of today’s HDI designs create challenges in the circuit formation processes, so there is a need to provide the required fill in the laser micro-vias, as well as through-hole reliability at a lower surface copper thickness. Reduced surface copper thickness allows for formation of finer features at an improved yield.

The requirement for complete via-fill of laser micro-vias while simultaneously plating through holes presents challenges in the design of the chemistry system. Typically, for via-filling, the inorganic composition of the plating bath is high copper/low sulfuric acid. For through hole plating the inorganic composition is low to moderate copper content with high sulfuric acid to provide adequate throwing power, ensuring sufficient copper in the middle of the through hole to meet reliability requirements. Development of a plating process to fulfill all the needs of the HDI product requires the R&D group to carefully create the organic additives that are the key to the process. These additives consist of carriers, levelers and brighteners: the creation, and balance of these constitute the science and art of the process.

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Image 1: Additives used for bright acid copper plating are divided into 3 categories: (1) Carrier: make deposit a tighter grain structure, improve plating distribution & TP; (2) Brightener: direct enhance physical properties of the deposit, ex: tensile strength & elongation; (3) Leveler: adsorb preferentially at high points in the plating topography, allow low current-density areas to catch up with high area.

A new generation of via fill, Microfill™ EVF15, has been developed to meet these requirements with a plated Cu surface thickness of 15 microns. This new electrolytic chemistry was designed to be a drop-in solution for existing vertical continuous plater (VCP) lines, offering complete fill at lower Cu thickness, reduced dimples and skip-rate, and improved surface appearance. Product produced using this chemistry passed all reliability tests including elongation and tensile strength, IR, thermal shock, and thermal stress. The EVF15 requires no changes to waste treatment or handling compared to existing via fill processes. All bath additive components can be monitored by cyclic voltametric stripping (CVS), when proper analysis methods are used.

Dow’s Microfill EVF process has been used in high volume manufacturing (HVM) in more than 100 lines over the last 10 years. This new generation EVF15 is already being used at multiple sites, as demanded by leading-edge customers who need the ability to create finer lines and features with higher yield. Our team is committed to continuing the pioneering tradition of plating innovation, and we continue to work on our pipeline of new products to meet the needs of the leading-edge HDI PWB manufacturers.

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