As modern life becomes more and more intertwined with electronic gadgets – from mobile devices to movies on demand, the Internet of Things (IoT), e-commerce, etc., Internet traffic and storage demands increase exponentially. At the heart of the systems supporting Internet traffic and storage are a wide variety of routers, switches, and storage units, all of which continue to evolve with higher functionality per unit area.
These devices rely on state of the art chipsets, which increase in ball count, increase in number, and may also decrease in pitch. This increase in density generally drives layer counts higher, with pitch decrease leading to smaller holes, clearances, and lines and spaces. As layer count increases, board thickness increases. As the hole size decreases with pitch, the aspect ratio (AR) increases, bringing with it a new set of challenges. Aspect ratio is defined as thickness/through hole diameter and is normally expressed as a ratio to 1. For example, a board with 4.0 mm overall thickness and 0.25mm holes is 4.0/0.25, or a 16:1 AR. Today’s high aspect ratio product routinely gets to this number in mass production, and much higher in specialized production.
High AR products present many challenges, one of which is the ability to reliably plate the through holes without over-plating the surface. Typically, PCBs require 1 mil (25 microns) of copper in the hole in order to assure reliability over the life of the product. Ideally, we want the same amount of copper to be plated in the middle of the hole as on the surface. This presents a challenge due to electrochemical properties of plating processes combined with the AR of the product being plated.
Throwing power (TP) defines the ratio of the plating thickness in the middle of hole compared to the plating thickness at the surface. Ideal TP% for a conformal plating process is in the 80-120% range, and for manufacturing control it is best when the TP% has minimal variability dependence upon hole diameter.
As ARs have increased, one response in the use of DC plating systems is to go ‘low and slow’ – meaning low current density (CD) for an increased amount of time. While this has been effective, it also has an effect on throughput as the plating times can be many hours for high AR work.
Meeting TP and productivity requirements drove development of a new approach: Electroposit™ 1500 Acid Copper for use in conventional DC plating applications for either standard air agitation systems or eductor-based systems for panels with thicknesses up to 4.8mm.
In the figure above, the new technology is compared to a conventional low CD application in vertical hoist equipment with air agitation. The Electroposit 1500 Acid Copper process is able to provide equal or better TP at higher CD on the parts up to 12:1 AR, and provide nearly a 20% improvement in TP at 8 ASF in the 4.8 mm part.
As seen in the figure above, this technology was used to plate through-holes of a 6.0 mm backpanel using hoist-type equipment with eductor at 8ASF. The cross-sections show that top corners, middle sections, and bottom corners of all through-holes are smooth and continuous, with the 17:1 AR holes showing a TP of 75% with good knee shape and thickness. With this chemistry it is possible to plate 12-13:1 AR product at 12-16 ASF depending on board thickness.
In addition, the Electroposit 1500 Acid Copper has been evaluated in a Vertical Continuous Plater application, for product up to 3.5mm in thickness; the results are seen below:
When converting from conventional copper plating formulations used in hoist-type equipment with air agitation, the new formulation allows for operation with relatively high CD for shorter plating times. With all other parameters equal, Electroposit 1500 Acid Copper can achieve 20-30% through-hole TP improvement compared to conventional formulations. On mechanically drilled holes, both blind-vias and through-holes can be coated with TP% >90%.
Electroposit 1500 Acid Copper is able to plate both through-holes and blind-vias with exceptional TP, and can do so at higher current densities to improve throughput. It meets advanced reliability criteria including multiple (up to 15X) thermal stress and IR reflow testing.
This new solution is a turnkey upgrade for existing hoist-type and VCP-type equipment; the three-component system (carrier, leveler, brightener) is easily analyzed and controlled by conventional CVS techniques. Benefits like these showcase how Dow is innovating to respond to industry and OEM requirements.