Friday, December 9, 2016
VOLUME -26 NUMBER 2
Publication Date: 02/1/2011
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Archive >  February 2011 Issue >  Partnering > 

Vertical Acid Copper Plating for PCBs

There are some dramatic changes taking place in the North American market's landscape, as we see the migration of manufacturing to the Far East particularly to China. Most of the big runners are made overseas.

But there are certain categories of products that remain in North America. These are primarily prototype boards, the large backplane (>40 layers), and the boards for military use. These boards are classified as high value-added.

The prototype boards require very elaborate front-end engineering to bring the part to its final configuration in a reasonable time frame. Prototype boards are never made in numbers, but they may go through a series of iterations before finalization. These boards cover wide range of complexity with some being highly sophisticated with high layer count, small holes, and buried and blind laser drilled vias. Less than 3 mil lines and spaces and "Via Fill" are first encountered here.

The backplane type boards may reach a thickness of 400 mils, and although the holes are usually >20mils, it is the highest aspect ratio demand; 15:1 is actually being manufactured and plated today. This is one of the highest value-added products in PCB manufacturing.

To meet these specification requirements, the board shop is forced to seek new and advanced processes in every department in the manufacturing process. Acid copper plating comes under heavy scrutiny, as it is the process that forms the traces and the thru hole connectivity that convey the signal from end to end of the final device.

Surface uniformity is sometimes plagued with the occurrence of nodules, which come from a variety of sources. Gold wire bonding applications have no tolerance to any level of nodulation. New developments are helping meet this challenge. A major part of the developments are focused on:

  • New chemical additive packages for improved distribution.
  • Mass transfer improvements to complement the additives.
  • Nodule elimination by use of insoluble anode.
  • "Via Filling", a specific application.

High Throw DC Plating
A new generation of "High Throwing Power" acid copper systems have come to the market to fill the void. These baths are designed for today's plating currents, which are lower than the traditional 25 - 30 ASF, which was common in the days of double-sided, and simpler products.

High throw baths are designed to give the desired physical properties at current densities as low as 5 ASF and as high as 20 ASF. They produce bright ductile deposits.

These bath types are characterized by a specific combination of organic additive package that includes a unique leveling agent. The leveler plays a key role in improving throwing power particularly if it is coupled with eductor (airless) agitation.

Some of these baths can give a throwing power >85 percent for a 10:1 aspect ratio drilled hole plating 1.0 mil in the hole, in a plating time of 90 minutes, in a vertical dip tank mode. Higher aspect ratio boards (15:1) can be plated in the same system with the same results at reduced current density and increased plating time.

Mass Transfer
Mass transfer becomes a key parameter that must be understood and managed for high aspect ratio plating. An example is plating a 330 mil thick board with a 22 mil hole diameter a 15:1 aspect ratio, an 8 mil hole in a 93 or 125 mil thick board, or a blind via with an aspect ratio greater >1.0. Mass transfer is influenced first and foremost by diffusion, also affected by solution agitation, and part or rack agitation. Of course a reduction in plating rate will always improve distribution by maximizing the role of diffusion for mass transfer.

Eductors
Eductors are used today in many of acid copper plating tanks. They create turbulent solution flow without the use of air sparging. The design and layout of the eductor sparging system is important to maximize the solution shearing action at the surface for the board to be plated. If properly designed eductors can preclude the need for part agitation. The increased solution flow at the surface as compared to the middle of the hole could be effective in improving the throwing power, provided the chemical additives used are designed to respond preferentially to solution movement. Eductors eliminate the need for compressed air or air blowers, and also provides a safer environment where acid is not constantly been blown into the air, or the exhaust system.

Nodule Elimination
The use of insoluble anodes is well established in the acid copper conveyorized equipment. It offers a series of advantages over the conventional copper slugs/balls in titanium baskets. The most prominent advantage is the absence of copper anodes, which need to be filmed and bagged to contain naturally occurring sludge from getting on the work.

Anodes, even in their purest form, are prolific sources of suspended matter that leads to nodules. Insoluble anodes eliminate the need for dummy plating to film the anode. The need for standard anode maintenance is also eliminated. The anode shape and dimensions are not altered throughout the life of the insoluble anode, this allows for very consistent plated copper distribution.

The insoluble anode requires a continuous supply of copper ions brought into the system from an external source. Some of the methods used to generate copper are the dissolution of copper oxide into the electrolyte, the electrolytic dissolution in an external rectified cell and the use of ozone to oxidize copper metal. Most users of the insoluble anode use copper oxide to replenish the plated copper. Copper oxide is added in small increments based on Ampere hours of plating into an external dissolution system that feeds the plating tank.

Via Fill Plating
To meet the demands of high density interconnect, "via filling" is quickly becoming a clear choice for connecting the different layers in buildup technology (stacked vias and vias in pad) manufacturing. They result in an overall improvement in long-term reliability of the PCB and the package.

Suppliers have developed new electrolytes for plugging vias shut. Both pulse plating and DC plating proprietary "Via Filling" chemistries are available in the market place. DC plating offers a series of advantages as it does not require pulse rectification and avoids the complexity of managing a pulse wave. In addition DC plating systems are stable and do not require the constant regeneration of pulsed electrolytes.

Via Filling is based on high depositing ion concentration coupled with a low concentration of a leveling agent, in addition to the carrier and the brightener additives. Most of these systems require optimized solution flow. The flow allows the leveler to accumulate on the surface, thus inhibiting plating while the bottom of the via continues to plate. The plating dynamics in the bottom of the hole are very different than those on the board surface. Eventually as the hole fills, the plating dynamics even out.

Acid copper plating has come a long way since the early days of double- and single-sided Printed Wiring Boards. Plating challenges will continue to increase as new product demands (lighter smaller and more reliable) come to market. As one leading edge milestone is conquered, a new one is established.

Contact: Uyemura International Corporation, 240 Town Line Rd., Southington, CT 06489

860-793-4011 Web:
http://www.uyemura.com

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