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The Perfect Copper Surface



The Perfect Copper Surface
To provide the functionality in today's electronics, printed circuit boards are approaching the complexity of semiconductors.
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Authored By:


Eric Stafstrom
Technic Inc

Garo Chehirian
Tech-Etch

Summary


In order to provide the functionality in today's electronics, printed circuit boards are approaching the complexity of semiconductors. For flexible circuits with 1 mil lines and spaces, this means no nodules, no pits, and excellent ductility with thinner deposits. One of the areas that has to change to get to this plateau of technology is acid copper plating. Acid copper systems have changed in minor increments since their introduction decades ago. However, the basic cell design using soluble anodes in slabs or baskets has for the most part remained the same. Soluble, phosphorized, copper anodes introduce particulate and limits the ability to control plating distribution.

The companies worked together to evaluate a new approach using insoluble anodes that are isolated from the main plating bath. Insoluble anodes are known to eliminate the particulate, provide consistent anode area and shape the anode to match the plated part. But isolating the insoluble anode dramatically reduces high consumption of organic additives typical with insoluble anodes. This new approach limits additive breakdown and & consumption normally seen at the soluble anode surface. The end result is a surface free of nodules, pits, and precise control of copper thickness distribution minimizing the impact of breakdown products.

This paper is to document the results from prototype testing through implementation into production. The system was first tested in pilot tanks at the companies to determine the impact on nodules and surface distribution. Data was generated looking at impact of anode design on plating distribution and surface for any defects. This data was utilized to design a full scale production line that is being used to quantify process improvement over existing production equipment. The goal for the work being done is a perfect copper surface.

Conclusions


The use of insoluble anode assemblies with race track agitation has made it possible to meet the copper plating requirements for advanced PCB fabrication. Eliminating phosphorized copper anodes gets rid of the number one source for nodules in acid copper plating and the biggest variable in plating distribution. By using an anode assembly with proprietary membrane technology, insoluble anodes become a workable system by separating the IrO anode from the plating solution. The insoluble anode assembly also provides better control of current flow, which when properly applied, will give a plater control over copper distribution.

This work confirmed that all the benefits of the insoluble anode assembly may not be realized without proper equipment design. Work agitation is critical and the introduction of "Race Track" agitation provided improved surface distribution while insuring good throwing power. Basic cell design criteria like anodes properly placed with good connections, or solution agitation with controllable and uniform flow are all as essential to success as the insoluble anode assembly.

As a result of this testing, the product is now being produced easily, in a production environment, exceeding distribution requirements and providing a defect free surface. Getting to a perfect copper surface required more than just replacing soluble anodes with IrO anodes.

It required:
New membrane technology to isolate the insoluble anode.
Testing and implementing advanced equipment designs.
Understanding the root causes for defects.
Understanding the impact of basic cell design.

The conclusion is it takes more than just new equipment. It takes an understanding of the whole process and a holistic approach to move acid copper plating from current practices to an advanced level where it is possible to plate in production, The Perfect Copper Surface.

Initially Published in the IPC Proceedings

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