Sunday, March 15, 2009

Acid Copper Through-hole Plating


Once the through-holes have been activated, your board is ready for acid copper electroplating (short for "electrolytic plating"). Considering the amount of trepidation that seems to surround the entire topic of through-hole connectivity, and the lengths that some people will go to avoid wet chemistry, putting a uniform, reliable sheath of copper on the insides of every hole turns out to be quite straight forward if not downright easy. Thanks to decades of work by the major electrochemical suppliers, the various chemical systems are well understood and readily available in most industrialized nations. Most of the equipment, like acid copper plating tanks, is easy to make and will last for many years if properly maintained. The items that are not easily fabricated in a home shop are available from a variety of sources. A high-performance plating solution can be mixed using readily available materials.

Basic principles

An acid copper electroplating solution is a mixture of water, sulfuric acid, copper sulfate, and a trace of hydrochloric acid. To this is added a number of organic constituents that serve to regulate and distribute the delivery of copper to the surface being plated. The two basic organic additives are commonly referred to as the "brightener/leveler" and the "carrier".

A basic electroplating cell consists of a tank full of the above electrolyte with arrays of copper anode bars arranged along two opposite sides. These bars are referred to as the anodes, and, as you might expect, are connected to the positive terminal of a current source. This supply must be capable of continuous sourcing into a near short circuit load (a typical copper electroplating bath has an effective full load operating "impedance" that ranges between 0.025 Ohms and 0.015 Ohms). Situated halfway between these anode "banks" is the copperclad substrate that is to be plated. It is variously referred to as the cathode (duh!) or the workpiece.

In the simplest terms, copper deposition occurs when an electrical potential is established between the anodes and the cathode. The resulting electrical field initiates electrophoretic migration of copper ions from the anodes to the electrically conductive surface of the cathode where the ionic charge is neutralized as the metal ions plate out of solution. In the Think & Tinker process, a thin layer of conductive ink extends the conductivity of the surface foil layers into the through holes. This ink forms a highly reliable surface for efficient electrolytic copper depositon. The figure at right shows a photomicrograph of the mechanically active surface that results when the ink is cured and the uniform layer of smooth, bright copper that has been deposited inside the through-hole.

At the anode (in a properly maintained bath), sufficient copper erodes into the electrolyte, to exactly make up for the deposited material, maintaining a constant concentration of dissolved copper. This all sounds quite nice, except for the annoying tendency of electrical charges to build up on the nearest high spot, thereby creating a higher electrical potential. This area of increased potential attracts more copper than the surrounding areas which in turn makes the high spot even higher. If this process were allowed to continue unchecked, the resulting plated surface would resemble a random jumble of copper spears instead of the smooth, bright surface needed for reliable electrical circuit formation. Inhibiting and controlling this nonlinear behavior is where the organic additives come in to play. This situation is especially critical at the rims of the through-holes. Here the field concentration is sufficiently high, that, in the absence of some mediating mechanism, electrodeposition would completely close off many of the smaller diameter holes.

Organic additives

In a well controlled plating bath, the carrier supports the formation of a black skin on the anode material which serves to regulate the diffusion of copper ions into the electrolyte. The material is also attracted to, but not co-deposited on the cathode (work piece) forming a layer (film layer) in close proximity to the surface that controls the rate of copper grain growth.

The brightener works within the film layer to control copper deposition on a microscopic level. It tends to be attracted to points of high electro-potential, temporarily packing the area and forcing copper to deposit elsewhere. As soon as the deposit levels, the local point of high potential disappears and the brightener drifts away. (i.e. brighteners inhibit the normal tendency of the plating bath to preferentially plate areas of high potential which would inevitably result in rough, dull plating) By continuously moving with the highest potential, brightener/levelers prevent the formation of large copper crystals, giving the highest possible packing density of small equiaxed crystals which resulting in smooth, glossy, high ductility copper deposition.

Mark Brelsford of QMS in Toronto, ON likens the action of the carrier to the function of a doorman at a theater who regulates the flow of people into a theater but doesn't really care where they go once inside. The brightener would then be the ushers who politely lead each person to a vacant seat until the theater is uniformly filled.

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