Environmental PCB Etchant: Updating the PCB Manufacturing Process

“Double, double toil and trouble

Fire burn and cauldron bubble.”

If we didn’t know better, we might conclude that the preceding lines are from some type of Shakespearean-like play or something.  However, the poisoned entrails, eye of newt, toe of frog, and lizard tails that go swirling into the cauldron are actually the ingredients for old-style PCB etchant.

Oh, just kidding.

But…we may want to stifle our gasps just for a minute while learning about the new wave—not wave solder—of environmental PCBs.

A Witch’s Brew: PCB Manufacturing and Etchants

At one time or another, PCB manufacturing processes use solvents such as ferric chloride, cupric chloride, ammoniacal etchant, sulfuric acid + hydrogen peroxide, sulphuric and chromic acid, cupric chloride, alkaline etching, and copper ammonia complex ion as etching fluids.

We can cut the list to include the two most-used types of etchants: ammoniacal etchant and cupric chloride. In terms of etching capabilities, the use of those chemicals has worked well as part of the subtractive method for producing PCBs. Before that, many manufacturers used ferric chloride as an etchant liquid.

So…why create this list and why care? After all, the etchants work and we have jobs.

The waste streams from the PCB production processes include spent etchant and waste rinse water that includes ammonia, chromium, copper, iron, and acids. Most of the materials that make up the waste streams consist of suspended solids, metals, fluoride, phosphorus, cyanide, and chelating agents. Manufacturers use different stages of treatment to recover and reuse the water. Those processes include the addition of other chemicals that react with soluble pollutants, filtration that removes sludge, reverse osmosis, ion exchange, membrane filtration, and different types of rinsing techniques. Some of the sludge goes into landfills.

Now that we’ve taken a walk on the wild side, let’s think about the economic and environmental impact. Both are sizable. Manufacturers invest significant resources to comply with federal and state regulations that govern liquid wastes. After all, none of would appreciate the extra added flavor of waste etchant in our morning tea or the sensation of etchant mixed with our shower soap.

Table one compares the impact of three well-known etching materials.

In addition to the etchant, many manufacturers supplement etchants with chemical additives to achieve better etching characteristics. For example, the addition of ammonium chloride, monomethanol amine, ethanol, acetonitrile, acetone, or dimethylformamide to cupric chloride produces a better etching rate and increases the amount of dissolved copper.

Cupric Chloride Could Work as an Answer

If you or your kids have played with lizards, you may have witnessed their interesting ability to regenerate a tail—for later use in that special etchant mix.  Although probably not as interesting, PCB manufacturers can chemically regenerate waste cupric chloride. Regeneration allows chemicals to recover and use cupric chloride. Chemical regeneration, however, requires the addition of at least one chemical. The available chemicals for cupric chloride regeneration include chlorine gas, hydrogen peroxide and hydrochloric acid, and sodium chlorate and hydrochloric acid.

The full regeneration of cupric chloride eliminates pollution with the etchant material. However, it may not work as a complete solution for the environment because of the introduction of chemical additives that may present other challenges. For example, the use of chlorine gas as an additive requires specialized techniques and a holding tank to prevent its release into the environment.

Manufacturers often use a buffered sodium chlorate solution as an alternative to chlorine gas. Easier to handle than chlorine gas, sodium chlorate also eliminates the need for chlorine gas storage.

Clean production process technologies use an electrolytic divided cell that simultaneously regenerates the etching solution and recovers unwanted copper. To prevent the discharge of toxic wastes into the environment, the process uses a membrane that allows hydrogen and chloride ions through while blocking any copper. The process recovers pure copper for later use and eliminates the cost of disposing of the copper effluent.

But…Other Answers May Exist

Subtractive PCB manufacturing processes include a dozen stages that range from designing the conductive pattern, casting lacquer onto a sheet with solvent-based carriers, eliminating volatile organic content (VOC), and applying the resist to etching the PCB and removing the copper. In contrast, additive processes join materials layer-by-layer to make objects from 3D model data. In PCB design and manufacturing, additive manufacturing stacks circuit traces and integrated circuits on the substrate or any irregular surface.

Additive PCB manufacturing integrates mechanical and electrical CAD design and does not require photomasks, processing, or any intermediate tooling. With this digital process, the PCB design moves from the software to board processing. Any image compensation can occur during the process.

Additive manufacturing improvements will continue to improve efficiencies and reduce waste

Additive processes that use digital inkjet and laserjet printing reduce material waste, save costs, conserve energy and water, and reduce storage requirements. In addition, the digital processes do not contact the PCB and serve as an ideal method for working with thin substrates.

Inkjet printing deposits a thin layer of highly viscous silver nanoparticle ink on the PCB through the use of piezoelectric heads. Ink goes only where the ink is supposed to go. The viscosity of the ink allows it to retain the desired shape after printing. After the ink hardens, a high-speed drill drills vias and through holes. Additional ink fills inside the vias to interconnect top and bottom layers. Sintering the ink obtains the necessary electrical conductivity. Then, another process mounts surface mount device components on the PCB with solder paste.

Along with inkjet printing, additive PCB manufacturing processes also involve laser printing. The laser process fuses conductive elements to the substrate with heat. Laser printing also adds component labels, the dielectric layer, and additional protective coatings. Laserjet PCB printing works as a solution for a limited run, high-value PCBs used in safety-critical applications.

No matter the chemistry you're working with, keeping a greater eye on the effects of materials on environmental relationships throughout the design and fabrication cycle can contribute to a more long-lasting contribution. Whether you're making tiny ICs for satellites, large-scale transformer arrays, or contributing to the efficient manufacturing of plastic pallets, make sure your design is addressing this appropriately. 

With Cadence’s available suite of design tools, you’ll always find room to design as efficiently and smartly as you can imagine. Allegro’s PCB Editor is capable of accurately defining your manufacturing process and moving your board to production in the capacity that you and your product demand. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.