Using Eductors to Achieve Uniform Plating

Q: Mechanically speaking, what’s the greatest challenge to achieving consistently high plating uniformity?

A: Putting aside the chemistry and focusing strictly on the mechanics, getting plating results that are always within specification first requires the right agitation, either from air pumps, ultrasonics or turbulent-flow devices.

Properly designed, agitation does three things: It removes air bubbles that form on the underside of the part, it removes spent solution from the part surface and replaces it with new, and it cools the part.

When choosing an agitation method, the first consideration should be part geometry. For a captive shop with a finite number of relatively simple-shaped parts, air and eductors will both work. Shops that plate more complex parts—those whose shapes trap air or that have undersides or fold-overs, or parts with some inner feature—are better served with eductor technology. This is because, unlike air pumps, eductors won’t complicate the situation by introducing more air into the bath. Job shops almost universally choose eductor agitation, because they want the ability to reliably plate parts of any shape without being concerned about the agitation factor.

If eductors are used, they should be oriented so that they target spots where air bubbles can form and provide flow along the entire part surface to prevent “dead spots” next to the parts. The shape of the process tank helps determines where the eductors should be placed to distribute the agitation around the parts most effectively.

Eductors are manufactured in many different materials; bath chemistry and temperature are what matter most in determining which eductor material is best for a given plating process. Polypropylene is the most commonly used material; stainless steel and polyvinylidene fluoride are preferable for higher bath temperatures; Hastelloy is used when extreme corrosion resistance is required. A “natural” (colorant-free) Kynar typically is used for printed circuit board plating applications.

Here’s a fact that often causes confusion: Eductors are sometimes called nozzles. They actually have a nozzle section and a diffuser section, however. We manufacture both eductors and what I’ll refer to as “spray nozzles.”

Eductors (and spray nozzles) are an important component for wash and pretreat tanks, as well as plating. Chosen carefully, eductors can prevent a myriad of woes. Following are four common causes of part rejects in plating; often, these causes involve how the eductors are selected, installed or maintained.

Uneven coating of phosphate, caused by:

Plugged spray nozzles, poor nozzle choice (inadequate or inappropriate spray pattern) or improper racking, which causes parts to shield each other.
Poor agitation in the dip tank, a plugged or broken eductor, a poor eductor choice, system leaks.

Flash rust, caused by:

Plugged spray nozzles that cause poor spray quality and inconsistent deposition.
Excess humidity on entrance or exit to treatment line.

Streaks on parts, caused by:

Inadequate cleaning due to plugged eductors or spray nozzles.

Bare spots, caused by:

Inadequate cleaning due to plugged eductors or spray nozzles.
Bath chemistry that is off.
Excess sediment in the bath caused by insufficient filtration.
Poor rinsing between cleaning and phosphating, often caused by plugged eductors or plugged spray nozzles.

Agitation is as critical to plating results as chemistry. Mechanically, agitation balances the chemistry, so that bath components, concentrations, temperature and pH are uniform throughout the bath.

Agitation balances chemistry: Think of it as the ABCs of good plating practice.

Dave Keely is an applications engineer with Bex Inc. Visit bex.com.