Electroless Nickel Plating of holes, threaded holes, and blind holes

A. Hello, Jim. The biggest problem that you will have it that all of these electrochemical processes operate out of solutions which provide the necessary ions for the reaction to proceed--and the amount of solution within a 1/8" dia. hole is obviously very limited.

So, if you try to hard anodize it and you don't get fresh solution in the hole, the acid will grow depleted and unable to perform the necessary dissolution process. Similarly, to electroless nickel plate it, you must get sufficient nickel in the solution immediately proximate to the deposition point.

Maybe an anodizer or electroless nickel plater can offer you a practical solution exchange mechanism based on high agitation, ultrasonics, etc., but my feeling is you'll probably need special rigging to pump solution through these holes.

Zincate is an alkaline solution, sometimes proprietary, with zinc dissolved in it which immersion plates onto aluminum and is used as a prep step because of the difficulty of keeping an aluminum surface active for electroplating. Zincate is used as the first plating layer nearly universally, but it is not a final finish.

A. Try high phosphorous electroless nickel. To insure the hole is coated uniformly you will have to cut a part at the halfway point of the 1/8 in ID and have the plating measured with a microscope. This is called a microsection and should cost about $100.00 at a testing lab.

A. First build a jig that will hold a significant number of tubes in the vertical position of a manifold. Then pump Aluminum Cleaner into the manifold and up thru the tubes and spilling back into a catch basin. Repeat the pumping with each of the following solutions, (2) water (3)Aluminum Deoxidizer (4) Zincate (from "cook" book or proprietary mix) (5) water (6) alkaline electroless nickel (7) water (8) acid electroless nickel (experiment with times until you get enough thickness to withstand whatever corrosion media that you did not mention).

A. First off ... if your plater is worth his salt, he should gladly take the parts back and make them good. If you did the plugging on the holes, that's the risk you take! This is one plater who won't accept anyone else's masking without a specific waiver that if there is leakage, it's not our fault! Now on with a couple of solutions....

White salt corrosion so I'm presuming that your EN plated part is made of aluminum or zinc. Once you've seen the salts, the corrosion mechanism is in high gear - water from air has been adsorbed with corrosive plating and/or pretreatment chemical residue. If indeed caustic residue (i.e. alkaline cleaner, etch, etc.), then you need something mildly acidic that will neutralize alkaline salt without harming aluminum or nickel - try acetic acid [on eBay or Amazon] (a.k.a. vinegar [in bulk on eBay or Amazon]) or boric acid (eye wash solution from blue bottles). If acidic residue from acid deox or EN solution itself (except this would be characteristically green salts), need something mildly alkaline that will dissolve salts. Sodium carbonate (washing soda [on eBay or Amazon] ) or bicarb (baking soda [on eBay or Amazon] ) might do the trick. Either way, dilute with warm water and apply in holes with eye dropper. Obviously we've been here ourselves. Masking holes is tricky and never 100%.

Good luck!

A. I'll take a wild guess that you have an aluminum substrate. Then, the white corrosion could be aluminum and/or zinc hydroxide. If so, the corrosion could slowly spread beneath the EN. Try the following methods [test one part or one hole first]:

Short procedure [chromic acid]: Soak part in 1 wt% chromic acid/DI water at 120 °F for 15-20 minutes, turning and draining as necessary to fill and empty each hole. Rinse in a dragout tank, then with DI water, then blow dry with clean air.

Long procedure [nitric acid clean & chem film]
Set up a working area next to a flowing rinse or sink. Use an eyedropper to put some 50 vol% nitric acid [or deox. solution if very little corrosion] into holes on the top surface. Use caution, the nitric may foam up and stain the EN [of course, you could play safe & put plating tape over each hole area, then work through a hole in the tape]. After foaming ceases, thoroughly rinse. Final rinse each hole with DI water. After thus cleaning all the holes, move your workstation next to your chromate conversion (chem film) solution tank and its rinse tank. Apply chem film via eyedropper, rinse when coated. Rinse again with DI water, then blow dry with clean air.

Let us know the results.

A. Galvanically, aluminum and stainless steel are not far enough apart to cause any serious problem. That said, I would recommend plugging (masking) the holes, because as you know, the oxide film forms half in and half out, so that on the sharp cutting edge of a thread the whole edge will be brittle oxide and it will break off and weaken the connection.

A. Since you said electronics, I assume that it is a 4-40 or a 6-32 thread. The probability is that it is for helicoils. If not, it probably should be. Small holes hold a lot of chemicals after each process tank which seeps out in the next. After the EN, you normally have ugly rings around the holes and sometimes , even runs. The only way to avoid that is time consuming flushing each hole with a small stream of water. With helicoils you can use a dab or zinc chromate and the helicoil creates a metal to metal contact and seals the non contact portions. Small blind holes are a challenge.

A. In letter 39153 James makes a suggestion regarding blind holes that will be plated that they be filled with clean rinse water before the process starts, to minimize entrapment of chemicals. I can't help but wonder then, in the case of plugged holes, if filling the holes with clean rinse water before plugging them wouldn't minimize stains, accidents, etc., from the plug not performing perfectly.

Regards,

A. Joe,
I would assume you are giving these parts a Woods Nickel Strike? I have found that front to back bar agitation in the EN tank helps with blind holes. With this rocker type motion, solution is forced in and out of the holes. Side to side agitation sometimes is not good enough. Good Luck!

A. Joe

I would agree with Mark's suggestion about agitation but I believe that your problems start at the cleaning stage. 3:1 is about the limit for plating blind holes and problems increase as the diameter goes down.
First, cleaning is just as important inside the hole as on the surface. Vapor degrease if possible. The next problem is to get the first of the aqueous process solutions down the hole. Beware of trapped air bubbles. Ultrasonic agitation is often recommended. In my experience this just leads to a vibrating air bubble! I have been forced to use vacuum occasionally to get the air out and the solution in.
Next rinsing: Because of the aspect ratio, the removal of chemicals is largely dependent on diffusion. This is slow so allow plenty of time. Use a water jet if possible, and hot water helps. If you leave any of the chemicals in the hole they will interfere with the next process.
Now to get the next process chemicals into a hole full of water, and again diffusion limits the rate. Effectively you have a diluted process operating in the hole so allow time and don't expect fast plating.
I used to visit our design department regularly to ensure that there were no blind holes designed in to cause me problems later. Even a tiny bleed hole helps enormously.

A. Geoff and Mark are kind! Plating will normally go into blind holes with a 1:1 aspect ratio (width over depth). Yours is 3:50.
Individually hand plating will not plate to the bottom of these holes for the reasons stated.

Exotic printed wiring board plating solutions are not even asked for this level of plateability and they are thru holes.

True zinc chromate paint/primer [affil links] is about the only option.

A. It's not easy but a good electroless nickel plating practice and fixturing will be able to solve your problem and plate those blind holes provided your substrate is plain carbon steel. Alloy and stainless steels as well as other substrates that require a galvanic activation will be subject to the described problems associated with the aspect ratio. We have successfully plated several molds with intricate geometries and very deep recesses and holes.

A. To cover all the blind hole it is necessary that the hole facing upward to allow the hydrogen gas to escape. It is aided by the use of "wetting agent" (surfactant) to facilitate removal of hydrogen bubbles.

If it it is impossible to have all holes face upward, then rotating is required.

A. With a proper fixture you will be able to electroless plate the blind hole.
Besides this, you must be sure that the Salt Spray test is done in a correct way.
Liquid must flow thru the surface and must prevent liquid stagnation inside the blind hole.

A. Hi Gabe,

Vibration (with vibrator) or knocking by lifting up rack then free falling (manual or automatic) is highly recommended for EN inside blind holes to get rid of trapped gas bubble quickly (mostly hydrogen gas generate from EN reaction). Similarly, post rinsing (with vibration and/or knocking system) and drying with strong hot air circulation is necessary to ensure chemical residue inside blind holes is fully removed and dried.

Regards,
David