Problems & Solutions in ENIG (Electroless nickel / immersion gold) and ENAG Plating

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A discussion started in 1996 but continuing through 2019

Q. I currently have wafers with electronic die that get an ENIG finish applied to the aluminum pads. This process is done with an outside vendor. We have had two problems that keep on occurring. I'm looking for answers, information, or directions on where to look for the cause. Any assistance would be appreciated.

1. There have been times where the ENIG process will selectively etch through the aluminum in small spots, but the rest of it will still be 2.0-2.3 µm thick (nominally 2.5 before the ENIG process). The aluminum clean is a very dilute (~0.02 mol/L). The zincation process, which we do a two step zincation, is about 25% NaOH. Is anyone aware of what would allow the process to selectively etch through the aluminum in select spots but not in others? The picture with the round pad shows some of this pitting after ENIG. They show up as shadowed areas on the microscope. Cross sections show nickel plated entirely in the void of the aluminum.

2. We are constantly dealing with rough ENIG. It comes and goes. We have made several changes to the ENIG process but have been unable to get rid of it. I've also attached images of this problem. One is the view under the microscope at 5x. The other is a 50x confocal scan of the surface to show what the roughness looks like. It appears to be irregularities in the nickel grain. What causes this type of appearance? What do we need to control better?

I appreciate any input. Thank you.

July 31, 2013 A. If you filter your EN bath, I seriously do not think that it is the problem. You may be over cleaning which can attack spots if the alloy is not truly homogeneous. Your definition of the zincate bothers me. I am not aware of a zincate that is 25% caustic. That is a pH of 14 and will seriously and rapidly attack the aluminum. James Watts - Navarre, Florida July 31, 2013 A. I do not see the photographs but it seems to me that the process is completely out of control. You need a process map and work on C&E. An Ishikawa Diagram wouldn't hurt either. Quick fix - change vendor. Long fix - make sure the zincate is in good condition and it should be mixed prior to use. Also, I suspect the problem is pre-treatment, specifically, poor drag-out and rinsing. When there are small pads like that, you have to use excellent rinsing and drag-out techniques. The double zincate is a good idea but you also might try moving the parts about a bit while in the bath. You shouldn't have to deal with this, get a new vendor, but be sure to inspect your parts at the same magnifications prior and after plating because if you are giving your vendor bad items you can't blame them. Blake Kneedler Feather Hollow Eng. - Stockton, California

photos added Aug. 2

Q. In regards to the zincation solution, I believe the reason for the high caustic is to get some surface roughening for better adhesion, but the reaction is limited once the zinc plates over. I know I have read about both acid and base zincation solutions. Most of the time we only see 0.2 µm removal of the aluminum. I was wondering if anyone had seen anything similar. I don't know why the pictures haven't posted.

What I would really like to know is what could be causing sporadic rough ENIG. This has been an off and on problem and process mapping never seems to produce answers. Hopefully the pictures will help.

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Ed. note: Pictures are posted now. Sorry for the delay.

Q. On the subject of my rough samples, I recently found articles that talk about Phosphorus content of the nickel and it's affects on nickel grain sizes. Specifically, above about 10.5% w/w of phosphorus the nickel becomes more amorphous. There was some mention that higher phosphorus was also better for soldering. Are these observations seen in industry? If so, what is the best method for measuring the weight percent of phosphorus in the metal. What is a good P concentration to have in the nickel that would be robust to fluctuations in the process. I appreciate any insight or advice.

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August 30, 2013

A. David,
Low phos. works best for soldering (1-3%). High phos. is not recommended for soldering.

Blake Kneedler
Feather Hollow Eng. - Stockton, California

Porosity in Gold

Q. Hello all,

What is the suitable bath composition, current density or electroplating method for gold electroplating on nickel? The gold must be very thin (100 nm) for good solderability with indium and there will be very low porosity.

Thanks to everybody

Q. Hello I just want to ask a question. Regarding ENIG a problem that we experiencing frequently is increase in current during nickel plating. That resulted to plate out. Normally we never encounter this problem and the bath is new. What do you think the cause of sudden increase of current?

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October 30, 2015

A. Hi Rhodora,

Possible causes as below:

1. Stainless steel rod (cathode) heavily plated with Ni without strip off regularly.
2. Stainless steel rod and tank wrongly connected to rectifier (rod accidentally become anode with tank become cathode).
3. Stainless steel rod too close to tank or rod tip accidentally hit tank wall due to basket movement.
4. EN bath too active due to chemical (Ni, hypo, etc) or operating parameter (pH, temp, etc.) out of control.
5. Poor filtration.
6. Plating article accidentally drop inside the tank.

Regards,
David

David Shiu
- Singapore

Q. Dear all,

I'm an engineer of ENIG process.
Currently I found serious problem about skip plating on small pad (round dia. 0.3 & 0.5 mm)

I've tried many items such improve cleaning, improve catalyst efficiency, EN efficiency but still not solve yet.

Does anyone have better idea to solve this issue?

Thanks in advance.

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July 2016

? Hi Kawin. Can you comment a bit more specifically on the skip-plating suggestions from Peter Fogel and Vipul Desai?

Regards,

Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha

July 20, 2016 Q. Dear Ted, Thank you for your suggestion but I already tried all actions that suggested before. 1. Make sure you don't have SM residues in the holes (helps trapping solution). Since our product is flex board, we don't use SM in our process. 2. Make a horizontal micro-etch outside the line (if the equipment is vertical); it improves rinsing of the holes. I've tried already but it still can not solved. (Outline micro-etch around 1.5 microns) 3. Improve rinsing in the vertical line with heated rinses and vibrators in the rinses after the micro-etch. After micro-etch I already increased fresh feed rate of rinsing water and we also have acid activation and another rinse water between micro-etch and catalyst zone. 4. Increase time in the activator (to make sure even the small surfaces is activated. We can't increase time in activator because limit of machine, anyway I've tried to increase temperature and concentration instead but still can not solved. 5. Increase activity in the Ni-bath, by lowering the stab. Already but still can not solve. 6. Increase surface in the bath with dummies. Most important again, check the rinsing of the holes. Already but still can not solve. Kawin N. [returning] - BKK, Thailand July 22, 2016 A. Hi Kawin, Is the small skip pad: 1. Isolated pad? 2. Pad connect to small via or plug hole? 3. Totally EN skip or with EN but much thinner than other pads? 4. Randomly happen or always on same location of each unit? Besides that, is there enough rinsing between Cu microetch and catalyst or over post dip / rinsing between catalyst and EN? Regards, David David Shiu - Singapore

Q. Dear gentlemen,

I would like kindly ask for some of your opinion. I work with ENIG system which is applied on steel material. EN and IG is done on separate plating lines and before IG we do an activation by acid. My issue: after IG plating we detect spots unplated with Au. These spots are small, rounded with sharp edges. This issue is coming randomly with new make-up of Au bath. Can somebody recommend what we can do to solve this issue or why is this happening?

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February 6, 2017

A. Jiri,
This can be caused by nickel oxidation. One might be sure the parts stay wet before the IG step. Also, the activation step might need to be adjusted. It would seem either the etch step is not working correctly or else the parts are partly drying out prior to IG.

Blake Kneedler
Feather Hollow Eng. - Stockton, California

Q. With ENIG we are facing severe issues like:
01. Oxidation / yellowing on Gold surface

02. Ni/Au voids: small spots where Ni is exposed and No gold deposition.
To avoid this we are cleaning Ni post rinses frequently.

Please provide your input towards resolving these issues.

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April 2, 2018

A. Hi Pradeepa,

Please find my input as below.

01. Analyze metallic impurities (Ni, Fe, Cu, etc) in Au bath and re-makeup if out of tolerance. Enhance DI rinse after immersion Au (with hot DI rinse before drying) and verify drying effectiveness.

02. Analyze EN deposit phosphorous content whereas too high might affect Au deposition. Avoid prolong rinsing or dry up after electroless Ni and before immersion Au. Improve immersion Au solution movement / PCB basket vibration.


Regards,
David

David Shiu
- Singapore

Immersion gold sticks to baskets

Q. Hello everyone! I want to ask a question: We are a manufacturer of printed circuit boards. And in ENIG process, we have trouble with baskets in the Immersion gold tank. They are painted green and black and very much get stuck with gold. Black basket is stuck by gold more than green basket. Can you offer any advice for us?
Thank you so much!

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August 24, 2018

? Hi Anna,

Please clarify if only immersion gold sticks onto basket or after electroless nickel (before immersion gold) already seen basket stick with nickel.

Regards,
David

David Shiu
- Singapore

Q. Dear all,
My product was electroless plating gold, after cleaning with solvent or IPA then on the surface appears water red spot. I would like to know mechanism of water red spot and how to prevent.

Thanks you all,
Thong.

Q. High gold on lower half of PCB

We run ENIG line and are finding that we will get higher gold deposits on the lower half of our panels. What would cause this and how can this be fixed? I was thinking that our gold bath might not be circulating as it should.

Vince

Q. We have the same issues. We also have large amount of plate up on our black baskets. After stripping with PSA we also have the white spots that appear to be etched into our baskets.

October 2, 2019 A. The end of the circuit in the plating tank is the high current density area, particularly with acid gold solutions. You must balance this current by shortening the anodes, or shielding the bottom of the anodes.. Robert H Probert Robert H Probert Technical Services Garner, North Carolina October 4, 2019   Robert Probert, you probably misread the question. Vince runs an Electroless Ni, Immersion Au line. Mark Baker Electronic Plating - Winston Salem, North Carolina USA

adv.