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Peel-off strength of Electroless Nickel on Aluminum

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Q. We have designed an aluminum (6063) terminal block which is EN plated then soldered directly to a circuit board but we've found that a smaller force than expected detaches it if given any load - the bond between nickel plate and aluminum fails.

My question is: What peel-off force range should I expect from this process (our plater advises only 1-2 PSI). We have done our best to isolate the part from any force but we are having failures during assembly.

Thanks

Matt Jackson
electronics manufacturer - Gateshead, Tyne & Wear, UK

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A. I can exert more pressure than 1-2 PSI with my fingernail. I doubt that figure (as stated) greatly. Poor adhesion on aluminum is a result of either the cleaning or the zincate step. There is no good visual indication of the quality of the zincate, so it is done poorly in a number of shops. We used an alkaline EN strike followed by a mid phosphorous EN and had excellent adhesion.

James Watts
- Navarre, Florida


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A. Shooting this plater might be an appropriate corrective action. More profitably, accept payment for not publishing their name with their inane 1-2 PSI comment on the Internet. Did they certify to some standard?

To withstand plating stresses (coating internal stresses and CTE stress cooling from plating temperature), EN-to-aluminum adhesion should be 5000-10,000 psi. Adhesion can be improved and internal stress relieved by baking (e.g., 120 °C for 1 hour for heat-treatable Al alloys), but this will reduce solderability. The 180° bend test and uses such as die/mold and engine piston coating suggest adhesive strengths of 25,000-50,000 psi are achievable.

Many possible causes of adhesion problems; see letters 878, 4074, 4487, 12880, 14954, 21801, 22414, 23303, 27627, 35325...

For an EN-plated Al block, the quench test can be used: Heat the item for 1 hour at 250 °C, then "quench in room temperature water. The appearance of blisters or peeling is evidence of inadequate adhesion." -- Electroless Plating, pp. 180-181, Mallory & Hajdu, eds. (1990) =>

Ken Vlach
- Goleta, California  

Ken received a special
"Contributor of the Year" award
from finishing.com for his numerous
helpful and well researched responses

Electroless Plating
by Mallory & Hajdu



A. The shear bond strength of properly pretreated aluminum is at least 16,000 psi. Proper pretreatment of aluminum requires cleaning, deoxidation, etching and a double zincate immersion to guarantee good adhesion.


Ron Duncan

- LaVergne, Tennessee

Ed. note:

It is our sad duty to alert the readers to the passing of Ron Duncan on Dec. 15, 2006. For those who would like to know more about him, a brief obituary opened Update No. 13 of our Metal Finishing Industry Update Podcast.


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thumbsup2Thanks for all the help. Unfortunately the plater is our manufacturing subcontractors supplier not ours so getting anything meaningful out of them is taking time.

Q. Certification to standards has been mentioned, what standard would be appropriate in Europe? A standard would probably be the most suitable.

I guess this might be 'a question too far' so some pointers as to how I can find out for myself would be equally useful.

Matt Jackson
electronics manufacturer - Gateshead, Tyne & Wear, UK

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A. I believe the current European spec. is ISO 4527 [link is to spec at TechStreet]:2003
( BS EN ISO 4527 [link is to spec at TechStreet]:2003 in the UK) Metallic coatings. Autocatalytic (electroless) nickel-phosphorus alloy coatings. Specification and test methods.

Description: Metal coatings, Coatings, Electroless plating, Nickel, Nickel alloys, Phosphorus, Heat treatment, Surface defects, Chemical composition, Chemical analysis and testing, Calibration, Density, Physical properties of materials, Adhesion tests.
Cross references to ASTM, EN and ISO specs.

This International Standard does not apply to autocatalytic nickel-boron alloy coatings, nickel-phosphorus composites and ternary alloys.

Ken Vlach
- Goleta, California

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UPDATE Through some testing and help from a local university we have discovered that our failures are actually due to shock fracture not peel off.

The failure is of the nickel/phosphorus coating which is made shock sensitive due to the interaction of phosphorus and aluminium during the soldering process and stressed by the relative thermal expansions of the aluminium and nickel. This stress is 'locked in' by the freezing of the solder attaching it to the circuit board and a total failure is then initiated by a fairly small shock to the assembly.

Matt Jackson
electronics manufacturer - Gateshead, Tyne & Wear, UK


Plating Adhesion Problem, Gold on Electroless Nickel on Aluminum


Q. I am having a problem with adhesion of my plating (electrolytic gold on electroless nickel over aluminum 6061). After environmental testing (moisture 85% temp 85 C) there is bubbling. When viewed, it can be seen that the nickel has bubbled off the aluminum. It is interesting because if the part is not gold plated, there are no adhesion problems. It must be something to do with the current applied from the gold plating. I bake my parts for one hour at 320 F after gold plating. This helps a bit but not much. I have tried both cyanide and non-cyanide zincate. I plate the gold at 5 ASF.

Any body have any suggestions?

Thank you very much.

Darwin R deleted
- Ottawa, Ontario, Canada


 

A. If the EN let go from the aluminum, I would question the quality of the zincate step. I am a very strong believer in the use of the alkaline EN strikes. It is excellent for improving adhesion. The ammonia smell is strong though.

James Watts
- Navarre, Florida


Good spec for Electroless Nickel on Aluminum?

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Q. Can anyone recommend the specification or callout for Electroless Nickel plating on 6061-T6 Aluminum? I need a coating with minimal heat distortion of the alum (chuck body lapped flat to .0005"), and .001 minimum thickness. The coating should be uniform in thickness, and is intended primarily for cosmetic improvement/corrosion resistance.

Thanks for any suggestions.

Bob Morey
Mfg. Engineer (Consumer) - Issaquah, Washington, USA

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A. Good adhesion is probably required, too.

If selecting an electroless nickel plater, ask to see their monthly lab test reports per MIL-C-26074; most EN platers have the thickness, adhesion and corrosion tests run on both steel & aluminum test coupons.

Specify 0.0010-0.0012" mid-phosphorous EN per Class 4 of either ASTM B733-04 [link is to spec at TechStreet] or Mil-C-26074 [link is to spec at TechStreet]. Class 4 require an adhesion bake suitable for heat treated aluminum, e.g., 120 °C (250 °F) for 1.5 hours. Have a scrap part or 6061-T6 test coupon included in the processing and adhesion tested (somewhat destructive) periodically. If adhesion is a problem, search this site for 158 Letters on 'adhesion electroless nickel aluminum'.

Any heat distortion is actually the release of residual stresses from the initial quenching which have been unbalanced by machining. If this occurs, add a mild stress relief bake slightly warmer than the final adhesion bake, e.g., 125 °C (260 °F) for 2 hours, prior to final machining.

Ken Vlach
- Goleta, California  

Ken received a special
"Contributor of the Year" award
from finishing.com for his numerous
helpful and well researched responses
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