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Some Notes on Nickel Diffusion Barriers

Contributed by Paul Stransky, Paul Stransky Associates

February 20, 1995

Info on the role of nickel as a diffusion barrier between copper and gold for electronic applications.

A. From some old notes:

Copper and gold have a propensity to undergo solid state diffusion into each other, however copper does so at a faster rate, which is accelerated by increased temperature. Intermetallics form at room temperature over time, bulk lattice diffusion occurs at 200 - 250C. Copper on surface oxidizes leading to increased contact resistance,nickel keeps this from happening, or at least minimizes it.

B. Portion of an article written about connectors from ELECTRONlCS - September 1983, pg 56, article incomplete so author unknown.

Nickel Underplating

A second reconsideration of contact plating material involves switching to the use of nickel underptate instead of tin/lead, for several reasons:

(1) Smear) Unlike tin, nickel will not smear or transfer to and thus contaminate the gold button or rlbbon inlay or gold plating on the contact point.

(2) Wear) Much testing has demonstrated that nickel underplate enhances the wear characteristics of gold.

(3) Porosity) The nickel barrier helps to reduce both the number and the effect of pores as opposed to plating gold directly over the copper alloy base metal (figure 2).

(4) Diffusion) Nickel acts as a diffusion barrier between the copper alloy base metal and the gold because it keeps the copper from migrating up into the gold.

Nickel is cleary superior to tin/lead as an underplate; however, some additional consideration should be addressed to its thickness. Tests indicate that nickel underplatlng performs best at a thickness of approximately 50 micro-inches, regardless of the thickness of the gold. Additional thickness of nickel is therefore not helpful, and because of the following three reasons, could in fact, be detrimental:

- Because nickel requires longer plating time, cost is Increased.

- As nickel plating thickness increases so does roughness of the surface. This emerges as decreased wear resistance because the high points wear off.

- A thicker nickel plating increases porosity.

A further savings may be realized through elimination of the one to five micro inches of gold flash commonly found on posts. Despite customer demand, this amount of gold does not provide adequate pro- tection and serves only a cosmetic function. For wrap type posts,the use of tin/lead is recommended. If the posts are to be mated with plug-gable connectors, they should be considered as connector contacts and plated accordingly.

C. From an old mil. circuit spec. where thermocompression wire bonding was to be done calls out 50 microinch min. - 75 max. nickel barrier layer.

D. References on nickel barrier layers, #3 particularly good

1. Panouis N.T., Hall P. M., "The Effects of Gold and Nickel Plating  
   Thicknesses on the Strength and Reliability of Thermocompression Bonded 
   External Leads," IEEE Trans on Parts, Hybrids, and Packaging, Vol. PHP-12,  
   No. 4, Dec. 1976.
 
2. Hall P. M., Panousis N.T., Menzel P.R., "Strength of Gold Plated Copper 
   Leads on Thin Film Circuits Under  Accelerated Aging," IEEE Trans. on 
   Parts, Hybrids, and Packaging,   Vol.  PHP-11, No. 3, September 1975. 
 
3. Wang C. A., "Interactions Between Au and Cu Across a Ni Barrier  Layer,"  
   J. Appl. Phys., Vol. 60, No. 3, 1 August 1986,  pp. 1220-22. 
 
4. Pinnel M.R., Bennett J.E., "Qualitative  Observations on the Diffusion   
   of Copper and Gold Through a Nickel Barrier,"  Metallurgical  Trans. A, 
   Vol. 7A, May  1976,  pp. 629-635.