Blog - Advint Incorporated

This is the second year of short papers release. Last year, Advint released papers on cyanide ions, properties and behaviours of anions, trends within the groups and periods of the periodic table, electroless Ni and current distribution (primary and secondary), quality and management. A year ago, I wrote about electrode potential and electromotive force (emf). As an example, nickel and gold possess distinct electrode potentials, and so are their deposition mechanisms.

 

Reel-to-Reel Plating

Electronic industry regularly uses gold, tin, palladium, palladium/nickel and copper on a continuous reel-to-reel plating application. Similar to gold, tin and indium possess comparable properties and advantages. Tin, silver, copper, indium, and gold all possess good conductive property. Indium, a precious metal, we primarily use as an alloying material in plating applications, though they use it as a deposit by itself (without alloying element).

 

Palladium and nickel are neighbors in the periodic table. We consider palladium to be a precious metal and it also possess properties comparable with nickel. It is broadly used in electronics and jewelry industries. On many applications, palladium or palladium/nickel acts as a good undercoat (an intermediate layer). Palladium/nickel deposit is recognized for its cosmetics, and wear and corrosion resistance properties. The ratio of palladium and nickel in the deposit varies between 80 and 20 to 95 and 5, respectively. Palladium also gained prominence because of nickel release and nickel allergic properties. Upsurge in dominance of palladium is apparent when we look at the price of the metal in the market (today’s value - ~ USD 2213 per troy ounce).

 

hard chrome plating

 

Plating on Plastics (PoP)

The demand for nickel on other hand is rapidly growing in the automotive industry, particularly in plating on plastics applications. The wide acceptability of nickel is because of its refractory properties, though it does not belong to elements of the refractory group (periodic table). Nickel release and its allergic properties are an issue in certain demographics and on applications such as eyeglass frame, earring, necklace, ring, bracelet.

 

On PoP applications, plastic substrate preparation using a colloidal catalyst is one of the most important steps. On acrylonitrile butadiene styrene (ABS) plastics, it follows preparation of the substrate with electrolytic copper, nickel and chromium deposits. Among Ni use, electroless Ni is gaining wider acceptance in recent decades. The industry also now replaces hexavalent Cr with trivalent chromium.

 

Electrode potentials are distinct between hexavalent and trivalent Cr plating applications. The trend is the same on other metals such as Ni, Au, Sn, In, Cu and Pd. The properties are distinct not only because of electrode potentials but also because of transportation of ions and ionic mobility. Similar to chromium, ionic mobility of electrolytes of electro-polishing and anodizing are also less. This is one reason these require a very high DC voltage from the rectifier during processing. Note: A paper on rectifier is coming soon.

 

Summary

Different metals deposition mechanisms vary because of their electrode potentials and other properties like ionic mobility and concentration of metal ions.


Among many elements of reel-to-reel plating and PoP applications reviewed in this paper, Pd is used on both applications, and has gained significant recognition in the recent decades.


The recognition goes beyond plating applications.

 

 

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Electroless Ni-P and Ni-B process management had improved significantly within the last two decades. In this paper I’ll write about the science of autocatalytic plating as practiced now and the criterions for advancement at a higher level.


The purpose is to offer reader’s deeper information to achieve consistent deposit physical characteristics and the functional properties of electroless nickel plating.


We will go over the functions of a reducing agent, stabilizer (or catalytic inhibitor), and complexing agent related to thermodynamic property of an electrolyte and mixed potential theory.

 

electroless nickel

 

Temperature (energy), electrolyte flow (current density), solution volume, and the concentrations balance determine the effectiveness of the deposition mechanism and consistency of the deposit in the long term. Gibbs free energy and its relationship with volume, temperature, and energy influence the required concentrations and ratios of a reducing agent, stabilizer, and the complexing agent. An understanding of Gibbs ∆G value with the stability, instability and absolute stability of the electrolyte is imperative. Per mixed potential theory, the deposition reaction is a combination of cathodic and anodic partial reactions taking place at separate electrodes. Redox potentials of sodium hypophosphite, dimethyl amine borane (DMAB), and sodium borohydride play a significant role in a reduction step and the concentrations of stabilizers and complexing agents.

 

To confirm sustained deposit characteristics, electrolysis mechanism and control or management of by products release are important. Geometrical properties such as

  • tank and pump design,
  • electrolyte volume, flow rate and pattern,
  • process control of temperature, pH, metal ion concentration, reducing agent, stabilizer, and complexing agent

will ensure stability.

 

When Gibbs free energy and partial electrode reactions (anodic and cathodic) are within the optimum range plating rate, deposit composition (Ni and P or B weight percentages), and formation of uniform eutectic compounds of Ni-P or Ni-B will be effective and consistent.  Corrosion and tribology properties depend on the consistency of the deposit composition and phase formation. Out of range ∆G value can lead to unstable or absolute stability of the electrolyte. This will lead to either electrolyte decomposition or very low plating rate. The concentrations of a reducing agent, stabilizer, and the complexing agent determine the stability of the electrolyte. Plating rate depends on electrolyte temperature and pH.

 

Summary:

 

Other than simple control and changes done to temperature and chemical constituents, due diligence on the following variables considering the aforementioned concepts will advance the attained deposit characteristics:

  1. Optimize tank design, flow pattern and temperature stratification
  2. Aim for consistent deposit composition
  3. Observe deposit phase formation and aim for consistency if it is economically viable.

One can improve the bath life and the effectiveness of any deposit by giving more attention to the fundamental concepts and conscientiousness on the input variables. 


Though the examples cited in this paper are for electroless Ni, the recommendations apply to all electroless processes including Cu, Ag, Pd, Au, etc.

 

Readers - You are welcome to post questions, comments or thoughts. 

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