Results of a two-year study of various electroless nickel baths.

Many Pb-free alloy formulations, including SAC alloy (consisting of 3-4% silver, 0.5-1% copper and the balance tin), have been developed to replace leaded solder (typically, SnPb), and used successfully to manufacture and assemble printed circuit boards. As Pb-free solder can leave bumps on the traces during HASL (hot air solder leveling), many alternate surface finish processes have been investigated to overcome this challenge.

Alternate surface finishes for Pb-free solder have been developed and used for leadless components with pitches of 4 mils (0.01mm) or less, on which BGAs, TSOPs, etc. are mounted and soldered. These alternate surface finishes include:

• ENIG (electroless nickel immersion gold).
• ENIPG (electroless nickfel immersion palladium gold).
• Immersion tin.
• Immersion silver.
• OSP (organic solderability preservatives).

Several formulations of ENIG plating have been tried. Here, we discuss a successful formulation of ENIG that has been in continuous use in large-scale PCB production for more than two years.

Formulations of electroless nickel on copper. The following Bath I showed good results for electroless nickel plating on copper pads and PTH holes.

Bath I:
Nickel chloride (Nicl2.6H2O): 20g per liter.
Sodium hypophosphite (NaH2PO2.H2O): 15g per liter.
Acetic acid (CH3COOH): 20ml per liter.
DL-Malic acid, CH2 (COOH).CH (OH).COOH: 20g per liter.
Glycine (NH2CH2COOH): 5g per liter.
Boric acid (H3BO3): 2g per liter.
pH – 6.5 to 6.7 (adjusted with NaOH and HCl) 
Temperature: 70o to 72^oC.
Time to get 5 to 6µm of eNickel on copper: 25 to 30 min.

Process sequence. The process sequence was as follows: 1) acid cleaner, 2) swill, 3) micro-etch, 4) swill thoroughly, 5) pre-activator, 6) activator, 7) swill, 8) post-activator, 9) swill thoroughly, 10) electroless nickel plating, 11) swill, 12) immersion gold plating, 13) swill, 14) hot water dip, 15) dry.

This bath was further studied thoroughly by varying the various ingredient chemical compositions. Graphs were plotted to show the optimum condition to get the best and fastest results of deposition of electroless nickel deposit on copper. Each time, a single chemical ingredient (out of six chemicals) was varied and studied at temperatures of 70˚, 75˚ and 80˚C.

Cleaned, pre-weighed 1" x 1" copper specimens were used throughout the experiment, and the nickel deposit thickness obtained and calculated by the weight gain method. This was cross-checked using an x-ray fluorescence thickness tester, and the results were within ±5% difference.

Figures 1 to 11 show the thickness results as obtained by weight-gain method, demonstrating the optimum condition of the bath chemical composition.

Table 1 shows the optimum results.

Optimum composition of eNi bath (Bath II):
Nickel chloride (Nicl2.6H2O): 15g per liter.
Sodium hypophosphite (NaH2PO­2H20): 14g per liter.
Acetic acid (CH3COOH): 22ml per liter.
DL-Malic acid, CH2 (COOH). CH (OH). COOH: 18g per liter.
Glycine (NH2CH2COOH): 7g per liter.
Boric acid (H­3BO3): 2g per liter.
pH: 6.8 to 6.9 (adjusted with NaOH and HCl). 
Temperature: 70^o to 72^oC.
Thickness: 6.74 to 9.0µm of Ni in 30 min.
Time: 30 to 35 min.
Final (optimum) composition of eNi bath:
Nickel chloride, Nicl2.6H2O: 15g per liter.
Sodium hypophosphite, NaH2PO­2H20: 14g per liter.
Acetic acid (CH3COOH): 22ml per liter.
DL-Malic acid, CH2 (COOH). CH (OH). COOH: 18g per liter.
Glycine (NH2CH2COOH ): 7g per liter.
Boric acid (H­3BO3): 2g per liter.
pH: 6.8 to 6.9 (adjusted with NaOH and HCl). 
Temperature: 80^o to 82^oC to get 9.44µm of nickel.
Time: 25 to 30 min.

This bath was prepared and used for 12 months to verify and establish the results. It was consistent in providing a thickness of 8-9µm of nickel at 80^oC in 25-30 min.

Composition of nickel deposit. The deposited nickel was subjected to EDAX analysis to reveal the composition. Results are shown in Figures 13 and 14. The deposit contains 12.23 to 13.74% of phosphorus.

Formulation of immersion gold on nickel:
Gold potassium cyanide K Au (CN)2: 3g per liter.
Sodium hypophosphite: (NaH2PO2.H2O): 10g per liter.
Sodium citrate: 50g per liter.
Ammonium chloride (NH4 cl): 5g per liter.
Temperature: 90±2oC.
pH: 6.9 to 7.0.
Time to get 0.03 to 0.05µm of gold on nickel: 15 to 20 min.

The thickness of gold was measured using an x-ray fluorescence tester. With these two formulations, standard deposits of electroless nickel gold were obtained for large-scale production.

Before shutting down, add 100ml of glycine solution (1gm per liter of glycine stock solution) to the working electroless nickel bath to avoid the possible deposition of nickel on the walls of the process vessel. With this precaution, the bath can easily be maintained for more than 100 days.

Before the start of the bath, analyze the nickel content and add required nickel chloride solution, sodium hypophosphite solution, adjust pH and start the dummy to plate.

This formulation has been found useful for volume production of ENIG plating.

References

Arthur K. Graham, ed., Electroplating Engineering Handbook, second edition, 1962.
Kenneth E. Langford, Analysis of Electroplating and Related Solutions, 4th edition, Teddington, Draper, 1971.

Dr. T.S. Krishna Ram is chief executive (Techno-Commercial) at M/s. Meena Circuits Pvt. Ltd.; This e-mail address is being protected from spambots. You need JavaScript enabled to view it .