Not All Immersions Silvers Are Created Equal

For Immersion Silver, the Key to Success is controlling the process

The inherent nature of almost all immersion silver formulas did little to control the chemical reaction resulting in an aggressive attack of the copper. The consequence was field failures due to creep corrosion, champagne voids in soldering, and complete opens due to etching through of traces. To completely eliminate these issues requires a wholesale change of the formulation.

You would think there would be little difference between immersion silver chemistries, as the chemical reaction of plating immersion silver on copper is pretty simple. Copper is dissolved from the surface, supplying two electrons to the ionic silver atoms in solution. This allows the silver to return to its metallic state on the surface.

   2Ag⁺ + CU → 2Ag + Cu²⁺

However, let us not confuse simple with uncontrolled. This simple reaction is split into two half-reactions, resulting in a positive voltage potential of 1.259. This means silver will easily deposit on copper.

To control this reaction requires slowing the speed that Cu dissolves and/or inhibiting the ability for silver to deposit. Early formulas were focused on getting the silver to deposit and look uniform. The theory was a thicker deposit must be a better deposit. These formulas typically used silver nitrate as a silver source, which is readily available (and relatively inexpensive) but has some inherent deficiencies.

1. First, when using silver nitrate, the silver will precipitate with the introduction of ppm levels of chloride. Even with DI water, it is difficult to eliminate all chloride contamination. The silver chloride precipitant can then deposit with the immersed silver, creating porosity. 
2. Second, nitrates are not consumed by the chemical reaction and will build in concentration as silver is replenished over the life of the bath.  At an acidic pH, which all of these early formulations required, increasing nitrates means there is an increasing presence of nitric acid. Nitric acid is a strong oxidizing acid and aggressively dissolves copper.
3. Finally, high acidity resulting in low pH makes it easier for a solution to dissolve copper. As the pH becomes lower, removal of copper becomes even easier, increasing the aggressive nature of the immersion process. 

All of these early formula decisions created a process that easily deposited lots of silver on copper. Please keep in mind that a true immersion process should be self-limiting once the base metal gets completely covered. With these immersion processes, a high silver thickness is an indication there is either porosity and/or severe attack of the substrate metal.  As these early immersion silver processes became more prevalent in the commercial market, issues with these processes began to surface.

As these early immersion silver processes became more prevalent in the commercial market, issues with these processes began to surface.

1. Hyper-corrosion of copper traces. (Figure 1, MacDermid Tech report 211, Sept 2003)
2. Champagne or microvoids along the copper surface after soldering. (Figure 2, IPC EXPO 2005 D. Cullen)
3. Creep corrosion in corrosive environments. (Figure 3, SMT 2007 R. Schueller)

As always, we need to look at the entire picture and the multiple factors impacting these problems.  All of these issues are made worse due to the uncontrolled nature of these early formulations.

1. Hyper-corrosion is an aggressive chemical reaction where the base metal, copper in this case, is scavenged from a location to drive silver deposition, creating thicker deposits.
2. Champagne voids are copper voids/caves under the immersion silver deposit where copper has been removed from under the silver deposit due to an aggressive chemical reaction.
3. Creep corrosion is possible because of exposed copper caused by an aggressive chemical reaction opening up the solder mask to the copper interface, or due to porosity in the deposit. 

You can see the theme here. Uncontrolled immersion silver deposits can play a major role in most, if not all, of the field failures that have plagued immersion silver.

So why continue working with immersion silver or silver in general?

Silver is a unique metal, with better conductivity than copper, and is relatively low cost when compared to other precious metals.  Shown at the right, data published by John Coonrod of Rogers Corp.₄ demonstrates that thin deposits of silver generate an insertion loss almost identical to copper. Without going into detail, as 5G technology matures, we will continue to move toward much faster speeds, and silver is one of the metals that will have to be used.  To do this we will have to look at the entire picture to make silver viable.

One elegant approach to controlling immersion silver is to control the replacement reaction using complexants.  Experience with plating precious metals has taught us that control is possible by complexing both the precious and base metals, silver and copper in this case. Complexants can be specific, or at least selective, for individual metals.  If a complexant is chosen to tie up silver, it will slow the replacement reaction.  On the other hand, a complexant used for copper typically increases the replacement reaction and can actually help corrode the copper surface.  In both cases, the dissociation factor of each complexant indicates how strong the attraction is to the metal and its impact on the immersion reaction. In designing an immersion silver process, the choice of complexant can be utilized to control the deposition rate and, ultimately, the consistency of the silver deposit.

Complexing the silver provides some additional benefits, such as the elimination of nitrates. When silver nitrate is eliminated, so are its issues with chloride sensitivity and nitrates that build up over time.  A complexed silver also allows a wider range of pH options, including the alkaline side.  By utilizing a non-nitrate alkaline immersion silver, the reaction rate becomes controlled, eliminating the potential for hyper-corrosion of the copper substrate.  Also, it is almost impossible to plate a high thickness, indicating this immersion silver deposit has no porosity and is not attacking the Cu to solder mask interface. 

Alkaline immersion silver is being used in production today with no indication of the problems associated with previous formulations. It provides a surface with tremendous solderability and excellent reliability for the immediate future.

References

1: MacDermid, Excessive Galvanic Attack in Sterling™ Silver Process, Tech report 211 Rev1, Sept 2003
2: D. Cullen, Characterization, Reproduction, Resolution of Solder Joint Microvoids, IPC EXPO 2005
3: R. Schueller, Creep Corrosion on Lead-Free Printed Circuit Boards in High Sulfur Environments, 2007 SMTA
4: J Coonrod, The Impact of Final Plated Finish on Insertion Loss for High-Frequency PCBs, 2017 SMTA

Author

Denis Jacques is currently Technical Director of PCB Service at Technic and has over 30 years of experience in PCB fabrication.

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