Electronics Assembly Knowledge, Vision & Wisdom
Optimizing Immersion Silver Chemistries For Copper
Optimizing Immersion Silver Chemistries For Copper
Study evaluated several types of additives. Data on thickness distribution, copper attack rate, and deposit porosity resulted in new theories on how to control immersion silver.
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Authored By:
Ms Dagmara Charyk, Mr. Tom Tyson
Mr. Eric Stafstrom, Dr. Ron Morrissey
Technic Inc., Cranston RI USA

Summary
Immersion silver chemistry has been promoted as a final finish for solderability for several years now. There are different commercially available products that will deposit silver in a wide range of thicknesses. Some chemistries, because of their very aggressive nature, can produce a thickness distribution within a circuit board of 20-30 millionths and higher on small pads versus large ground planes.

A large difference in silver thickness will affect solder wetting times within the same board, especially with SAC alloys. A large variation in silver thickness can be overcome in assembly, but it typically requires a more precise assembly process with a longer and or hotter soldering profile. A study has been underway evaluating nitrate-based immersion silver chemistry against other potential
silver sources.

The study evaluated several types of additives against PCB design. Data on thickness distribution, copper attack rate, and deposit porosity has resulted in new theories on how to control immersion silver deposits. Specific formulas have been found that provide a pore free deposit with minimal attack on electroless copper. A specific test vehicle was chosen that takes into account ground planes, isolated traces, interconnected pads and different metallization processes. Data on thickness distribution, copper attack rate, and deposit porosity has resulted in new theories on how to control immersion silver deposits. Specific formulas have been found that provide a pore free deposit with minimal attack on electroless copper.

Conclusions
A new immersion silver chemistry has been developed that replaces silver nitrate with a manufactured silver complex and operates at a slightly alkaline pH. The result is a process that initiates quickly on copper and provides a much more consistent silver thickness over different PWB geometries.

The process is self-limiting and eliminates the aggressive attack on copper seen with nitrate-based systems. Based on this work, a theory has been developed on thickness distribution typically seen with immersion silver over different pad sizes. It appears that as the ratio of the perimeter of the feature to the surface area increases, the immersion silver thickness will increase as well.

The problem is that with very aggressive immersion plating processes, the thickness distribution can be over 20 microinches and the attack can cause notching of the trace or pad sidewall. The overall range in thickness with this new process is typically less than 10 microinches as measured by XRF. This provides a much more consistent surface which is extremely important with lead free soldering. During the assembly process the silver must be absorbed into the solder joint. Table 3 shows calculations for a % increase in silver based on the immersion silver thickness and the stencil thickness. Keep in mind that small pads/fine pitch will have the highest immersion silver thickness and for fine pitch typically the stencil thickness is lower yielding lower amounts of solder paste.

Based on these calculations, 30 microinches of immersion silver can change the weight % of silver in the solder joint by over 1%. With SAC alloys, typically the silver is 3-4 % so a thick immersion silver deposit can change the silver content by 25-30% of the total. As shown in table 4 a 1% shift in silver can change the liquidus temperature by as much as 10°C.

By design, the new immersion silver process is impacted much less by PCB design. It offers the fabricator a wider tolerance for chloride contamination and improved consistency of the silver deposit. To the assembler, this translates into a PCB that has the same soldering performance between boards and over the wide range of components used in today's advanced electronics.

Initially Published in the IPC Proceedings

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