Surface Finish——Immersion silver

    There’re different surface finishes for PCB boards, including immersion silver, immersion gold, OSP, HASL and so on. Here I would like to introduce the surface finish—immersion silver. Sterling immersion silver is an immersion process for plating a silver deposit on copper, designed to be used as a final finish for printed circuit boards. Immersion silver is quickly becoming the surface finish of choice for PCB fabricators, assemblers and end-users. Immersion silver finish can offer outstanding solder ability, contact functionality, aluminum wire-bonding, solder-joint strength, and is a drop-in replacement for HASL. Some PCB boards may apply both HASL finish and immersion silver finish. The apparent difference between HASL and immersion silver is the color of immersion silver is more matte than HASL. Also, immersion silver can be touched more flushed than HASL.
Features& benefits:
Features:                                  Benefits:
High stability                               Long solution life
Uniform bright deposit                       Improved cosmetics
Wide operating window                      Process control simplicity
Replenished                                Reduced make-up costs   
Using the immersion method of metal deposition, sterling silver is deposited on printed circuit boards as the surface finish. Immersion deposition is also employed in the process of the deposition of gold in the process of ENIG. As the widespread use of immersion silver, a main problem may occur in the PCB fabricating. We called it SMIA (Solder Mask Interface Attack). Silver is deposited to a thickness of 6-18microinches, with a proportional copper dissolution. In some extreme cases, the copper is excessively attacked at the solder mask and copper interface, particularly on conductor traces. That’s what we called just now SMIA. SMIA is a problem which will cause a reduction in conductor width; negatively will affect controlled impedance and extremely will cause open circuits. So when making the immersion silver finish, we should control the volume of chemical material and control the technique.
   Solder Mask Interface Attack (SMIA) can pose a problem to the PCB manufacturer. However, several options for minimizing and eliminating the phenomenon are readily available and easy to implement. To improve the solder mask profile and its adhesion to copper is the most effective avenue in resolving the SMIA. Improving the solder mask side wall profile prevents corrosive chemistry from being entrapped under the solder mask and excessively corroding the underlying copper. Solder mask process parameters are the most successful avenue to investigate and optimize for elimination of SMIA. So having a good command of the solutions for SMIA can commendably avoid the problem.