I would recommend ENIG for both leaded and lead free.

Most of my designs are specifying Lead Free Solder (HAL-LF) and/or ENIG. HAL is good for low tech components and the ENIG gives you a nice flat surface.

There are a few options all with their related good and potentially bad points. Immersion tin is a thin tin layer which normally has good solderability however is easily oxidised or contaminated.

Immersion silver has similar issues with the added potential of atmospheric sulphur tarnishing. The best option in my opinion is electroless nickel / gold (ENIG). This provides a long lasting and durable finish which can withstand storage, pre-baking and double sided reflow with few problems.

It works well with tin/lead or lead free processing.On the down side it is sometimes affected by so called "black pad" and I'll let you research that one on the internet since books could probably be written!

My advice would be to use ENIG with a PEC supplier who hasgood process controls in terms of finish plating chemistry.

Not knowing where this product is being used, there are many alternatives to select. The easiest one that I would recommend would be a Tin HASL process. This board finish can be assembled with either tin/lead solder or lead-free solder and should not impact the process at all.

Keep in mind the differences in the thermal requirements of the lead-free process however, as the laminate material may have to be changed to prevent laminate damage during the lead-free assembly and I would recommend reviewing IPC-4101C, Specification for Base Materials for Rigid and Multilayer Printed Boards, to make sure the correct laminate is being used for your application.

Conversion to OSP should not be a problem for the existing SnPb process due to the superior wetting of Pb containing solder. For lead free, it will be a slightly more challenging, especially if they require a double aided process.

There are numerous lead free solder pastes with characteristics that have been fine tuned for various cases. For OSP, and double sided reflow, it sometimes is necessary to choose a lead free solder paste with slightly more activity, since the OSP finish on the second side is compromised during the first side reflow.

The exposed copper will slightly oxidize with time, so the time delay between reflow processes can effect the amount of copper oxidation, which is an issue for lead free solder paste. Cookson, and other suppliers with a broad product offering, can recommend and apporpriate lead free paste.

Given the higher price of lead free solder, one suggestion is to examine the low silver versions, namely SACX Plus. The lower silver content helps to lower the price, while the presence of some silver and the overall formulation provides excellent performance.

I would suggest that you use Lead Free (SN100CL) Hot Air Leveling as this is one of the most robust finishes on the market today.

One of the advantages of the SN100CL HAL process is that the finish will be flatter than what you are currently seeing and since your boards aren't that complex and don’t have any issues with coplanarity the obvious answer is SN100CL HAL.

This finish easily works for both lead free and tin lead reflow and there are plenty of board shops capable of this final finish (also many service companies).