Hi Pete, Thanks for your response. My post was somewhat confusing, as by OSP over Immersion Silver I actually meant to use Semblant SPF or SCI for corrosion protection. I was advised by a member to re-post it with correction. I really wanted to know if Immersion Silver can be selectively applied to certain part of circuit, which I know now is not correct. Rest of the board will still have solder mask, so SPF or SCI can be applied only to the part of the circuit with no solder mask. I have been adjusting trace impedance to take in to account the solder mask effect. However it still produces non-uniform impedance due to uneven application of solder mask. Regards Ravinder Ajmani HGST, a Western Digital company [log in to unmask] 5601 Great Oaks Pkwy. San Jose, CA 95119-1003 www.hgst.com Pete <[log in to unmask]> Sent by: TechNet <[log in to unmask]> 09/07/2012 06:21 AM Please respond to TechNet E-Mail Forum <[log in to unmask]>; Please respond to Pete <[log in to unmask]> To <[log in to unmask]> cc Subject Re: [TN] PCB Fab process Ravinder, If you are using both Immersion silver and OSP, I believe that the Immersion Silver will be the replacement for the HASL, OSP can be applied selectively (not the entire board). But neither provide good long term corrosion protection over traces. If you must route high speed traces on outer layers, consider adjusting the trace width of the high speed traces to account for a solder mask cover. You may want to subscribe to the Designer's council listserver for more advice on PCB design questions. Pete ______________________________________________________________________ This email has been scanned by the Symantec Email Security.cloud service. For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________