It has always been my sense that some solder masks will absorb moisture. I guess my original point should be modified to say that solder mask is not conformal coating. The function of solder mask is to prevent solder shorts between traces not seal the circuitry. I think Denny is correct. Oxidation protection is accomplished by a surface protection applied to copper from the fabrication process and oxidation protection from the solder mask chemistry but is not particularly a moisture barrier. Then again there may be some solder masks that are impermeable to water. I would wonder what happens when an impermeable solder mask is applied to a board that has absorbed moisture. At assembly I expect the mask would blister due to the pressure of the escaping water vapor. Paul Reid -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Dennis Fritz Sent: Monday, May 11, 2009 9:06 AM To: [log in to unmask] Subject: Re: [TN] Solder Mask Well, I disagree to a point.? Many printed circuit?formulations contain benzotriazole as a copper "adhesion promoter" and that functions as an antitarnish also.? I am not saying solder mask contains benzotriazole, but I wouldn't be surprised.? Used to be, solder mask only had to adhere to traces through high temp excursions of soldering.? Now, you also have to adhere through tough plating cycles like ENIG.? Copper antitarnish is applied from water base solutions, so I don't see how that can be a moisture barrier. I think it is wrong to equate oxidation protection and moisture barrier. Denny Fritz -----Original Message----- From: Victor Hernandez <[log in to unmask]> To: [log in to unmask] Sent: Mon, 11 May 2009 8:31 am Subject: Re: [TN] Solder Mask If solder mask protect exposed copper from oxidation then it must be a moisture barrier. Inert protective coatings serve a variety of purposes. They: Protect the copper traces from oxidation. You must passivate every exposed piece of metal that you can't protectively coat. Examples of structures requiring passivation include solder pads and edge-connector fingers. Protect the substrate from humidity. Most substrate materials readily absorb water vapor from the air. In epoxy-glass materials, the water vapor can enter microscopic interstitial voids between the glass fibers and the epoxy matrix, traveling great distances into the interior of the substrate. The water reduces the dielectric strength of the substrate and greatly increases its dielectric loss. Increase insulation resistance between traces. The inevitable accumulation of dirt and water degrades the insulation resistance between exposed metallic contacts on the surface of a board. In high-voltage circuits, it can cause arcing. An insulating coating covering the traces prevents conduction from one trace, through the dirt, to the next trace. Decrease spurious shorts between traces. During the reflow-soldering process, any water trapped within the solder paste explodes into steam, spewing tiny solder balls all over your board. If you cover the traces, the solder balls are less likely to short anything out. Control movement of solder during soldering. During reflow soldering, a via placed too close to a solder pad tempts the solder paste to slide over to the via and disappear down the hole. A thin line of solder mask drawn between the solder pad and the via acts as a solder dam, preventing the solder from moving to the via. Victor, -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of John Burke Sent: Friday, May 08, 2009 4:45 PM To: [log in to unmask] Subject: Re: [TN] Solder Mask Nice one Joe Victor here is a link describing the effect of solder mask on trace impedance : http://www.sigcon.com/Pubs/edn/PassivationandSold erMask.htm As Joe alludes it is indeed a dielectric with a typical dielectric constant value of 3.3. You will find in RF designs that often the boards are specified without solder mask for this very reason. Have a great weekend, John John Burke (408) 515 4992 -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Joe Fjelstad Sent: Friday, May 08, 2009 1:11 PM To: [log in to unmask] Subject: Re: [TN] Solder Mask Victor How/why do you conclude that solder mask is not a dielectric? Cheers, Joe In a message dated 5/8/2009 12:37:48 P.M. Pacific Daylight Time, [log in to unmask] writes: Fellow TechNetters: Besides being a moisture barrier and solder dam what else is the function of solder mask. It is not a dielectric. Victor, --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- **************A Good Credit Score is 700 or Above. See yours in just 2 easy steps! (http://pr.atwola.com/promoclk/100126575x1221322931x1201367171/aol?redir =htt p://www.freecreditreport.com/pm/default.aspx?sc=668072&hmpgID=115&bcd =May5509AvgfooterNO115) --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ------------------------------ ----------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 -----------------------------------------------------