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/PassivationandSolderMask.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,
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