Seems to me that this may be another item that falls in the "it depends"
bin. Do I correctly sense need to make a distinction between and insulator
and a dielectric?
From the web:
snip> Although "dielectric" and "insulator" are generally considered
synonymous, the term "dielectric" is more often used when considering the
effect of alternating electric fields on the substance while "insulator" is more
often used when the material is being used to withstand a high electric
field. <snip
A dielectric can absorb moisture and still be a dielectric. Polyimide is a
good example. Granted there are concerns about what happens to that
entrapped moisture when you go to reflow, hence, we are all now practitioners of
the Betty Crocker "Better Boards Through Baking" approach to manufacture...
;-)
When I started in this industry, solder mask was typically used only on
one side of a board (not every board required it). There appeared to be three
reasons for it 1) to save on solder 2) to reduce weight and 3) to prevent
shorting as trace were getting closer together.
Later, military designers often called for "pads only" surface features
eschewing the use of solder mask because of concerns of solder mask lifting
off making the traces vulnerable to inadvertent shorting in the field (among
others).
By the time of SMT, solder mask was a virtually universal requirement and
not on just one but on both sides of the board. Since we crossed over the
100 MHz barrier in the mid 1994, like it or not, solder mask is often
playing a dielectric role.
Things have obviously changed a lot over the years but most of the basic
concerns remain the same and reading the posts here regularly, solder
continues to be the cause of a great deal of our industry woes. (or so it seems)
Joe
In a message dated 5/11/2009 6:46:36 A.M. Pacific Daylight Time,
[log in to unmask] writes:
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,
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