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January 2001

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Subject:
From:
"McGlaughlin, Jeffrey A" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 11 Jan 2001 12:19:57 -0500
Content-Type:
text/plain
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text/plain (175 lines)
Soldermask thickness and dielectric are of a tertiary level of effect in the
impedance calculation. Therefore effect of soldermask, especially LPI
soldermask, is negligible. The quick rule of thumb is that it adds about
1-ohm to the impedance, far less than the 10-20% tolerance that is generally
specified for controlled impedances. This does not mean that its effect
should be ignored in tight designs.

As to Steve's original problem of how to deal with the boards he has - hand
assembly is probably his best choice. From what I see in his photo, he is
not in as bad of shape as he could be; I worked with one RF Engineer who
demanded no thermal relief at ground connections, No soldermask and HASL
finish talk about a nightmare for the tech's.

Jeffrey A. McGlaughlin, C.I.D.
Sr. PCB Designer
Battelle Memorial Institute
Columbus Ohio
[log in to unmask]



-----Original Message-----
From: Beerman, Dennis [mailto:[log in to unmask]]
Sent: Thursday, January 11, 2001 11:58 AM
To: [log in to unmask]
Subject: Re: [TN] RF Surface mount...


James,
Are you saying that the addition of a soldermask has no affect on a
controlled impedance microstrip?

        ----------
        From:  Terveen, James @ NARDAEAST
[SMTP:[log in to unmask]]
        Sent:  Thursday, January 11, 2001 11:56 AM
        To:  [log in to unmask]
        Subject:  Re: [TN] RF Surface mount...

        Steve, I do alot of this kind of boards seeing that we are an RF
house.  For
        some reason the design guys seem to think that the mask will effect
the RF
        performance of the boards,  I have yet to see that happen when we
have
        changed over to an all soldermasked board and some of our boards go
up to 26
        GHz.   From what I saw on your picture you will probably have some
        tomestoning but not nearly as much as you may think.  The problem
you have
        is they are trying to go to ground on the components and those are
the areas
        that may cause you the most problem  But I don't think it will be
nearly as
        bad as you think it is going to be.

        Good Luck
        -----Original Message-----
        From: Stephen R. Gregory [mailto:[log in to unmask]]
        Sent: Thursday, January 11, 2001 11:00 AM
        To: [log in to unmask]
        Subject: Re: [TN] RF Surface mount...


        In a message dated 01/11/2001 9:49:00 AM Central Standard Time,
        [log in to unmask] writes:

        > Steve,
        >
        >  Ina very past life we produced a product similar.  We were able
to
        control
        > the solder running and maintain the solder tension at the pad site
by
        having
        > the trace necked down considerably (bare minimum width) to force
and
        create
        > as large a solder dam as possible.
        >
        >  Kathy

        Hi Kathy!

        It looks like they've done that with some of the pads but not with
all of
        them. You can see what I mean by looking at a picture I've taken of
a
        section
        of the PCB. Go to:


http://www.driveway.com/share?sid=4eced0a4.94f42&name=Pictures&view=3

        There you'll see the footprint for 4 components, a large cap, two
1206
        parts,
        and a 0805 part. I'm not sure if they necked down the traces to help
with
        soldering, but I still think I'm going to have a lot of problems if
I try to
        print paste and reflow...don't you?

        Thanks!

        -Steve Gregory-


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