Just curious, Dan.....how tight to the board are you able to place the chem
etched shields (repeatedly)?  With board warpage, and chem etch being very
"rough" sometimes, the tolerance stack-up gets us pretty badly on occasion.
We have wound up with some .020 - .025 gaps very recently, they were long
boards.  Is this ever a problem for you?
thanks, jb





                      "Dan R. Johnson"
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                      >                        cc:
                      Sent by: TechNet         Subject:  Re: [TN] Smt Shields for an RF board
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                      .ipc.org>


                      06/23/2003 03:57
                      PM
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                      "TechNet E-Mail
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                      R. Johnson"






Hi Mike,
We use a lot of SMT Rf shields
Points to remember on shields.
1) Alloy- 770 (Cu55/Zn27/Ni18) or 752 (Cu65/Zn17/Ni18) work well. good
solderabiltiy and long shelf life.
2) We use chemical etch forming. Probably the least expensive.
3) Let the fab house do the design of the shield, I give them a mechanical
drawing of the *finished* part *not* a pattern  drawing. If the parts don't
meet the drawing it is their nickel (groan).

Just re-read your message and realized you were talking about PCB design. I
would highly recommend tabbed parts, as mentioned else where there are some
real reliability issues with soldered butt joints. Just make sure there are
enough tabs and solder attach points to prevent microphonics (electrical
noise resulting from mechanical motion of the shield). We don't often
reflow the shield, it is hand attached, when we have I used a "pad" 4X the
material thickness (design allowing). The ratios for solder mask and paste
were the same as we use for any SMT part.

Hope you find this helpful,
Dan
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