Dorothy,
thanks for the info. The main reason for placing and reflowing these shields during the SMD
process is one of cost - the shields are cheap and we can use standard P&P machines. We tried
looking at things like shield clips, shields that were half etched in the tops so that you can
peel off the tops, removeable lids, shields with a conductive injected material for inner
walls. All proved to be prohibitively expensive - I'm not sure quite why - maybe there are
patents on all these things pushing the price up.
We also looked at using a chassis with segregated compartments but the RF guys didnt think
that we could get good shielding this way.
Have you had any experience with conductive adhesive? Presumably you use a dispensing robot
in line with a P&P machine? How easy is it to rework the cans after the adhesive has set?
Is there any calculation to work out the maximum hole size that a shield will allow for a
certain frequency? This product range I think will operate between 66 and 900 MHz.
Finally what do you think of IR rework for RF shields - just get the heat to the tops and let
the shield conduct the heat down to the solder joint?
Thanks again,
Chris Murphy
Jack Crawford wrote:
> >>> <[log in to unmask]> 09/18/01 06:16PM >>>
>
> Chris,
>
> Have you thought of using shield clips or fences or conductive adhesive so
> the shield cover can go on as a final mechanical process? If shield covers
> were a final step then they would not be exposed to high temperatures or be
> sucking heat from all the components under its domain and exposing the rest
> to sometimes excessive heat. What frequency is this assembly going to
> operate at? Can some of the shielding be in the box it is installed in
> rather than on the PCB itself?
>
> As for rework, a nozzle can be made for every shield shape that is
> nonstandard and there are many rework stations that could then remelt or
> remove/replace a shield. Of course you could always get a laser if you got a
> lot of extra money in your back pocket.
>
> Dorothy Lush
>
> > ----------
> > From: Jack Crawford[SMTP:[log in to unmask]]
> > Reply To: TechNet E-Mail Forum.
> > Sent: Tuesday, September 18, 2001 9:49 AM
> > To: [log in to unmask]
> > Subject: Re: Rework of RF shields
> >
> > >>> Chris Murphy <[log in to unmask]> 09/17/01 11:11PM >>>
> >
> > Hi Technetters,
> >
> > I have got a board covered with SMD RF shields in front of me, ranging
> > in size from 15mm x 15mm to 140mm x 40mm to 70mm square. The spacing
> > between them is only 1.5mm, and they are up to 12.5mm high.
> >
> > I am working through the many issues with these, such as methods of P&P,
> > board / shield warpage, and how we are going to reflow parts under
> > them. My question is does anyone know of equipment out there that we
> > can use to rework these shields? I vaguely recollect seeing a machine
> > that moved a hot air nozzle rapidly around in a rectangular pattern, but
> > I am not sure how effective it will be for larger shields that are
> > spaced so closely together.
> >
> > Is there anyone else doing this type of board, and does anyone have any
> > supplier contacts?
> >
> > Any replies greatly appreciated.
> >
> > Thanks,
> >
> > Chris Murphy,
> > Production Engineering,
> > Tait Electronics,
> > Christchurch,
> > New Zealand
> >
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