First... the fixture idea sounds like the right direction... dimensional
stability would be a concern in fixture design because of the thermal
profiling of the parts... I would go for the graphite.
I have done some RF/Microwave boards and we left the copper bare. The only
thing we did to control spread of solder when assembling the boards is we
provided 'solder dams' at the areas where components were connected to the
traces by their mounting pads or lands. This was easy to do by making a
special mask that had lines that traversed at right angles to the traces
that approached the part at the distance from the pad that we wanted to
limit the solder wetting to. Soldermask has an effect on High freq RF and
Microwave boards too, although it's very limited... but it can vary from
batch to batch and consistent performance was a requirement... Copper oxide
seems to have no affect on the Microwave propagation down the trace  and
it's self healing. We used OSP to protect the copper until assembly and then
let the copper oxidize after assembly... it worked well, (not very pretty
though...)
Nickel would not be a good choice for RF. It's lossy to RF signals... just a
word of caution...

- Bill

-----Original Message-----
From: Marsico, James [mailto:[log in to unmask]]
Sent: Thursday, September 28, 2000 4:57 AM
To: [log in to unmask]
Subject: [TN] DENSE ASSEMBLY (PART II)


I want to thank those who responded to my request for suggestions regarding
the assembly of my dense board.  If you recall, it is 6002 Duroid, .005"
thick, 1.5" x 2", with 270 0402s on it.  The direction I'm leading to is to
panelize the boards (6" x 9").  The panel will be placed on a solid plate
(.050"?) and held in place with another plate (window frame) on top.  This
would allow me to go through pick & place and reflow by handling the fixture
only.  How does this approach sound?  Would you suggest the fixture be made
of stainless steel or should I consider a more thermally stable material
like graphite?

Second question...  This assembly is being used for a high frequency RF
application, so solder coating the boards will effect performance.  (Just
try and HASL this board!)  The RF guys like bare copper, gold, silver, maybe
even nickel or tin plating.  Does anyone have any recommendations regarding
surface finish for this assembly?  (from a board fabrication and assembly
process perspective... I'll deal with the RF guys).

Thanks again,
Jim M.

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