Regarding RF Boards - here's my little collection of thoughts...
Actually Gold is not really necessary to get good RF performance. We built
boards in the 6 GHz range with no surface protection at all... We used OSP
(Organic Soldering Preservative) prior to assembly and just left the
surface as copper after assembly. Papers have been written on the subject
that claim the gold requirement actually is unnecessary, and the copper
oxide that forms on the surface does not seem to affect the performance of
the RF circuit. (of course you will need a good conductive surface where the
board interfaces with shields and such). As far as conductors go, Silver is
the best for RF applications, it has the best conductivity, (better than
gold) and Silver oxide is conductive as well. Silver plating is used inside
High powered cavity filters and in RF applications. Just be aware that it
will combine with sulfur and u must have a sulfur free environment to use
silver or it creates silver sulfide.. a black mess.
We were using a rubber based compound for RF Absorber to reduce reflections
in a RF cavity and discovered this black oxide forming on the silver plated
areas... this was due to the sulfur content in the rubber...
We found that soldermask affects the RF losses in higher frequencies and
chose to use thin stripes of soldermask at right angles to the circuit
traces near the component lands to create solder dams instead of covering
the entire circuit with solder mask. This improved the RF performance. RF
boards will many times have printed components, inductors, capacitors,
filters, mixers, power dividers, couplers... These features are sensitive to
etching tolerances, the final pattern must be held very tightly to the
dimensions in the cad file. Slight changes in the geometry will affect the
performance of the printed component and require tweaking to bring it into
spec. Most other standard techniques for assembly still apply. Some RF
materials are sensitive to moisture and should be handled as such. We used
the LM (low moisture) PTFE based materials from ROGERS for our boards, the
moisture affects the loss tangent of the material... Roger's 4003 or 4004
was a good choice in the bands between 2 and 6 GHz. Peal back of the etch in
handling is a real concern... its not easy to bond anything to Teflon...take
precautions in handling... I remember dropping a PTFE PCB once, it fell on a
corner and wrinkled up like a Ruffles potato chip...
Bill Brooks
PCB Design Engineer , C.I.D.
DATRON WORLD COMMUNICATIONS, INC
3030 Enterprise Court
Vista, CA 92083
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
mailto:[log in to unmask]
IPC Designers Council, San Diego Chapter
http://www.ipc.org/SanDiego/
http://home.fda.net/bbrooks/pca/pca.htm
-----Original Message-----
From: Earl Moon [mailto:[log in to unmask]]
Sent: Wednesday, December 19, 2001 8:18 AM
To: [log in to unmask]
Subject: Re: [TN] Assembly of RF boards
If the design is manufacturable, much concern can be abated. Above 2 GHz, or
so, gold is needed to support the skin effect requirements getting the
electrons, or holes, on down the line. Gold is the key word here so watch
out for quality issues concerning it.
With a good design, there will be no through holes in SMT pads. They will be
necked down when possible, capped or plated over when blind vias filled, and
MLB constructions will meet specified requirements in terms of material
types (DC, loss tangent, etc.). Some materials may have problems depending
on how well suppliers manage their processes. This has been discussed many
times before but watch out for warpage (especially when using dissimilar
materials such as CLTE laminated to GETEK, as an example). Look also at
board surface conditions and those conditions found in x-sections. Don't
want excessive voiding or any delamination as these conditions will ruin
your day.
Though signal integrity will have been thoroughly thought out and verified
at the design level, TDR testing will be required as well as correlating
findings to MLB constructions, materials, dielectric thicknesses, etc.
As always, limit the rework and or repair when chip devices are involved.
Also, ensure the right devices have been selected through the DFM/CE
process. Just went through a very interesting time doing failure analysis of
GaAs devices though finding the devices not the problem as much as the
circuit protection.
MoonMan
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