TECHNET Archives

November 2001

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
Frank Kimmey <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 21 Nov 2001 06:57:14 -0800
Content-Type:
text/plain
Parts/Attachments:
text/plain (101 lines)
Steve,
Bear with me while I try this.
        First of all there is an issue in RF design that has priority over
just about everything, that is the "return path". As a signal is sent down a
RF pathway (components, transmission lines, etc.) there is an equivalent
signal traveling the return path. If the return path is of a different
length than the signal path then there is a degradation of the signal (this
is a bad thing). So as RF goes the closer I get ground (my return path of
choice) the better integrity I have for my RF signal.
        Secondly, there is the issue of RF ground as a part of the RF magic
(I think it is mostly the return path issue) having an effect on the
electromagnetic fields surrounding the RF signal path. Any disruption in
this field can cause integrity issues and performance failures.
        Thirdly, there is the need to develop and market product rapidly in
todays world that tends to make the extra XXX hours needed to refine a
circuit enough to pull vias back impossible to get.
        Lastly there is the RF factor. This is the fact that even the top RF
guys will admit to "it's magic" sometimes.
Even though as designers we hate to make the boards harder for you to
assemble, we must make the circuit work. This means vias near lands, poor
thermal relief around components (poor hell, usually none), and extremely
tight component placements are all caused by the driving need to maintain
the signal's integrity (so you can hear that cell call, static and all).
Though as a designer I feel bad for what we do to guys like you, know you
are appreciated for making us look good when the customer has a product that
achieves spec.
Hope it helps,
FNK

Frank N Kimmey, C.I.D.+
Senior PCB Designer
Powerwave Technologies
PH. 916-941-3159
Fax 916-941-3195


-----Original Message-----
From: Stephen R. Gregory [mailto:[log in to unmask]]
Sent: Tuesday, November 20, 2001 6:18 PM
To: [log in to unmask]
Subject: [TN] RF "Black Magic", why do vias need to be beneath
components?


Hi all!

I'm a ignorant process guy, I just build stuff. Don't have the "wizard"
knowledge about RF stuff. I gotta question though...

Why do RF components HAVE to have vias beneath the footprint of the devices?

This creates a problem with certain conductive adhesives that are used to
bond the PCA's to the chassis, from what I'm seeing, trying to adhere to
solder that has flowed out from initial reflow, or during rework when trying
to change a suspected failed component after the bloody thing has been
bonded
to the chassis.

Solder flows out and wets to the gold planes and de-bonds the adhesive
that's
been used to provide ground to the chassis.

Why the 'friggen vias? Can't they be placed just outside the component
footprint where the solder if it goes to the bottomside won't interfere with
whatever black-magic the the RF stuff needs?

Conductive adhesives don't seem to like solder that is liquidous, which will
naturally flow and spread during rework of a suspected failed part...if you
lose ground, you lose god...I've at least learned that, don't quite still
understand it, but ground is god in RF talk...

Those of you that haven't had to deal with this, consider yourselves
lucky...

-Steve Gregory-

----------------------------------------------------------------------------
-----
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send the following message: SET
Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases >
E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700
ext.5315
----------------------------------------------------------------------------
-----

---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send the following message: SET Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------

ATOM RSS1 RSS2