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 ---------------------------------------------------------------------------------