Marty, sounds like you have to use a fishbone modelling. So many parameters that could have impact. You don't say what kind of part you are making, something for under 2GHz or much above? And nothing about dielectric and surrounding components, ground planes, distriubution kind of traces, noise level requirements, spurioses, I/O-loading etc. Not easy for an RF engineer to give advice. Have you modelled the circuit and simulated variations in a computer? If you are talking very high frequencies, even length of bondwires on inside of package plays a role. And what's the package made of, ceramic or plastic? If you ask an RFspecialist to engage you need prepare to give information about:
epsilon as function of temperature for your board material, as well as for all the components
RF-parameters (S e.g.)as function of temperature
possible hysteresis of your ceramic caps, if you use such (behave different on way up and down T)
identification of the parameter that gives your freq shift (Temp sensitive part on inside of 20leg)
checking if you really use valid test set up (microwave is tricky)
checking possible ferroelectric phenomenons in ceramic components, if you use such
what kind of tuning components do you use? Are they temperature sensitive?
As James said, check all return paths, also crosstalking and feedback traces.
and many more questions, depending on the complexity of your circuit
diffusion phenomenons that can change chip data
Tin, Silver migration that can cause bridging
I recommend you to see nearest RF engineer in person. RF problems are rarely solved by mail. If it is a RF-problem at all! Try not to solder the 20pin at all. Mount some 10 boards using silverepoxy mounting the 20pin after first soldering all other components. Aftermounting with other words. If all 10 get center freq where it should be, then the 20pin could be temp sensitive.
In fact your questions seems to reveal that you are not making an advanced HF circuit but something for the commercial 'low freq' market, a radio board or so. Important for you to tell TN, because the 'RF' guys are either ordinary and sound radio/TV/equipment makers..or Telecom/MIL/SPACE 'real' RF neurds with doctor's grade and at least 500 empty Coca bottles on the wall..2GHZ is called DC among the later...
Ingemar
-----Original Message-----
From: Marsico, James [mailto:[log in to unmask]]
Sent: den 29 mars 2001 14:50
To: [log in to unmask]
Subject: Re: [TN] RF Assembly Issues
Most of the time, RF components need the body (under the component) soldered
to the PWB for proper grounding. I'm no RF engineer, (God forbid) but I
understand that this grounding can be critical for performance. Perhaps
this is a good place to start.
Jim Marsico
Senior Engineer
Production Engineering
AIL/Electronics Systems Group
An EDO Company
[log in to unmask] <mailto:[log in to unmask]>
-----Original Message-----
From: Marty Brooks [SMTP:[log in to unmask]]
Sent: Wednesday, March 28, 2001 4:41 PM
To: [log in to unmask]
Subject: [TN] RF Assembly Issues
We are experiencing a concerning variation from board to board in
the
performance of our RF product. We have investigated several causes
of the
problem and are currently of the opinion that soldering quality of
the
transceiver chip is the most significant contributing factor. As an
example, on the last run of prototypes that was manufactured, the
signal
frequency was at the upper limit of the allowable range. We removed
and
replaced the transceiver chip in-house and decreased the frequency
closer to
what we would expect.
I'm going to be making a visit to our contractor's facility next
week to
investigate this problem and would like some feedback on possible
causes or
process controls that I should look into.
The board is 4 layers, HASL finish, single side smt, solder paste
used is
Alpha's UP78 no-clean. The transceiver chip is a 20-pin device with
terminations flush along the bottom of the component.
Thanks for any advice.
Marty Brooks
Manufacturing Engineer
IDENTEC Solutions, Inc.
102, 1860 Dayton Street
Kelowna, B.C., Canada V1Y 7W6
Ph:(250)860-6567, Fax:(250)860-6541
www.identec.com
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