Flatness out of the pwb process and warpage in reflow are common issues.
Extended cooldown press times and flatbaking prior to shipment are common approaches to mitigate the former.
Fixturing and playing with metal pattern in rails around the pwb image to maintain flatness during reflow for the latter.
By "playing with metal pattern in the rails" I mean a trick we'd do (Phil Bavaro needs to get the credit) where we'd leave the pwb (pwb's) with a breakaway rail all the way around. Then we'd leave full metal fill on one side of the rail and clear away or reduce % metal on the opposing side of the rail. We had good success in many cases using that trick to add counteracting forces to the board's natural warpage. The first one we tried it on (in desperation) warped badly in reflow that it hung up in the heating elements and we had to open the oven to pry it out. We referred to that as the taco salad shell C-band board. Phil came up with the idea of leaving a frame/border on the pwb with an unbalanced front/back metal pattern.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Frank Kimmey
Sent: Tuesday, September 5, 2017 7:13 AM
To: [log in to unmask]
Subject: Re: [TN] Multilayer PCB of Different Substrat Thickness RF Design
Ricardo,
Many of us who have worked in designing RF PCBs have had to use differing dielectrics and stack ups.
Send me your proposed stack up directly and I'll try to see where you may have issues affecting manufacturing and yields.
Use of different materials and/or asymmetric dielectrics is doable but as always there is a cost.
Lower yields may have to be considered if your proposed is the only way your circuit works.
Let's see if improvements are available.
Thanks,
FNK
Frank N Kimmey CID+
Electrical Engineer / PCB Design
VeriFone Inc
1400 W Stanford Ranch Road, Suite 200
Rocklin, CA 95765 USA
W: 1-916-625-1818
M: 1-916-833-9877
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ricardo Moncaglieri
Sent: Tuesday, September 05, 2017 6:55 AM
To: [log in to unmask]
Subject: Re: [TN] Multilayer PCB of Different Substrat Thickness RF Design
Richard,
Thank you very much for your feedback.
I´m affraid I´m not subscribed to Desiners Forum.
Could pls send to me its url to subscribe to the server list?
Again, thanks a lot.
brgds,Ricardo
>>> "Stadem, Richard D." <[log in to unmask]> 05/09/2017 10:50
>>>
Hi, Ricardo
I do not have the capability to post your picture, but I am sure Steve can, and I see you have copied him.
Being that many people are taking vacations here this week as part of Labor Day weekend, not sure when Steve will be in. You may need to wait until later today.
In looking at the stack-up, I do not have a lot of expertise in this area, but I do know that having differing layer thickness can maybe cause some issues with different coefficients of thermal expansion during reflow and other heat excursions. This can put stresses on the vias passing all the way through the board. But I am not an expert in those issues. One suggestion; you would be wise to also pose the question to the Designers Forum. I am sure they can give you plenty of advice on that.
Richard
From: Ricardo Moncaglieri [mailto:[log in to unmask]]
Sent: Tuesday, September 05, 2017 8:34 AM
To: Stadem, Richard D.; Steve Gregory
Subject: Re: Multilayer PCB of Different Substrat Thickness RF Design
Steve, Richard,
Would you pls upload the herebelow quoted image inord to be more clear on my quetion to the forum?
Indeed hope to receive your comments / advise.
Keep awaiting.
Thank you very much, brgds,Ricardo
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Estimated colleagues,
Nice to get in contact again with all of you after a bit long time.
We are deploying a RF design and by first time we intend to use a multilayer pcb with different substrat thickness as shows the stack-up herebelow detailed.
Questions here are:
As per your experience, is it recomendable such a kind of substrat design?
Some pcb provider reported it is very difficult to achieve. Can you recommend or share your background about pcb provider who can assure high reliability on this pcb provision?
Keep awaiting for your unvaluable feedback.
brgds, Ricardo
Ricardo Moncaglieri
QA Electronic Service Manager
INVAP SE
www.invap.com.ar
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