Good question! Like any other material, the final expansion/contraction can cause permanent or long-lasting dimensional changes. It depends on a number of factors, such as the slash type of the PWB material and the temperatures seen during the cycling.
I do know that an FR-4/polyimide mix circuit board that is .500" thick, consisting of 48 layers of copper (which measured 36 miles in length if it was laid out in a continuous .007" wide trace), measuring 40" wide by 34" deep, weighing 45 lbs before the components are loaded, will expand .190" in the long direction and .0893" in the short dimension during reflow on a Summit rework station that is heating the entire PWB to 140 deg. C (preheat temp) and 220 deg C (localized target temperature). After cooldown, it returned to its original dimensions within -.000+.004" overall. I would imagine over time it would return even closer to +-.002" overall.
The reason I know that is because a long time ago when I was very young (45) and naïve I needed to determine if it could be reworked inside of a metal frame (which added another 50 lbs to the weight). Turns out it couldn't due to the much higher CTE of the frame. It literally pulled the PWB in every direction, and not uniformly, causing trace stress/shearing on vias located in the outer perimeter of the PWB.
So much for that time-saving idea.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: Thursday, May 31, 2012 12:22 PM
To: [log in to unmask]
Subject: Re: [TN] PCB dimensions
Thanks Richard,
What about the influence of thermal excursions? Will they affect final dimensions?
Ioan Tempea
Ingénieur principal de fabrication / Senior Manufacturing Engineer
950 RUE BERGAR, LAVAL, QC, H7L 5A1
T+ 1 (450) 967-7100, EXT244
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Lauréat Dunamis 2012
Entreprise manufacturière / Gestion du capital humain
-----Message d'origine-----
De : Stadem, Richard D. [mailto:[log in to unmask]]
Envoyé : May-31-12 1:19 PM
À : [log in to unmask]
Objet : Re: [TN] PCB dimensions
Yes. It is possible. Standard tolerance of all features with respect to the PWB zero datum reference is .003". We have had certain CCAs that needed all features AND the outside dimensions held to within .0015" of the 0,0 reference point, and the fabricator we chose was able to do it. It all depends on the capabilities of the fabricator. Some may not have the capability.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: Thursday, May 31, 2012 10:45 AM
To: [log in to unmask]
Subject: [TN] PCB dimensions
Dear Technos,
Is it possible to keep the PCB dimensions within +/- 0.003"?
I mean, even if the PCB fab does it, will these boards still be within tolerances after soldering?
Thanks,
Ioan Tempea
Ingénieur principal de fabrication / Senior Manufacturing Engineer
<http://www.digico.cc/>
950 RUE BERGAR, LAVAL, QC, H7L 5A1 <http://g.co/maps/2gh3f>
T+ 1 (450) 967-7100, EXT244
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<http://www.facebook.com/pages/Digico/277076778479>
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Lauréat Dunamis 2012 <http://digico.cc/dunamis-2012/>
Entreprise manufacturière / Gestion du capital humain
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