Hello Robert,
I totally understand where your frustration comes from. You spend a lot of time designing a complex board only to discover later that it has warped during assembly.
I will try to add my two cents based on my manufacturing experience and I must warn you that I do not know of any "magic solution" or "magic stackup" that will answer your needs.
Printed circuit boards are complex systems that consist of various materials, each with his own thermo-mechanical properties: Polymeric resins with different degrees of cure (and sometimes - several polymer systems in the same board), inorganic filler matrix, copper of different weight and surface area and I'm sure there is something I forgot.
When subjected to a thermal profile, each element of the system wishes to expand/contract according to its specific value. Luckily for us - the movement of one element is constrained by the others and thus our boards do not brake apart. This is the whole concept of composite materials and diffrent glass styles have different constraining effect on the resin. Since many times innerlayer that differ in thickness also differ in the glass style used - each will expand a little differently.
Lamination engineers in PCB production work with the assumption that similar constructions will behave in a similar manner and this is where the balancing requirement originates. The balancing is usually designed respective to board centerline and usually takes into account as many parameters as possible - the copper weights and copper distribution (in design stage) and laminate thicknesses and glass styles (in the lamination stage).
The PCB lamination process gives you not only your multilayer board but also a bunch of inherently induces stresses that are kept "locked in" and just wait for a chance to express themselves.
These chances usually come around when the board is heated (many times above its Tg) and that is why your perfectly flat and nice board suddenly wishes to become a "strudel" in the assembly oven. Actually, if your lucky - finishing operations at the PCB shop such as HASL (Hot Air Solder Leveler) might start the warping and allow you to detect it before you waste time and money on assembly.
I hope that I succeeded in giving a peak on the subject.
Have a great day,
Yehuda Weisz
Yehuda Weisz
Tel: (972)-3-6342045
Cel: (972)-53-556897
----- Original Message -----
From: "Robert Sefton" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, January 27, 2004 7:33 PM
Subject: [TN] PCB warp
> I'm a design engineer, not involved the PCB industry other than as a
> consumer. I want to understand the mechanisms of bow and twist in
> multi-layer PCBs so that I can specify stackups that will stay flat
> during fabrication and assembly. I want is to understand the mechanisms
> well enough to be able to get away from the constraint of always
> building perfectly balanced PCBs with symmetric stackups. There are
> times when unbalanced stackups have advantages, and I'd like to have the
> flexibility to use them with confidence.
>
> I've heard a lot of different opinions on how to prevent warping. Copper
> balance, balance of laminate and prepreg thicknesses and types, copper
> thieving, solid copper ring around the perimeter of each layer, etc.
> Others say there is simply no way to predict whether a board will warp
> or not. There has to be some science to this. Have there been any
> detailed studies of the causes and prevention of PCB warping? Are the
> stresses that lead to warp always created during board fabrication, or
> can stresses be introduced later on during assembly or rework? Lots of
> questions. Any info or pointers to more info would be much appreciated.
>
> Thanks,
>
> Robert Sefton
>
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