You are right, there is some science to this. All the items that you have
mentioned (balance, balance of laminate and prepreg thicknesses and types,
copper thieving, solid copper ring around the perimeter of each layer, etc.)
are DFM issues and need to be reviewed and 'optimized' for the design.
After that it is the fab manufacturer's responsibility to maintain process
control to deliver a product that meets the requirements. Lay-up (grain
orientation, balanced constructions, ( no substition of cores)and multilayer
lamination heat-up and cooldown rates become extremely important. Not to
mention that some process upsets get an 'extra' bake to flatten them only to
have the warp re-occur at the first the fab sees temperature during
assembly.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Dave Chapman
Sent: Wednesday, January 28, 2004 3:34 PM
To: [log in to unmask]
Subject: Re: [TN] PCB warp
What about boards that are warped before assembly that are not HASL but
Immersion Silver or Tin? So they didn't see the extra heat cycle.
Not the entire lot but half of them are warped. 50 out of 100.
Dave C.
-----Original Message-----
From: Yehuda Weisz - Netvision [mailto:[log in to unmask]]
Sent: Wednesday, January 28, 2004 1:32 AM
To: [log in to unmask]
Subject: Re: [TN] PCB warp
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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