Dave,
They do see a thermal excursion although not extreme during solder mask
curing. During this time the boards are normally placed in a horizontal or
vertical rack. If they are in a vertical rack and not properly supported the
panels have a tendency to flop back on each other during baking. Since you
have 50 out of 100 that are warping I might suggest that these might be part
of the panels that were stressed during the backing cycle, if indeed they
were baked vertically with poor panel support. Also during the lamination
cycle these could have been the outer most panels in a lamination book (top
and bottom panels). Panels are normally stacked in books of 10 to 15 panels
in one press opening. If the cool down cycle was to quick the outer most
panels in the book cool at to fast of a rate and that can cause them to warp
while the panels in the center do not warp. Finally they might also be
boards that were lay up incorrectly during lamination, with the weave
direction of the laminate going right to left and the weave direction of the
pre preg going north to south. This is called cross weave. Laminates are
made with epoxy, copper foil and woven glass. The glass is made like cloth
fabric which is done by glass thread being held in place under tension
(Grain Direction) while another glass thread is woven under and over these
glass threads (Fill Direction). If the laminate and the pre preg are not
going in the same direction this causes warp because of the cross stresses.
The first two possibilities are difficult to determine by you however the
last one can be confirmed by burning the board and separating the layers.
Talk to a laminate manufacturer and they can give you the procedure.
Hope that this helps. If you want to discuss off line let me know
Scott Westheimer
----- Original Message -----
From: "Dave Chapman" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, January 28, 2004 3:33 PM
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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