My guess is that given that the material grain and the design are constant, it's probably more related to the heat-rise during lamination as Guy as indicated -- boards closest to the platens will have a steeper curve than the ones on the inside of the books. This will introduce internal strain in the boards which may release during subsequent thermal excursions. You will then see variation within the lot even though everything appears constant -- material, process, press cycle, etc...
The real question is what can you do...
Plan A -- a fix
A bulk stress-relief cycle -may- work -- load the bare boards into a low-pressure press, heat them and hold them under pressure while the cool -- this may relax and some of the internal stress and retrain the material somewhat -- keep in mind that FR-4 is thermoset and once it is fully cured there's not a whole lot you can do to change its mind about these sorts of things...
Plan B -- a screen and sort
A mass thermal shock may work as a pre-screen -- run the unpopulated boards through your reflow line and pick out the warped ones and use the flat ones...run a test to ensure that subsequent thermal cycles do not cause the flat ones to warp further. Usually the do not, in this case the thermoset properties are our friends...
Hopefully your board shop is your friend
My $.02 -- Ian
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Geoff Layhe of Lamar Group
Sent: Friday, July 13, 2007 9:06 AM
To: [log in to unmask]
Subject: Re: [TN] PCB Fab Question Regarding Bow and Twist
Especially if the warp/weft are in different directions in the cores and pre-pregs
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: 13 July 2007 13:59
To: [log in to unmask]
Subject: Re: [TN] PCB Fab Question Regarding Bow and Twist
Bow and twist in not exclusively a design problem. Poor process control
during the cure cycle of a multilayer board can lead to bow and twist. It is
also possible to flatten the board, only to have the twist return after a
thermal excursion.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leland Woodall
Sent: Friday, July 13, 2007 7:59 AM
To: [log in to unmask]
Subject: Re: [TN] PCB Fab Question Regarding Bow and Twist
Dale,
All of the 10 boards were from the same manufacturing lot...
Leland
-----Original Message-----
From: Dale Ritzen [mailto:[log in to unmask]]
Sent: Friday, July 13, 2007 7:55 AM
To: 'TechNet E-Mail Forum'; Leland Woodall
Subject: RE: [TN] PCB Fab Question Regarding Bow and Twist
Were the two that remained flat from a different PCB date code than the
others that didn't?
Dale Ritzen
Quality Manager
Austin Manufacturing Services
-----Original Message-----
From: Leland Woodall [mailto:[log in to unmask]]
Sent: Friday, July 13, 2007 4:20 AM
To: [log in to unmask]
Subject: Re: [TN] PCB Fab Question Regarding Bow and Twist
Ido, Ian, Lee, and others,
We've already tried an experiment with ten bare boards. These were
essentially flat at start, and went through two reflow excursions. Only two
remained flat, and the others saw twist measuring between 0.28 and 0.87 mm.
If two can remain flat, why can't the others?
Today we ran 488 through only first side reflow and had 21 boards we were
afraid to further populate. Five escaped that process and went on to
receive top side components before being discovered. These ranged in
measurement between 0.88 and 1.96 mm. Our 256 pin QFP with good coplanarity
characteristics has had problems with unsoldered leads at 0.60 mm, but the
majority of our problems have appeared when the overall twist is between 2
and 3 mm.
IPC standards dictate a maximum twist of 0.75% for SMT applications after
soldering. That would equate to approximately 3.2 mm overall for our 187 mm
board.
Internal standards from our mother company say that 1 mm of twist should
give us about 1000 microstrains upon casement (which I haven't yet
verified). Anything above that, in their opinion, could cause a reliability
concern, particularly with ceramic caps.
With today's reject rate above 4% we're experiencing a disaster.
We had two confirmation jigs built during the last two days that cost us
over $1,000. I'm sending four samples out to AkroMetrix
(www.warpfinder.com) tomorrow to perform shadow moiré testing beneath
thermal application of our present profiles. That's about another $10,000
for expedited service. I can only test bare boards with this process, so
I'm not certain what the resultant data will tell me. Two of the samples
will be flat, and the other two will measure 1 mm.
What else can I do in order to solve this problem?
Sorry for being such a burden...
Leland
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ido Mashall
Sent: Thursday, July 12, 2007 3:40 PM
To: [log in to unmask]
Subject: Re: [TN] PCB Fab Question Regarding Bow and Twist
I suggest reflowing the board without component to see If its connected to
component or residual stress from the fabrication processes (faster cooling
time after lamination, or "flattening boards"
after fabrication to send them flat...)
(surely in appropriate and validated reflow parameters) Ido Mashall Beyond
Materials
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ian Hanna
Sent: Thursday, July 12, 2007 10:11 PM
To: [log in to unmask]
Subject: Re: [TN] PCB Fab Question Regarding Bow and Twist
The problem is that the unpopulated boards meet the requirements -- they do
not warp until the assembly process -- this does not violate the
(6012) bare-board or the (4101) raw material acceptability specs...I suspect
that the root-cause lays in the lamination, however there are many other
factors that can influence of course -- but spec-wise, the bare-boards
pass...Ian
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Lee Parker
Sent: Thursday, July 12, 2007 2:27 PM
To: [log in to unmask]
Subject: Re: [TN] PCB Fab Question Regarding Bow and Twist
Leland
I would first characterize the boards using the parameters in the 620
document. If the boards are out of spec, I would inquire as to why these
defective boards are being shipped. The cause is often best left to the
manufacture (you would probably have to do an in-plant audit to begin to
identify the root cause). The first place I would look is at the raw
material; does it meet your specification.
Best regards
Lee
J. L. Parker Ph.D.
JLP Consultants LLC
(804) 779 3389
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leland Woodall
Sent: Thursday, July 12, 2007 12:03 PM
To: [log in to unmask]
Subject: [TN] PCB Fab Question Regarding Bow and Twist
Everyone,
I have a product that ran for almost a year with little or no problem.
Recently we've started seeing unsoldered pins on a 256 pin QFP, and found
the cause to be excessive twist in the board.
We have verified that no changes have been made to our process. Reflow
temps, cooling rates, conveyor speeds, rail parallelism, and transfers are
all good. We've checked incoming raw material and found only slight
deflection in a few specimens.
A couple of examples:
Incoming: 0.13 MM
B reflow: 0.66 MM
A reflow 1.98 MM
We've identified assemblies that have excessive twist after each of the
reflow operations, and our once good product is becoming an expensive
stepchild. We cannot mount twisted boards into cases because of too much
strain, and are forced to scrap this material.
What in the PCB manufacturing process might have changed that would cause
this scenario?
Thanks in advance for your input.
Leland
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