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July 2007

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TechNet E-Mail Forum <[log in to unmask]>, Leland Woodall <[log in to unmask]>
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Fri, 13 Jul 2007 19:05:50 +0300
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TechNet E-Mail Forum <[log in to unmask]>, Ido Mashall <[log in to unmask]>
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Ido Mashall <[log in to unmask]>
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There are endless reasons for such problem.
It will be very hard try to find the root cause problem in your PCB
manufacturer working process or materials.
If you totally verified that your process are unchanged
Surely something had changed in his process.
Send the problem to him, he have much better ways to see if something had
changed in his process or material supplies.
If you have old boards to test so you can show him the difference it will be
better and if you can find the lot that things had started to change its
much better.
To my opinion other way you will end with tens of expensive tests not sure
that you will get the right answer and if you will find a direction to the
problem where it will take you?

Good luck

Ido Mashall
Beyond materials

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leland Woodall
Sent: Friday, July 13, 2007 2:59 PM
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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