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From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Tue, 14 Jul 2009 10:50:12 -0500
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I understand now what you mean.

IPC-6012B states the following on bow and twist:

3.4.3 Bow and Twist Unless otherwise specified in the
procurement documentation, when designed in accordance
with 5.2.4 of IPC-2221, the printed board shall have a
maximum bow and twist of 0.75% for boards that use surface
mount components and 1.5% for all other boards. Panels
which contain multiple printed boards which are
assembled on the panel and later separated shall be
assessed in panel form.
Bow, twist, or any combination thereof, shall be determined
by physical measurement and percentage calculation
in accordance with IPC-TM-650, Method 2.4.22.

The standard does not discriminate amongst CCAs that have been subjected
to any other post reflow process, including HASS, thermal cycling, or
rework. I believe the intention is to prevent CCAs that exhibit severe
bow and twist (more than .75% per inch for SMT, and 1.5% per inch for
PTH) from being used as production goods, especially in a high
reliability (Class 3) applications, because of the expected reduction in
reliability due to the flexural stress the solder joints would see if
the is CCA stressed when clamped in the frame.

The fact that some of your CCAs warped, and not all, is probably
indicative of a laminate condition during fabrication, not their
position in the chamber. The direction of the fiberglass weave can have
a very great impact on the amount of bow and twist seen after reflow, or
any other thermal excursion. For example, if the PWB fabricator
maximized the number of PWBs (or panels) from a single flat by aligning
an array 3 across by 5 rows, and had enough material left over in the
flat to get two more across the bottom if they were turned 90 degrees,
that could possibly explain why you get two or three that warp every so
often. I'm not saying to rule the chamber location out, I'm just saying
it is more likely to be something related to fabrication. Design and
process issues also come into play. They can combine to bring some PWBs
"over the fence" from a warpage standpoint.

As far as whether you can use the CCAs that do not meet the
specification, you need to decide whether they are usable for your
particular application. I note you work for Dell, so I am assuming this
is a computer motherboard, judging from the size. When your CCA is
screwed to the bosses in the bottom of the cabinet, how much deflection
takes place? There will be a one-time relaxation in the solder joints,
but the amount and severity of that relaxation depends on the amount of
warpage and location and the type of components. The initial relaxation
or creep could reduce the expected product life by a small amount, but I
don't have any documentation that states just how much that would be. I
don't think anyone does, because a CCA with a pile of QFNs and LGAs and
larger MLCCCs soldered with lead-free on an ENIG PWB is going to be much
more of a risk if badly warped than one where most of the components
have compliant leads (SOICs and QFPs) and are soldered with SN63/Pb37 to
a HASL PWB. 

I would suggest you contain all CCAs that are warped and document them
as non-conforming and investigate the flat layout, prepreg, design, and
process factors to find the root cause. Concurrently, you can take the
worst-case warped CCAs and screw them into a metal frame that precisely
emulates the plastic cabinet. Then thermally cycle to failure those and
compare the results with two or more non-warped CCAs in the same frames
cycled inside the same chamber. That should give you some data as
whether there is any loss of reliability as a result of the flexural
stress applied and the basis for disposition. Once a decision is made,
either scrap or maintain traceability on the suspect lots in case you
need to get them back, either as a recall or by attrition through your
repair outlets.

The fixtures I described could be designed as reflow fixtures that would
hold the PWB planar during reflow, and possibly during any post-reflow
cycling as well. 


-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]] 
Sent: Tuesday, July 14, 2009 9:47 AM
To: Stadem, Richard D.
Subject: RE: [TN] Maximum Allowable Bow & Twist for Post T/C Reliability

Post soldered assembly were placed in a Thermal cycle environment in
which very noticeable bow/twist was observed after the test.   The
assemblies did not exhibt signs of bowness prior to the Thermal Cycle
event.   Only a portion of the assemblies exhibited the bow.   Perhaps
the area within the chamber where they were located.   Does the 1.5%
bow/twist criteria apply to these samples?

Victor,

-----Original Message-----
From: Stadem, Richard D. [mailto:[log in to unmask]] 
Sent: Tuesday, July 14, 2009 9:37 AM
To: TechNet E-Mail Forum; Hernandez, Victor G
Subject: RE: [TN] Maximum Allowable Bow & Twist for Post T/C Reliability

I am not sure what you mean by Post T/C?

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Victor Hernandez
Sent: Tuesday, July 14, 2009 9:31 AM
To: [log in to unmask]
Subject: [TN] Maximum Allowable Bow & Twist for Post T/C Reliability

Richard,

I did not receive any comments on my Post T/C Reliability inquiry so I
will post again.

What about Bow & Twist on Post reliability testing, T/C?
Does the 1.5% still applied for SMT/PTH assemblies?

The assembly is a rectangle, 10.5 x 8 inch, form factor.

Victor,

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, July 13, 2009 10:07 AM
To: [log in to unmask]
Subject: Re: [TN] Maximum Allowable Bow & Twist for PWB-PCB

Bev, I totally agree with you, but the point they are getting at in IPC
2221 is that no section of the PWB should have more than .75% per inch
as a rate of warpage, extending in both the X and Y directions, after
reflow. This is because if you have more warpage than that you are
putting a stress on the resultant solder joints that leads to a
reduction in reliability, especially with a lead-free joint and also
especially if you need to straighten the CCA when mounting it into a
chassis or frame.
I think this specification, like so many others that were based on
leaded solder, should be revisited for unleaded solders.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian
Sent: Friday, July 10, 2009 3:24 PM
To: [log in to unmask]
Subject: Re: [TN] Maximum Allowable Bow & Twist for PWB-PCB

I contend this is old school and if you aren't measuring warpage at
reflow temperatures how do you know you aren't suffering from head on
pillow?

I suggest the term head in pillow be dropped because you don't actually
put your head IN a pillow.

Bev
RIM

PS Waiting for my Dewism.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Genny Gibbard
Sent: Friday, July 10, 2009 4:20 PM
To: [log in to unmask]
Subject: Re: [TN] Maximum Allowable Bow & Twist for PWB-PCB

IPC-2221 5.2.4 
...
Unless otherwise specified by master drawing, max bow and twist shall be
0.75% for boards that use surface mount components and 1.5% for all
other board technologies.
...
Values are measured as per IPC-TM-650 Method 2.4.22 


Genny Gibbard, P. Eng 
Manufacturing Engineer, Vecima
PTS dept. x.4229 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Victor Hernandez
Sent: July 10, 2009 2:15 PM
To: [log in to unmask]
Subject: [TN] Maximum Allowable Bow & Twist for PWB-PCB

What is the maximum allowable Bow & Twist value?   Not able to locate in
IPC-2221, 2222, & 6012A

Victor,

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