Hello All,
1. What are typical dewarping procedures that are used? Is there a concern with OSP finished boards?
2. Do we need a tighter warpage spec than 0.75% for a board heavily populated with BGAs? What determines warpage requirements - component placement or joint reliability?
3. What kind of pad finish offers the least warpage?
Thanks.
Kamalesh
TRW Automotive
>>> Fred Paul <[log in to unmask]> 03/25/99 12:47pm >>>
Richard,
SEE DIRECT RESPONSES TO YOUR SURVEY TOWARD THE END OF THIS EMAIL.
We don't change the criteria based on smt pitch. We do a quick check of our smt MLBs
using pin guages based on the .75% criteria in IPC 6012 para. 3.4.4 in accordance with
IPC-TM-650, Method 2.4.22. Occaisionally some panels will fail this standard and require
rework. We will test them again after rework (Dewarping Procedure) and pass or fail them
depending on the results. It is rare that boards will fail after rework. We also monitor the
feedback from our customers to measure the effectiveness of our methods.
Over the years I have classified some known failures (some here, some at other companies
where I have worked, some learned of from others), as follows:
1. Older designs with "cap" constructions.
2. Designs with unbalanced planes in z-axis distribution, including multiple part panels
where some parts had planes, some not, some 4-layer, some 6-layer, all on same panel.
3. Hybrid MLBs laminating Polyimide 'caps' to FR4 innerlayers.
4. Designs with layers of differing copper weights.
5. Uneven copper distribution over surface area (planar areas on one side of panel, etc).
6. Laminate received with bow and twist from vendor.
7. Cross plied laminate from vendor.
8. Different prepregs used in the same construction having different %cure, flow, and gel
characteristics.
9. Lamination press malfunction -- causing non-uniform heat distribution and transfer
through the stack in the press, including power failures.
10. Too fast a heat rise. Not matching heat rise with cooling rate.
11. Transferring into a cold press subjecting panels to thermal shocking.
12. Curing solder mask in convection ovens leaning in slotted trays at 300 F.
13. Placing hot panels on cold surfaces.
14. Not paying attention to the effects of grain direction on the various components of a
stack-up (construction).
The following MAY have been a contributor, but wasn't verified:
MLBs that were subjected to IR Tin Lead reflow may have had bow and twist induced by
uneven heat distribution within the chambers.
I'm running out of time. I'm sure I've overlooked something, but perhaps this will help you.
-- Fred Paul
Staff Engineer, PCB Operations
FLUKE Corporation, P.O. 9090, Mail Stop 55, Everett, WA 98206
VOICE: 425 356 5734, FAX: 425 356 6070, [log in to unmask]
>>>>>>>> You wrote previously <<<<<<<<<<<
Date: Mon, 22 Mar 1999 19:55:39 -0700
From: Richard Hamilton <[log in to unmask]>
Subject: Warp and Twist
Morning TechNetters,
Since everyone was at IPC last week, I am re-submitting this question for
those of you who took a vacation on the TechNet last week. Those of you who
did read this, I apologize. I was underwhelmed at the response, so I am
desperately grasping for life here!!
I would like some feedback, either directly or through TN.
I am trying to get a feel for what people are generally using for acceptable
warp and twist in relation to the pitch that is being used. If there is
interest, I would even make the results public if there is a significant
number of responses. The thought being that the more responses, the more
valid the data.
1. What amount of warp and twist per inch do you allow at your facility?
****** no more than .75% for smt panels. (assembly equipment driven)
2. What amount of warp and twist total do you allow at your facility?
****** no more than 1.5% for non smt. (assembly equipment driven)
3. What is the finest pitch produced by your facility?
****** .01"
4. Do you have different requirements based on different characteristics?
If you do, please answer 1-3 for each characteristics.
5. Is the warp and twist requirement based on the equipment used in
production or on the technology/pitch in use?
****** Yes
Thanks in advance for the assistance.
Richard Hamilton
Clemar Mfg. / Rain Bird
[log in to unmask]
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