Lum Wee Mei,
In addition to what Ramon answered, the amount of warpage is also
determined by the weave direction and lamination methods of the B-stage
prepreg.
As to the amount of warpage, the amount must be taken over a portion of
the laminate. For example, the overall warpage could be less than .5%,
but if that amount were concentrated in a corner where the BGA resides,
as an example, you may have problems.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dehoyos, Ramon
Sent: Thursday, April 27, 2006 1:14 PM
To: [log in to unmask]
Subject: Re: [TN] Bow & Twist
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Lum Wee Mei
Sent: Thursday, April 27, 2006 8:56 AM
To: [log in to unmask]
Subject: [TN] Bow & Twist
Hello,
I have several questions to ask regarding bow and twist. They are :
1. How were the value of 0.75% or 1.5% determined for Bow and Twist? If
my boards consist of SMDs, must 0.75% apply for both bow and twist or we
can have one value for bow and another value for twist.
<<<<<< I do not know how they were exactly obtained, but I would do a
series of experimentation and find out what would be a safe threshold
for most boards >>>>>>>>>>>>>
2. John Lau did share with me that 0.5% should be used if the printed
board have BGA. However, to date, this value has yet been officially
adopted in the IPC, why?.
<<<<<<<< IPC moves slower than a Snail >>>>>>> Pardon me snails,
but you do move slow
3. What is the rationale to measure bow on the short side if bow / warp
has the tendency to occur on the longer side?
*****Bow can be in both directions, long or short dimension***
4. Very often, people specify "bow & twist shall not exceed 0.75%". In
general, am I correct to say, the statement is interpreted as 0.75% on
the side that experience bow or twist. If I specify my statement as "
bow and twist shall not exceed 0.75% on the short side", how would the
general public interpret this statement? Is this statement practical?
<<<<<< Yes, which is the longest dimension for the most part.
No, I would not specify short or long side, it can be in either
direction >>>>>>>>>>
5. IPC Test Method :
For bow & twist, why IPC states that we need press the 4 corners with
the convex facing upward for bow and 3 corners down for twist? Why can't
we let the sample rest on the reference plate without constraint?
What is the reason behind such procedure?
<<<<<<<<<<"Convex up as shown in fig. 4 allows the four corners
to rest on the plate. The Go No-go gauge will be pushed in the middle of
the dimension to see if it slides under the bow. If it does, it fails
>>>>>
6. Should not board thickness be included in the calculation for bow and
twist?
I have not see any of this parameter being mentioned. I have notice that
for the same board size with 0.4mm and 1.6mm board thickness, the former
will be more prone to twist. If I apply the same twist formula to both,
the thinner board will definitely fail. As it is natural, I cannot
reject it. Does it not sound contradicting.
<<<<<<<<<<<< The thinner the board the more chances of bowing, if it
is not balanced and the same amount of copper and same pattern are in
both of them. Of course you can have a perfect thin board and a warped
thick board, depends on the design >>>>>>>>>>>>
7. I have a board with width = 100mm and length = 300mm. Board thickness
= 1.6mm. Applying the IPC twist formula, the pin gauge diameter =
4.74mm.
Assuming the maximum twist is found to be 4mm. This mean the board pass
the twist requirement. If it is mounted with components and later
mounted down flat with mounting holes along its length including the 4
corners, will the pressure pose solder joint reliability? If yes, which
area or part of the board will be stress out?
<<<<<<<<< IPC gives us safe guidelines based on manufacture
experiences. If you are within those guidelines, the board should be
fine. But everything is possible, specially with this new SAC solder.
160 mils twist is not too bad at all, sleep well. >>>>>>>>>
I am very sorry that this message is pretty long but I really need the
information to discuss with my manger.
A million thanks in advance to all who answer
Regards,
Wee Mei
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e To
unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or
(re-start) delivery of Technet send e-mail to [log in to unmask]: SET
Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the
posts: send e-mail to [log in to unmask]: SET Technet Digest Search the
archives of previous posts at: http://listserv.ipc.org/archives Please
visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for
additional information, or contact Keach Sasamori at [log in to unmask] or
847-615-7100 ext.2815
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e To
unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or
(re-start) delivery of Technet send e-mail to [log in to unmask]: SET
Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the
posts: send e-mail to [log in to unmask]: SET Technet Digest Search the
archives of previous posts at: http://listserv.ipc.org/archives Please
visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for
additional information, or contact Keach Sasamori at [log in to unmask] or
847-615-7100 ext.2815
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
|
