Hi Bill,
Whether or not anyone admits it, EVERYONE who uses PBGA's has this problem.
It is due to the fact that the plastic encapsulate has a different CTE
(coefficient of thermal expansion) than the BT laminate. If you look at the
CTE ratings for both materials, you will notice a range for the CTE. This
is due to many factors such as filler to resin loading or resin to glass
loading and will vary from lot to lot and even within the same lot. The
bottom line is, PBGA's can have a plastic encapsulate that is either poorly
or closely matched to the BT laminate with respect to CTE.
I'm sure you are aware what happens when CTE's are not matched. For the
benefit of those who haven't had their morning coffee yet... Mismatching
CTE's is how old thermostats work. Two metal strips with different CTE's
are bound together. As the temperature increases, the metal with the higher
CTE will lengthen more than the other metal. Since the two metals are
bound, the longer metal must make up the extra length by taking a longer
path, or in other words, the longer metal will bend the strip.
If your PBGA is consistently lifting up in the corners, the PBGA manufacture
can modify the warp by changing the size of the plastic encapsulant. Keep
in mind, however, that the CTE's will still vary from lot to lot, so you
will still get the warp, but sometimes it will now warp down in the corners
instead of up.
To make the problem even more complex, both the plastic encaplsulant and the
BT laminate have a Tg that is lower than reflow temperatures. When either
material reaches its Tg temperature, the CTE changes radically. If you
watch your PBGA (or any PBGA) in your rework machine, you will see it
"wiggle" as the corners move from curving up to curving down to curving up
etc. This is due to the CTE mismatches before reaching Tg temperatures
warping the BGA in one direction. Once either the plastic or laminate
reaches its Tg, it will either exacerbate the problem or make the BGA warp
the other direction. And finally, once the both the plastic and laminate
reach their Tg the BGA will warp again in the direction of whichever CTE
dominates.
CTE is temperature dependent and not temperature changing rate dependent, so
you are pretty much hosed by the physics. However, I do like trying like
hell to get the top and bottom of the BGA the same temperature before
entering reflow. Moving into reflow slowly so the paste has time to become
liquid before the BGA begins to dance. This means a long soak before
reflow. Although I will admit this is probably wishful thinking since the
physics of CTE's doesn't care about the rate of temperature change.
Good luck and BTW, shorts and warp are NEVER caused by moisture. (Not to be
confused by popcorning due to moisture) But that's a different discussion.
Kind Regards
Ryan Grant
Advanced Technology Engineer
MCMS
(208) 898-1145
[log in to unmask]
> -----Original Message-----
> From: William Raymond [SMTP:[log in to unmask]]
> Sent: Wednesday, September 05, 2001 10:08 AM
> To: [log in to unmask]
> Subject: [TN] BGA Warp
>
> Hi all...
>
> We are placing a number of PBGA's onto 8 and 10 layer PCB's for a number
> of
> years. We have one paticular brand of PBGA, a 492 ball device that the
> corners of the chip are warping upwards causing unsoldered connections
> near
> the corners of the chips.
> The parts are in proper dry pack from the manufacturer and we have even
> tried baking these parts at 125 deg C for 24 hours just to make sure we
> are
> not having a moisture problem with these devices.
> We place other 492 ball devices on the same PCB with no problems and no
> measurable warp.
> We are not having much luck with the manufacturer of these parts, lastest
> word from them is "they allow up to 8 mil warp" ... does not help me, I
> only print solder paste with a 6 mil thick stencil.
> This problem "comes and goes"... some reels of parts are fine, some are
> "so-
> so", and some are terrible.
>
> My question is, has anyone had a similar experience? I'm reluctant to
> mention the manufacturer of this part in the "open", however they are a
> well known PC supplier.
>
> Thanks in advance, Bill...
>
> --------------------------------------------------------------------------
> -------
> Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
> 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 delivery of Technet send the following message: SET
> Technet NOMAIL
> Search previous postings at: www.ipc.org > On-Line Resources & Databases >
> E-mail Archives
> Please visit IPC web site (http://www.ipc.org/html/forum.htm) for
> additional
> information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700
> ext.5315
> --------------------------------------------------------------------------
> -------
---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
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 delivery of Technet send the following message: SET Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------
|
