We have talked about this previously, but [as I recall] the comments were
anecdotal, with no references to analysis.  This references no rigorous
analysis either, but raises questions about the raison d'etre of an entire
segment of the electronic assembly equipment business.

AN1231 - Plastic Ball Grid Array (PBGA)
 http://e-www.motorola.com/brdata/PDFDB/docs/AN1231.pdf ] has to be the one
of the most "reused" application notes in history.  Parts of it have been
published countless times in various trade publications.

Continuing with Jeff Ferry's question [on using flux only or paste when
replacing BGA], it makes two interesting points.

<snip>"Typically, the method chosen to apply flux is done to maintain
compatibility with current processes. Due to the fact that the solder ball
is comprised of eutectic or near-eutectic solder and its entire volume is
molten during reflow, it is not necessary to add solder volume to the joint
with solder paste. The 30 mil [sic thousands inch] diameter ball provides
enough volume to give a 16 to 24 mil average stand-off across the device
depending on package and device/board solder pad configuration. This is
enough collapse to ensure that no opens will occur due to device or board
warpage at elevated temperatures (more discussion on device warpage in the
Coplanarity section). Applying an amount of solder paste equal to 14% of the
ball volume (i.e., eight mil thick stencil, 25 mil diameter apertures, final
solder volume = 1/2 solder paste volume) will generally increase the
stand-off by one to two mils. In some cases additional solder volume may be
advantageous to increase stand-off and subsequent device solder joint
reliability. However, applying larger amounts of solder paste with the use
of thicker stencils and/or larger apertures has the potential to result in
joint voiding," bla bla bla

"DEVICE REBUMPING TEST AND POSSIBLE REUSE AFTER REMOVAL
Much discussion has taken place within the industry on the subject of
rebumping and reusing BGAs which have been removed from an assembly for a
variety of reasons. After a BGA is desoldered or removed from a motherboard
with heat, the resulting solder balls are typically non-uniform at best and
in some cases the pad is completely void of solder. Methods, supplies, and a
variety of equipment currently exist to successfully remove the remaining
solder residue from the pads and replace the solder with new preform spheres
to make the device suitable for reuse. Motorola does, in fact, re-bump
packages that are returned from the field to allow them to be tested.

However, Motorola does not recommend this process be used to allow
reassembly of the BGA into actual product. BGAs that are shipped by Motorola
are typically qualified to withstand from two to four reflows depending on
the intended application and the specific Motorola product group involved.
That is to say, the packaged devices are subjected to from two to four
reflows of preconditioning prior to any reliability stressing as part of
qualification testing. When a BGA is removed from a board, rebumped, and
reflowed back onto a board it has potentially seen from a minimum of four up
to six reflows and may have exceeded the number for which it was qualified.
Additionally, there is no traceability to discern a BGA that has been
rebumped versus one that has not. Any questions in this area should be
directed to the reliability and quality assurance group dealing with the
particular BGA in question."

Dave Fish
----- Original Message -----
From: "Jeff Ferry" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, January 31, 2002 11:59 AM
Subject: Re: [TN] BGA Rework Using Paste vs. No Paste


> Thanks for the replies (see below) to my original posting (see below).
>
> Seems like several of you are having good luck using flux only vs. solder
> paste during BGA rework. The thing I wonder about is the reliability of
> reworked BGA solder joints. I thought that proper BGA solder joint volume
> was based on the original BGA ball combined with the solder added during
> the solder paste screen printing process. Thus, what effect does solder
> volume and solder joint height have on the reliability of a BGA solder
> joints for plastic BGA components?
>
> Thanks again,
>
> Jeff Ferry
> CEO
> Circuit Technology Center, Inc.
>
>
> Some Replies to prior posting included:
>
> ---------------------------
>
> My "rule of thumb" is plastic BGA's with eutectic balls - I use stick flux
> only.  For ceramic BGA's with non-eutectic or "hard" balls, I must use a
> microscreen and apply solder paste.
> Mark
>
> ---------------------------
>
> For our eutectic PBGA's, we only flux (no clean).  Have only done about 50
> or so over the past year with very good (97% yield) results.
> Have not seen any studies or papers.  Just worked closely with our CM to
> create the profile for each type of BGA.  We use Air-Vac.
> Kerry
>
> ---------------------------
>
> It's the only way to go. I've been designing, placing, reflowing and
> reworking BGA's for nearly 8 years now. It took me five minutes to make
the
> discovery and, as I've said far too often, I'd use flux only on initial
> production if feasible.
> Regards and respect,
> Earl Moon
>
>
> ---------------------------
>
> We have been working and reworking BGA and Micro BGA for quite sometime.
> This is a requirement in a prorotye environment. We tried using micro
> stencil when we first attempt to replace the BGAs and found out that using
> micro stencil was not a good method. We then tried using no-clean tacky
> past fux and applied to both the BGA and the PWB substrates. This process
> has been adopted in our process and has been working well for us. Here are
> some great benefit of using flux paste only.
>      - No micro stencil cost.
>      - No cleaning is needed (No-clean flux).
>      - No aligning equipment is needed, since we can place the BGA into
its
> footprints without worry about     smearing the solder paste, and if the
> hand alignment if off ( no more than 50%), the liquid tension of the
> reflowed solder balls will pull it back.
>      - Save time and money. ( stencil cost, cleaning, and aligning).
>
> If the solder balls on the BGA are made of non-eutectic, solder paste is
> required.
>
> Tuan Bui
> Conexant Systems Inc.
> Prototype Process Dev. Eng.
>
> ---------------------------
>
> It is my opinion that pasting the board enhances wetting and a superior
> joint.
> Jason Gregory
>
> ---------------------------
>
> On Thu, 31 Jan 2002 10:54:24 -0600, Jeff Ferry <[log in to unmask]>
> wrote:
>
> >Fellow TechNetters,
> >
> >We nearly always apply solder paste to the circuit board when we replace
> >BGA components during rework. Yet, after speaking with a handfull of BGA
> >equipment companies at APEX, seems like they all recommend using flux
only
> >vs. paste, unless the circuit board is used in a high rel application.
> >
> >What do ya'all do/recommend for BAG rework? Can you point me to any
> >studies/reports on the subject?
> >
> >Thanks,
> >
> >Jeff Ferry
> >CEO
> >Circuit Technology Center, Inc.
> >
>
>---------------------------------------------------------------------------
> ------
> >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 e-mail to [log in to unmask]:
> SET Technet NOMAIL
> >To receive ONE mailing per day of all the posts: send e-mail to
> [log in to unmask]: SET Technet Digest
> >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 e-mail to [log in to unmask]:
SET Technet NOMAIL
> To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
> 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 e-mail to [log in to unmask]: SET Technet NOMAIL
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
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
---------------------------------------------------------------------------------