I totally agree with Mr. McGee that one must be very careful and have a
fully qualified process for re-balling of BGAs. The same is true of any
rework.
I personally have reballed many (thousands) of BGAs as a part of setting
up the process and qualifying it for several companies, including parts
with SAC alloy balls intended for use in processes using standard 63/37
solder. These companies routinely perform re-balling of BGAs as a normal
rework process per IPC 7711/7721 and 7095, and have done it successfully
without any major fallout.
But they used a qualified process. A qualified process means that the
parts are baked properly and/or are stored in MSD-safe packaging if they
are MSDs per J-STD-033, and most BGAs are MSDs.
The proper ramp-up, minimum TALT, minimum peak temperature to assure
full liquidus and collapse of all balls (but no hotter), and optimum
cooldown rate are established. The optimum cooldown provides a tightly
arranged microstructure within the alloy, as seen in a good microsection
at 500X minimum magnification.
Qualification boards built with the qualified process are then subjected
to temperature cycling along with parts that were also
reflow-soldered-only (not removed and reballed or reworked) for the sake
of benchmark comparison. A sample is also inspected thoroughly with a
good endoscope and then microsectioned to inspect for good wetting
characteristics (defined as good creation of IMF).
Usually, the grain morphology changes are the same after cycling between
the reflow-only parts and the reballed/reworked parts, with a single
major difference; they often differ along the ball to BGA pad interface
and the ball to board pad interface, and this is because the reworked
pads are typically pre-soldered during the removal and site preparation
processes inherent to the re-ball or rework process. This is especially
true with ENIG finishes. Solder joints formed on the reworked pads on
both the re-balled BGA and on the pwb typically show better wetting than
on those that have been subjected only to reflow. This is because
nothing solders to solder like solder does. You are essentially
pre-tinning the pads during the site preparation (removing the old
solder) on the parts when they are re-balled. The same is true for the
board pads when a rework/reball/resolder process is done.
Within the industry, large QFPs, LGAs, TSOPs, and other packages are
reworked/re-used on commercial product on a regular basis. BGAs really
are no different. If the rework on any components is not done properly
there will be reduced reliability and premature failures.
All too many re-ball and rework processes are not fully qualified and
controlled. The process quite often is not done the same way twice. This
of course will lead to failure.
Many military providers are not allowed to re-ball BGAs because the
particular standards prohibit any re-use of reworked components.
However, in order to improve durability in certain COTS components we do
have processes where the solder balls are removed and the component is
then soldered to compliant leads. This allows us to greatly increase the
time-to-failure of the resultant subassembly by adding the compliancy
between the part package and the board to better absorb differences in
CTE mismatch. I have not seen any difference in the package failures
(non-solder-related failures) between the interposed parts and
noninterposed.
Within the commercial industry, defective components (premature
failures) caused by improper rework/reballing do not always manifest
themselves in the form of defective solder joints. They usually show up
as dead components due to improper rework profiles or methods/materials,
as shown by popcorning or delamination of the die/substrate bond inside
the package, or other issues, such as failing to prevent moisture
absorption prior to rework/reballing. Improper materials (fluxes) used
as part of the rework or reballing also cause problems. Standard liquid
benchtop fluxes are totally inadequate for these processes.
If you are going to reball, take Mike's advice and go down that road
very carefully, with a fully qualified process using the appropriate
materials, methods, trained and certified operators, and the appropriate
equipment to do the job. Else leave the job to someone who knows how to
do it properly with a certified process, but fully qualify the reballed
parts on test assemblies prior to building production hardware.
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of McGee, Mike (MN51)
Sent: Wednesday, May 24, 2006 8:24 AM
To: [log in to unmask]
Subject: Re: [LF] Reball or not to reball
We chose not to allow reballing for just that reason. Most manufactures
will not support reballed parts. If you do reball, make sure you can
test the parts before placing on the board. I know of a shop that was
having approx 25% fall-out of the part they were reballing. I didn't
work directly with them, so I don't have the details, but I heard a lot
about their pain. It was further justification to our management that we
made the right decision. I know several manufactures offer reball "kits"
but for a high-rel environment, I would weigh the risks very carefully
before going down the reball path.
-Mike McGee
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Anslow, Phillip
Sent: Wednesday, May 24, 2006 4:33 AM
To: [log in to unmask]
Subject: Re: [LF] Reball or not to reball
Loa,
My company was planning same strategy of reballing BGAs - in process of
reassessment in the light of following disclaimer from one major
supplier in following link - see page 6.
http://www.xilinx.com/bvdocs/appnotes/xapp426.pdf
Extract for convenience: "Xilinx does not recommend reballing. Xilinx
parts that are reballed will not be guaranteed by Xilinx. A maximum of
three reflow cycles are allowed."
Not sure if this is isolated or general across component suppliers.
Suggestions for high-rel welcome...
Regards,
Phil.
-----Original Message-----
From: ALLEN, LOA [mailto:[log in to unmask]]
Sent: 22 May 2006 20:57
To: [log in to unmask]
Subject: Re: [LF] Reball or not to reball
*** WARNING ***
This mail has originated outside your organization, either from an
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Keep this in mind if you answer this message.
John,
Thanks for the reply . . . you pretty much answered my question
regarding the BGAs. We plan to replate/pre-tin the leaded comps because
of tin whiskers. However, guess I was looking for some reassurance that
re-balling the BGAs was the best course of action to take & was
wondering what others are doing.
Also - thanks for being the voice of the silent majority with Pushback.
Loa
-----Original Message-----
From: John Burke [mailto:[log in to unmask]]
Sent: Monday, May 22, 2006 9:45 AM
To: '(Leadfree Electronics Assembly Forum)'; ALLEN, LOA
Subject: RE: [LF] Reball or not to reball
HI Loa,
I will let others advise on the rest but you will have to re-ball those
BGA's if you are going to solder them using leaded solder.
There is a lot of published work, if you need examples ping me off line.
John
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Loa
Sent: Monday, May 22, 2006 9:22 AM
To: [log in to unmask]
Subject: [LF] Reball or not to reball
Is there a consensus among the listserv on using Pb-free comps in a SnPb
assembly?
We are a U.S. OEM claiming exemption to the RoHS directive (design & mfr
of data storage/transfer equipment with no current EU customers) and as
predicted, the supply of SnPb terminated components are drying up with
many of the new design comps not even being offered in SnPb. Based on
the published work of industry experts and monitoring of the IPC LF &
TechNET listservs, we have decided to "hot solder dip" Pb-free SMT comps
with legs and re-ball Pb-free BGAs (both with Sn~60/Pb~40); vastly
increasing the cost. Our assemblies are very low volume (5 to 15pcs
typ) with a large mix of complexity and assembly part numbers.
I have queried local contacts in similar industry/situations and
received the deer-in-the-headlight stare. I have sat through workshops
explaining how uncertain the future is regarding varying interpretations
of reliability - I just thought I was confused before.
My Other Question: What is the opinion of the masses - are we doing our
product unjust harm in an attempt to maintain reliability? We are
willing to deal with the cost of re-plate/re-ball as long as performance
and reliability are unchanged.
TIA (not the dog)
Loa
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