Gentlemen: As I said you need to develop the correct time, temperature
profile in conjunction with the flux activation system for this to work
properly. If it is set up properly it will not burn off the flux. Also
the theology has nothing to do with it. It's the thermal activation and
decomposition temperature ranges of the flux system that must be
considered.
You do not need to run a full blown leadfree profile. There is a middle
ground between a standard Sn/Pb and a Pb free profile. The key is in
determining what that is for the particular board design you are
processing
Yes, components on the board are going to see 230-240C. As I stated,
the problem is getting parts that are compatible with the higher temps.
If you can't you may have to redesign with lower temp parts on the
opposite side of the board as the BGA or if BGA's are on both sides do a
lower temp post reflow. Also you will start seeing more and more parts
with higher temperature ratings from the suppliers. Another thing is
don't take the component data sheet temperature ratings as gospel. Due
to the short duration of time at the higher temps many components do not
have a problem.
Reballing is certainly an option but if you don't want to screw around
with logistics, added cost, inventory, added supply chain complexity and
reballing quality issues then this is another route to take. These
issues become more of a player when running high volume lean production.
I never said it was easy and if you don't have the technical expertise,
material science and resources then choose an alternate path.
Regards
Michael Barmuta
Staff Engineer
Fluke Corp.
Everett WA
425-446-6076
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Friday, July 28, 2006 8:15 AM
To: [log in to unmask]
Subject: Re: [TN] BGAs; SnPb vice SAC Balls
Danielle,
Yes, excellent point. I do not think anyone makes a 63/37 solder paste
whose flux rheology is intended for those types of reflow profiles.
Thank you.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Danielle Casha
Sent: Friday, July 28, 2006 10:04 AM
To: [log in to unmask]
Subject: Re: [TN] BGAs; SnPb vice SAC Balls
Well put Richard.
I have one additional comment: I would caution you to watch the ramp
temps with PbSn paste and a Pb Free profile. You may burn off the flux
too quickly.
Danielle L. Casha
-----Original Message-----
From: TechNet [SMTP:[log in to unmask]] On Behalf Of Stadem,
Richard D.
Sent: Friday, July 28, 2006 10:59 AM
To: [log in to unmask]
Subject: Re: [TN] BGAs; SnPb vice SAC Balls
Mike,
I believe the actual melting temperature of the SAC alloy is
more like
220 deg. C or slightly higher. This means that the other
components are
seeing temperatures in the range of 230 deg. C to 240 deg. C,
because in
order to get the temperature under a BGA to 220 C minimum with a
longer
TALT, the other portions of the pwb that are more exposed to the
heat
are (typically) reaching much higher temperatures as a result.
Since you
need to hold this TALT for a longer time in order to achieve the
more
homogenous alloy mix on the resultant SAC/63-37 mixed solder
joints for
the BGA, you will be subjecting the rest of the components on
the board
to temperatures and dwell times that they should never see.
While your
particular assembly and set of components (including the printed
wiring
board itself) appear to be holding up to this process and you
have
undoubtly qualified it properly, I doubt that most other
standard SMT
assemblies in the industry can or will. This is why, in general,
it is
more often wiser to strip off the SAC alloy and reball the BGAs
if you
are tied to a 63/37 process. There is far less risk to
cycles-to-failure rates using BGAs that are reballed properly
than there
is using mixed alloys and higher reflow temps and TALTS.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Barmuta,
Mike
Sent: Friday, July 28, 2006 9:34 AM
To: [log in to unmask]
Subject: Re: [TN] BGAs; SnPb vice SAC Balls
Bogert: We are faced with the same predicament. As a
manufacturer of
Test and Measurement equipment we are not covered under RoHS.
Hence we
are free to use 63/37 Sn/Pb solderpaste. However we are seeing
more and
more BGA's switching over to SAC only availability.
Your statement:" There have been published test studies done
that show
that if one solders BGA's having SAC balls in an assembly
soldering
process using traditional Sn63 solder, an unreliable solder
joint may
exist." Is both true and false.
If you use a traditional Sn/Pb reflow profile it is true.
However the
profile can be modified to increase temperature and duration
above the
217-219C melting of the SAC alloy. By using the right time,
temperature
and flux activation system you can create a homogenous
dispersion of the
Sn/Pb throughout the SAC solderball. Thus creating a reliable
solderjoint. This is the route we have chosen to take.
We have conducted metallurgical and reliability testing and are
continuing to run long term reliability testing of SAC BGA's
with Sn/Pb
solder. We have not seen any reliability issues.
The real problem is getting all the other components on the PCA
to
withstand the higher process temps.
Good luck on whatever direction you take, I know it's a real
dilemma.
Regards
Michael Barmuta
Staff Engineer
Fluke Corp.
Everett WA
425-446-6076
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of - Bogert
Sent: Thursday, July 27, 2006 5:16 PM
To: [log in to unmask]
Subject: [TN] BGAs; SnPb vice SAC Balls
July 27, 2006
Folks, I have a dilemma regarding the transition to Pb free.
As a military user, we have no intention of transitioning to Pb
free
assembly soldering at this time, and probably never, unless
industry can
positively provide documented evidence, based on accelerated
life
testing acceleration factors, that establish that the use of
Pb-free
solders, such as SAC, will provide a product that will meet the
same
reliability life requirements that Pb based solders have been
proven to
meet, and that this level of reliability can be easily and
consistently
achieved on the production floor without hiring a "Rocket
Scientist" or
a team of 20 people to establish the soldering process using
Pb-free.
Given this, I am frustrated by some BGA manufacturers apparent
"Don't
give a dam attitude" about the military users of BGA'S since
some folks
are eliminating Pb from solder balls in favor of alloys such as
SAC.
Appears they are more interested in their bottom line industrial
and
commercial customers than us military folks. This is
understandable
since us military folks constitute a very small % of their
overall
business.
Although some manufacturers will still provide Pb BGA balls,
some will
only provide SAC balls. Also if Pb balls are requested, there
may be
long lead times involved.
There have been published test studies done that show that if
one
solders BGA's having SAC balls in an assembly soldering process
using
traditional Sn63 solder, an unreliable solder joint may exist.
Based on the above, our current intent is to prohibit the use of
any BGA
that does not use SnPb balls.
My question is, which is the least reliable alternative. That
is,
allowing BGA'S with SAC balls soldered using Sn63 solder, or
having
someone take the BGAs and have the SAC balls replaced with SnPb
balls?
What suppliers have the capability of doing this ball
replacement?
My preference is to stick with the prohibition on non-Pb BGA
balls. The
down side of this is that by doing this, we may not be able to
take
advantage of new technology parts that may only use SAC balls.
This Pb free issue is driving up costs. Since about 50% of the
part
manufacturers are eliminating Pb from their part finishes
without
changing their part numbers, we are forced to implement XRF
testing of
parts received by our OEMs to verify they contain the 3% Pb
mandated by
most military specifications.
Just because it is a mil spec part does not mean one will not
get Pb
free part terminations. There have been several recent GIDEP
Alerts
that indicate that some mil parts contained pure tin finish, in
violation of the mil spec.
While my experience to date is that part manufacturers who have
transitioned to a Pb free finish such as pure tin have
implemented
tin-whisker mitigation methods, there is no guarantee that all
folks
have done this. Additionally, even though JP002 tin whisker
mitigation
methods can reduce the risk for growing tin whiskers, if one
uses pure
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