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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