To my knowledge, Phase 2 of the iNEMI study has not published results for
reliability or mechanical strength. The time line I saw ran through the end
of the year. In addition to issues listed below there is the fact that most
tin/lead paste formulas are not designed for a time-temperature profile
required to get good mixing. The end result can be flux failure, serious
voids and dewetting.
I think it is certainly premature to say this is a "good practice". It may
prove to be acceptable in some applications, and probably okay for Class
one, or proof of concept proto-types.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Thursday, May 17, 2007 11:50 AM
To: [log in to unmask]
Subject: Re: [TN] Changing BGA solderballs from SAC405 to SAC105
And in all 4 of these papers discussed on the forum today, it is noted that
the MRT and TALT is required to be higher and longer in order to achieve
liquidus of the Pb-free solder ball and obtain complete homogenity of the
alloys in the solder. As a minimum, the reflow temperature must be at least
224 deg. C for a TALT of at least 40 seconds to get full wetting on the
coolest of these particular mixed solder joints on a given assembly.
I have soldered mixed assemblies of this type myself, including with
63/37 paste with lead-free flip-chips. The reliability life was always
reduced at least somewhat. I certainly did not gain anything, and I doubt
very much that anyone else ever has in this situation.
What is forgotten or not addressed in all 4 papers is the simple fact that
the rest of the assembly (the PWB and the other components) may not be
compatible with the higher max reflow temperatures and longer times above
liquidus.
My own experience has shown that even when the process is fully optimized,
there is some variation in the temperatures such that you do not always
obtain full homogenity of the lead throughout the solder joints of each and
every mixed solder connection, but perhaps it can be achieved on a single
BGA or flip chip. The more BGAs, and the more locations on the board that
they are placed, the more you will not always achieve complete homogenity,
and the higher risk of lead-rich areas leading to premature failure.
I do not have data showing the effects of the long term reliability of the
rest of the components on the assembly, except that several companies have
experienced PWB delamination as a result of attempting to reflow at the
higher temperatures.
Therefore, I would encourage anyone who is going to begin using lead-free
BGAs on an assembly, at least verify the PWB can withstand the hotter temps,
and it would be better to fully qualify the process by building a small lot
of assemblies first and make sure that some other components are not going
to be harmed before you jump in and start full production.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian
Sent: Thursday, May 17, 2007 9:36 AM
To: [log in to unmask]
Subject: Re: [TN] Changing BGA solderballs from SAC405 to SAC105
Richard,
You should also read "Mixed Metallurgy: Reliability of SAC Balled Area Array
Packages Assembled Using SnPb Solder", Heather McCormick et al, Celestica,
given at SMTAI, Chicago, 2006.
And Dave Hillman has a paper that he gave at the International Lead Free
Conference in Toronto (I think last year), that talks about when the
solder/ball metals do not mix completely. Contact Dave for more details.
Bev
RIM
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Thursday, May 17, 2007 10:10 AM
To: [log in to unmask]
Subject: Re: [TN] Changing BGA solderballs from SAC405 to SAC105
Thank you, Ioan!
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Tempea, Ioan
Sent: Thursday, May 17, 2007 8:48 AM
To: [log in to unmask]
Subject: Re: [TN] Changing BGA solderballs from SAC405 to SAC105
Hi Richard,
as communicated directly to you, one of the papers was written by G.
Wenger (Mixed Alloy Solder Joint Reliability 'Sanity Check').
Unfortunately I did not save the link, but maybe he can help.
Regards,
Ioan
-----Original Message-----
From: Stadem, Richard D. [mailto:[log in to unmask]]
Sent: Thursday, May 17, 2007 9:28 AM
To: TechNet E-Mail Forum; Tempea, Ioan
Subject: RE: [TN] Changing BGA solderballs from SAC405 to SAC105
Ioan,
Where are these studies showing that soldering of LF BGAs with leaded solder
is a good practice? Can you post a link?
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Tempea, Ioan
Sent: Thursday, May 17, 2007 6:58 AM
To: [log in to unmask]
Subject: Re: [TN] Changing BGA solderballs from SAC405 to SAC105
Well Phil,
I haven't really been following this, but I am worried. We already have too
many customers asking us to solder LF BGAs with leaded solder and the deal
was to get the BGA balls to melt, at around 220C. I was happy that studies
started to surface showing this was good practice and the result was
reliable.
How will I be able to solder the SAC105 with leaded? And we already have an
inspector full time on the XRF, just to make sure Pb is not there.
Now I will have to ask them to measure the composition of the balls...
When will it end?
Just my concerns!
Ioan
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Phillip Bavaro
Sent: Wednesday, May 16, 2007 8:31 PM
To: [log in to unmask]
Subject: [TN] Changing BGA solderballs from SAC405 to SAC105
I haven't been following the transition to use SAC105 so I would appreciate
any advice those of you who have already gone down this path might have.
SAC405 and SAC305 was considered compatible with our SAC305 paste, but now
there is a component which is changing the solderballs to SAC105.
SAC105 has a higher liquidus temperature (226 C) than SAC305 (217 C) .
Is this going to impact our assembly processes?
Does it only tighten up the reflow process window?
Is this transition something that the majority of manufacturers of
components are going to undertake soon?
Thanks in advance,
Phil
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