[tinwhiskers] Re: - RoHS & BGAs!!!

From: "Meschter, Stephan J \(US SSA\)" <stephan.j.meschter@xxxxxxxxxxxxxx>
To: <tinwhiskers@xxxxxxxxxxxxx>
Date: Mon, 23 Feb 2009 20:24:48 -0500
Alvin,
You will also want to pay attention to Re-work. The thermal profile
assoc. with hot gas rework is considerably different from SMT convection
reflow. The mixture of Pb-free and SnPb has a greater pasty range so
cooling rate and keeping track of the last region to cool are important.
I have heard that the solder process temperature window is tighter than
SnPb because you want to ensure good mixing, but not overheat the other
SnPb parts on the board. During the creation of GEIA-HB-0005-2, we spent
a lot of time on this topic. Unfortunately many of the papers had
conflicting results and there was hardly anything on vibration and shock
environments a couple of years ago.

There is a NASA DoD lead-free solder test underway that is evaluating
SAC305 and SN100C Pb-free alloys over military environments. Included in
the rework matrix are Pb-free BGAs soldered with SnPb solder paste.
http://www.teerm.nasa.gov/projects/NASA_DODLeadFreeElectronics_Proj2.htm
l 

There is also a fair amount of info on mixed solder reliability that
resides within the various consortia (CALCE Univ. of Maryland, Unovis
area array consortium, CAVE Auburn, STACK, HDPUG, iNEMI, etc.).

Good luck.
Stephan Meschter
BAE Systems - Johnson City, NY.

-----Original Message-----
From: tinwhiskers-bounce@xxxxxxxxxxxxx
[mailto:tinwhiskers-bounce@xxxxxxxxxxxxx] On Behalf Of Alvin Kevichusa
Sent: Monday, February 23, 2009 8:00 PM
To: tinwhiskers@xxxxxxxxxxxxx
Subject: [tinwhiskers] Re: - RoHS & BGAs!!! 

Hello all,
I came across this late December 2008. But the article has been around
since April 2006. It deals with the same scenario that Hani was thinking
of - SAC BGAs with Sn-Pb environment.

<<http://www.trafalgar2.com/documents/Columns-Shangguan/6.4-shangguan.pd
f>>

We're an avionics company out of Tucson, AZ and like everyone else, are
facing difficulties with getting BGAs in Sn-Pb. And since we're exempt
(for now), we're planning to go ahead and do some process qualification
for using SAC alloy BGAs in a Sn-Pb environment. 

We've invested in laying out a board with daisy chained BGA patterns and
some dummy components that are internally daisy chained. The plan is to
develop a reflow profile that stays under 235C and build some boards
using that profile with SAC alloy BGAs and eutectic Sn-Pb.

In the absence of much data out there for this combination, we're going
to do comparative analysis between this SAC and Sn-Pb combination and
the Sn-Pb and Sn-Pb combination.

Until I read through this string, the plan was only to do some thermal
cycling while monitoring the daisy chain. But now with the enrichment of
this very interesting string, we'll probably start with a 125C extended
exposure to increase intermetallics followed by a mechanical shock and
then do the monitored thermal cycling.

Will keep you all posted with what we determine from this. Any comments
and suggestions are welcome.


Alvin Kevichusa
Manufacturing Process Engineer
Universal Avionics
akevichusa@xxxxxxxx
520-434-3066



-----Original Message-----
From: tinwhiskers-bounce@xxxxxxxxxxxxx
[mailto:tinwhiskers-bounce@xxxxxxxxxxxxx] On Behalf Of Glen Herzog
Sent: Monday, February 23, 2009 3:08 PM
To: 'tinwhiskers@xxxxxxxxxxxxx'
Subject: [tinwhiskers] Re: - RoHS & BGAs!!! 

Hani,

That question should get you a mixture of responses!  

Using SnPb solder on SAC balls, known as backward compatibility, tends
to result in a product that may not be as reliable as either SnPb on
SnPb or SAC on SAC.  However, when you know the environment that the
product will see SnPb on SAC balls may yield quite satisfactory
reliability.  For example if you are in a constant temperature, no
shock, no vibration environment where a failure is not life threatening,
SnPb on SAC may be satisfactory.  That is the mix that we have been
using for several years and are satisfied.  We know that we will have to
do lead free (someday) but for the time being we can afford to let the
rest of the industry figure out which lead free it is going to be.

If you do consider SnPb on SAC I recommend a thorough literature search
and some experimentation on your part.

Another note, SAC solder in combination with SnPb melts at 207C-208C.
So, add 20C and you could solder at 225C to 230C and not stress the
components so much and not grow as thick of an intermetallic layer.  You
can get a good mix of solders at 220C and above.

Good Luck.


Glen Herzog
Quality Manager
Fairfield Industries
Ph. 281-275-7854
gherzog@xxxxxxxxxxxxx





-----Original Message-----
From: tinwhiskers-bounce@xxxxxxxxxxxxx
[mailto:tinwhiskers-bounce@xxxxxxxxxxxxx] On Behalf Of AL-Yousef, Hani E
Sent: Monday, February 23, 2009 2:20 PM
To: tinwhiskers@xxxxxxxxxxxxx
Subject: [tinwhiskers] Re: - RoHS & BGAs!!! 

Bob & Ed,

Here is wild thought, since we are [the industry I am in] exempt from
having to comply with the RoHS initiative, whose to say that I can't use
eutectic solder [Sn/Pb] to solder the SAC BGA into the board.

Of course, given that the rest of the parts on the board can withstand
240C needed to melt the SAC balls. 

So I will use Sn/Pb solder at RoHS temperatures

Any thoughts on that!    
       

Thanks,
Hani

-----Original Message-----
From: tinwhiskers-bounce@xxxxxxxxxxxxx
[mailto:tinwhiskers-bounce@xxxxxxxxxxxxx] On Behalf Of Parnagian, Edward
Sent: Monday, February 23, 2009 11:53 AM
To: tinwhiskers@xxxxxxxxxxxxx
Subject: [tinwhiskers] Re: - RoHS & BGAs!!! 

Hani,

When Bob said "middle of the board is the worst" what he meant was that
midway between supports is worst.  If you're only anchoring at the
corners of the board, then the middle of the board is worst.  If you
have a support in the middle board, then half way between the middle
support and the surrounding supports is going to have the problems.

Best regards,
Ed

-----Original Message-----
From: tinwhiskers-bounce@xxxxxxxxxxxxx
[mailto:tinwhiskers-bounce@xxxxxxxxxxxxx] On Behalf Of Bob Landman
Sent: 2009 Feb 22 4:51 PM
To: tinwhiskers@xxxxxxxxxxxxx
Subject: [tinwhiskers] Re: - RoHS & BGAs!!!

Hani,

Regarding underfill, if you do have to use SAC then underfill does seem
to help (depending on where on the board the BGA will be located (in the
middle of the board is the worst place - where mechanical stress is
greatest).  This article is a good one on the subject

http://ap.pennnet.com/Articles/Article_Display.cfm?Section=Articles&Subs
ection=Display&ARTICLE_ID=220942

Bob Landman
H&L Instruments, LLC




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[tinwhiskers] Re: - RoHS & BGAs!!!

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