There are two types of Pb-containing ball grid array components
(typically). They are standard BGAs with Sn63Pb37 solder and there are
C(eramic)BGAs that usually contain a Sn10Pb90 solder, which are not
intended to go into liquidus. I'm going to assume you are talking about
standard Sn63Pb37 BGA components that you wish to use on a Pb-free
assembly with a Pb-free profile and Pb-free solder paste and you wish it
to be RoHS-compliant.
You can reflow the 63/37 BGA with the hotter profile used to solder the
rest of the lead-free parts on the board, but you need to know the
impact of doing so.
First consideration:If you choose to do this, you need to make sure the
component itself can withstand the higher Pb-free processing
temperatures. The component manufacturer may not warrant that the parts
will work if subjected to the higher temperatures, but usually the
components themselves are the same, only the ball alloy has changed.
That is only if the parts are of recent manufacture. If the parts are an
older date code, they may not be able to withstand the hotter
temperatures. So the voiding of the warranty is one concern to think
about.
The second consideration is that if you do this, the resultant assembly
will NOT be RoHS-compliant, even if you use Pb-free solder paste.
3rd consideration:If you do not care, or you get a deviation from your
customer allowing you to ship non-RoHS-compliant product, then you
should block off the apertures on the stencil for the Pb-containing BGA,
and dip the component into liquidized tacky flux and place it directly
on the pads, with no solder paste. Use only the 63/37 solder that comes
with the component to make the solder attachment. Or block off the
stencil to prevent printing Pb-free paste and attach the components
later with the SRT utilizing only the solder balls that come with the
part. Mixing the leaded solder ball alloys and the unleaded solder paste
alloys will result in a non-homogenous solder joint with potentially
lead-rich regions. This type of non-homogenous solder joint is known to
have reliability issues. If your solder profile is of the proper length
and temperature you may be able to achieve fairly homogenous solder
joints with good reliability. The only way to determine if your process
will yield this is to qualify the process by performing microsectioning
and metallurgical analysis (SEM or Auger analysis). However, understand
that the result will STILL not be RoHS-compliant.
4th consideration. Please understand that if you re-ball the parts with
Pb-free solder and solder them along with the rest of the lead-free
parts YOU COULD STILL END UP BEING NON-ROHS-COMPLIANT. This is because
the IMF has already been previously formed with the leaded solder on the
BGA component pads; you may still end up with a part that is NOT
RoHS-compliant after reballing as there could be enough residual lead to
disqualify you. You cannot get rid of all of the lead by re-balling.
Also, consider that the component could very well have leaded
connections internally as well, and as such would still not be
RoHS-compliant from a lead-in-solder perspective. The component
substrate may have been manufactured with materials that are not RoHS
compliant. Unless you purchase a BGA that the vendor is certifying as
RoHS-compliant, you cannot assume that just because you had it re-balled
it is fully compliant.
If it was me, I would not try to use them in a Pb-free/RoHS-intended
process.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Darrel Therriault
Sent: Tuesday, January 30, 2007 12:17 PM
To: [log in to unmask]
Subject: Re: [TN] Leaded and unleaded BGAs
Richard/Technet,
Sorry for the confusion and my improper use of terms.
What I intended to say was we have BGAs with Pb balls. I was wondering
if there was an accepted process to attach those on a board that has
non-Pb components by either reballing them with nonPb balls, using SRT
rework or some profile that might allow both Pb and nonPb components to
be run at the same time.
If there is no acceptable method, I either need to consume them in Pb
assemblies (non-RoHS) or take an E&O hit on the Pb components.
DJT
"Stadem, Richard D." <[log in to unmask]> Sent by: TechNet
<[log in to unmask]>
01/30/2007 09:48 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to "Stadem,
Richard D." <[log in to unmask]>
To
[log in to unmask]
cc
Subject
Re: [TN] Leaded and unleaded BGAs
By leaded/non-leaded are you speaking of LGA arrays (leadless gate
arrays) or of BGAs with SAC alloy?
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Darrel Therriault
Sent: Tuesday, January 30, 2007 10:20 AM
To: [log in to unmask]
Subject: [TN] Leaded and unleaded BGAs
Technetters,
Question for the process folks.............
We have leaded BGAs that we could use and still maintain 5/6 RoHS
compliance.
Does anyone have a process for attaching leaded and nonleaded
components?
Using SRT for the leaded parts or even reballing them with nonlead?
Is there an acceptable method for doing this or are they E & O
candidates?
Any info appreciated................DJT
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