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From:
"Dehoyos, Ramon" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Dehoyos, Ramon
Date:
Tue, 19 Jul 2005 09:23:57 -0400
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        I totally agree with the idea that "avoiding" voids can result
in stronger joints. I do not see any advantage in having voids at all. I
understand that in reality it is something we have to live with due to
the constraints that we are faced with. Nevertheless, each void is a
weak spot. 
        P.S. Please do not shoot the guy, he might have some children.
Ha, ha, ha.
        Ramon

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard
Sent: Tuesday, July 19, 2005 8:55 AM
To: [log in to unmask]
Subject: Re: [TN] BGA voids

Voids in solder paste are normal, I do not disagree with Werner on that
point. However, what is "normal"? I see literally hundreds of X-rays of
solder joints per week, and based on experience I know that producing
void-free solder joints is entirely possible, but let me qualify that
with that now-famous line: "It depends.....".
There are a number of ways voids can form. You can have them in your BGA
solder spheres when the components come in the door, they can be created
from outgassing around the solder joint during reflow (especially in
soldermask-defined boards), they can come from chemical contaminants on
the board prior to print and reflow, they can be induced by the solder
paste, they can be created by contaminants caused by improper handling
of the board and/or components, they can be caused by improper reflow
profiles, and many other ways.
But most of them are typically caused by too much moisture in the solder
paste. If you get rid of that problem, your chances of producing solder
joints void-free, or nearly void-free, improve dramatically.
Are voids bad? IMHO they do not cause a problem if they are smaller than
the 25% limit, but again, it depends. I have worked at companies where
the voiding caused a component to break off during a gunshot. The
fracture went through the void and caused the component to lift off of
the board on one end. Now get this, every capacitor on the assembly
fractured on one end and lifted up during the gunshot, but only on one
end. This was the end where the split terminal induced a void, but the
solder wetting and IMF were otherwise good. 
There have been several published papers that state the void in a BGA
solder joint can prevent total failure if a crack is present, as the
void alleviates the pressure once the crack propogates into it.
But two of the papers ended up stating that it is less likely for a
solder joint to develop a crack if the grain structure is tightly
defined (not coarse), and no voiding is present in the first place, and
the pads are not soldermask-defined.
If voids can be avoided by the use of a solder paste that was formulated
to prevent it, why not take advantage of that?
If you don't have any voids in your solder joints, then you do not have
to depend on an operator to make judgement calls to determine if it is
over 25%. You do not need to worry about whether your X-ray system is
good enough to detect them or not.
My feeling is that if all of the solder joints on a given assembly are
extensively riddled with voids, there is something wrong somewhere in
the process.
Regarding which paste to use, I switched to a particular paste for a
previous company. It almost totally eliminated all voiding seen in both
SMT and BGA solder joints. Furthermore, it has been an
evaluation/qualification champion in three different companies that I
have worked with. There are other solder paste formulations out there
that were created to prevent voiding as well, but this one seems to be
the best overall performer in several different categories. There are
new formulations coming out from different vendors quite often. If you
are seeing a lot of voiding (or spattering, or wetting issues, or solder
strength issues, etc.) then I strongly urge you to undergo a paste
evaluation so you can go to sleep at night knowing that you have the
best solder paste on the market being used on your assemblies. A new
evaluation should be performed every so often to take advantage of the
improvements being made. I do not want to recommend any paste over the
other, as I think all solder process engineers should perform a paste
evaluation themselves to see what works best for their particular
process and product. The things you learn when you perform an extensive
and detailed paste evaluation are invaluable. It is well worth the time,
effort, and cost.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Monday, July 18, 2005 7:12 PM
To: [log in to unmask]
Subject: Re: [TN] BGA voids

Hi Richard!

I know one goal is to have zero voids. But is seeing them really a
problem?

This all came about when I've been looking at upgrading our current
x-ray system. To be honest, I haven't seen voids here using our current
equipment.

I've seen them now with some images from a sample that I've gotten with
a system that has more capability than our current system has. I asked
about the voids to the applications engineer, and was told that the
voids that he observed were normal, and not to worry about them, with
the caveat that if they should become larger, or more concentrated in
any one area. He told me that he has about 25-years experience working
with x-ray inspection...so I'm not that quick to go out and shoot him.
He didn't say not to worry about voids, but the voids that he observed
from the sample I sent him were not a concern...he said they were
normal.

It seems that Werner agrees...

From the differing opinions, it seems the jury is still out about what
is still acceptable, and what really is rejectable when it comes to
voids.

Right now, the J-STD-001 and -610 call out that more than 25% of the
area of the solder sphere is rejectable...it does not specify where they
are located.

Anymore thoughts? Anyone?

-Steve Gregory-

***************************************************************

>Hi Steve,

>Seeing voids is indeed normal.
>What you are referring to is in IPC-7095, Section 7.5.1.6 Process 
>Control Criteria for Voids in Solder Balls.
>This was a suggested process control example, and is NOT a requirement,

>nor was it meant to be one--otherwise, I would have voted negative on 
>the
document.
>Putting it like your customer did is absurd.

>Werner

Voids can be caused by a number of things, but the most common cause is
the moisture content of the solder paste, and its condition.
That being said, a certain paste that has been known to be the mainstay
for many years currently is the worst one today as far as voiding. A
different paste manufacturer made an excellent paste that was formulated
to prevent voiding.
I recently performed a paste evaluation of standard 63/37, a leadfree
water soluble, and a leadfree no clean paste. There were 27 different
tests, with 5 vendors. On the standard 63/37, the paste formulated to
prevent voiding had ZERO voids in 1330 BGA joints, multiplied by 15
samples. The previous paste vendor had voids in every single BGA joint.
All variables were strictly controlled, and I did not know which sample
was being tested (they were numbered by someone else to prevent any
bias).
While I do not want to give out vendor names on the forum, I would be
glad to share this info with anyone offline.
The application engineer who told you not to worry about voids should be
taken outside and shot. While some data exists that says voids are OK if
they are small enough, you should certainly strive for an alloy that
provides you with an excellent grain structure free of voiding, with
evidence of good IMF at the pad/ball interface. Sorry, pal, but you
cannot determine this with an X-ray machine. It can only be done with a
good microsectional analysis.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stephen Gregory
Sent: Monday, July 18, 2005 1:07 PM
To: [log in to unmask]
Subject: [TN] BGA voids

Good Day Technetters!

I have a question about BGA voids. We are bidding on some work and it
looks like they have taken statements that are in the IPC-7095, and
placed them as notes on their drawing.

One of the notes has to do with a limit on the percentage of balls on a
BGA device that are allowed to have voids. For example; "Less than 5% of
the balls can have voids, and the percentage of the void area must meet
class
3 requirements."

I can understand the void area being critical, but the percentage of the
number of balls that are allowed to exhibit them?

The reason that I'm asking, is that I was told last week by a
application engineer from a x-ray machine company (who shall remain
nameless), that seeing voids is normal. Just as long as they aren't too
big, or being concentrated at either the pad/ball interface, or the
ball/device interface.
He said you should worry if you don't see any voids on BGA's with
eutectic balls.

Just curious as to what you all think about this?

Kind regards,

-Steve Gregory-
Senior Process Engineer
LaBarge Incorporated
Tulsa, Oklahoma
(918) 459-2285
(918) 459-2350 FAX

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