Richard,
What testing regime did you use & what was the MTTF for both BGA's & non
leaded devices? Did you have any LGA's in your evaluation?
Regards,
Iain
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: 07 July 2011 15:01
To: [log in to unmask]
Subject: Re: [TN] BGA Repair Methods
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While a crack can propagate through a BGA solder joint due to delta CTEs
between the BGA substrate and the board substrate, some look at the void
as a stress relief valve where the crack stops, and thus assume that
because the remaining SJ hangs on awhile longer there is an improvement
in reliability.
However, my feeling is that if there was no voiding in the first place,
the amount of overall flexural stresses seen on the collective BGA
solder joints in the first place would be less.
I learned this when performing life-cycle testing on solder pastes. When
I soldered several BGAs to a test board using a solder paste with a
known propensity for inducing voids (and the voids were carefully
located and marked ahead of time with X-ray images), the overall time to
failure where any one of the solder joints would fail electrically was
much less than the test board where all of the BGAs were soldered with a
paste that had very little or no voids at all.
The amount of grain coarsening after 50, 100, 500, and 1000 cycles was
less on the boards with no BGA voids than was seen on the microsections
of the boards with BGA voids, even though both sets of boards went
through the same life-cycle testing, at the same time and in the same
chamber.
But the NON-BGA component microsections did not show the exact same
results. The leaded components were the same, with a small amount of
variation in the chip caps/resistors.
-----Original Message-----
From: Amol Kane [mailto:[log in to unmask]]
Sent: Thursday, July 07, 2011 7:39 AM
To: TechNet E-Mail Forum; Stadem, Richard D.
Subject: RE: [TN] BGA Repair Methods
Richard! Great e-mail compiling all the reasons. We do flux only rework
and I have a standard document I send our customers outlining the
reasons if they question the process. Some of your points are definitely
going into it! :-)
One item about voids however piqued my curiosity. You say " More voids
in the BGA solder joints. Solder paste is known to induce voiding in
BGAs. Never mind what they say about BGA solder joints with voids not
being a reliability issue; they are."
Why do you say that?.....can you share any
literature/insights/experiences that led to this opinion?
Thanks!
Amol Kane | Process Engineer
Catalyst Manufacturing Services, Inc.
941 Route 38, Owego NY 13827
Phone: (607) 687-7669 Extn 349 | Website: www.catalystems.com
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Wednesday, July 06, 2011 11:59 AM
To: [log in to unmask]
Subject: Re: [TN] BGA Repair Methods
I am sorry, Russ, but your EMS suppliers are right.
Additional volume of solder does NOT, and I repeat, does NOT increase
BGA solder joint reliability for BGAs whose balls are intended to
collapse during reflow (those with Sn63 and SAC alloy balls, but not
Pb90). A larger cheese loaf is just that, a larger cheese loaf. It does
not increase the modulus of elasticity.
The use of paste typically only increases the overall volume of solder
about 1% or 2%, anyway. There is no real reliability difference in a
solder ball that is .030" in diameter as opposed to one that is .031" in
diameter, provided both are fully wetted on the pads.
IPC 7095 states:
"7.1.1.3 Importance of Paste Volume
For plastic BGAs much of their solder volume is supplied by the solder
ball on the part itself and the paste volume is not all that critical.
For BGAs above 0.80 mm pitch, stencil thickness will be dictated by the
other component types used on the printed board assembly. Solder volume
and stencil thickness become more critical for ceramic and fine-pitch
BGA such as CSP. The solder balls used on ceramic BGAs are not eutectic
and do not collapse during the reflow process (see Figure 7-1).
Because the high lead content ball does not collapse having sufficient
solder paste is critical. The fillet between the land and ball depend
upon the solder paste volume.
Ceramic BGA requires a minimum of 0.08 cubic mm and a nominal 0.12 cubic
mm paste volume."
Therefore, a requirement that solder paste be used for standard BGA
rework is only going to lead to the following:
1. More Head-in-pillow solder joints caused by marginal reflow profiles,
where the paste and the ball do not both go into liquidus.
2. More voids in the BGA solder joints. Solder paste is known to induce
voiding in BGAs. Never mind what they say about BGA solder joints with
voids not being a reliability issue; they are.
3. More non-agglomerated (renegade) solder fines and particles. It is
difficult to manually screen paste and get it all on the pad.
4. More solder bridges caused by excessive or smeared paste prints,
leading to additional subsequent reworks.
5. More flux residue. The flux in reflowed solder paste is much harder
to clean than the tacky flux designed for rework.
6. Much more cost, from a rework standpoint. Applying a thin veneer of
tacky flux on the board pads as opposed to carefully aligning a metal
microstencil or stick-on stencil and printing paste is much less time
consuming.
Solder paste is printed prior to BGA placement only because it is a
convenient method of applying flux to the BGA pads during original SMT
assembly. The additional metal volume (from the solder paste) is
absolutely not needed.
If you are really concerned about BGA ball joint reliability, then
increase the modulus of elasticity by providing solder columns rather
than cheese loafs. This concept was explained in Werner Engelmaier's
article awhile back, which included an outline of my cornerbonding
process and my pictures of cornerbonded BGAs with solder columns rather
than cheese loaves. Once this is done, the BGA typically ran several
thousand more cycles before failure. The solder columns are not "added",
they are created using the solder balls that come with the BGA.
I have reworked hundreds of thousands of BGAs without using paste, for
many companies. None have shown any difference in reliability than BGAs
either reflowed with paste as part of the original process, or reworked
with paste. In fact, I have data that shows a reworked BGA using only
tacky flux is more reliable. Why? Because a reworked BGA will have the
benefit of pre-tinned pads with tacky flux applied, without any of
number 1-6 above.
One final note: There should be little or no no-clean tacky flux seen
after rework. A very thin "veneer" of a good tacky flux applied to the
board pads is all that is required. I prefer water soluble tacky fluxes,
and I am not afraid to state the Alpha WS 619 is an excellent
halide-free tacky flux to use, with excellent cleanability in water.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Russ
Sent: Tuesday, July 05, 2011 10:15 AM
To: [log in to unmask]
Subject: [TN] BGA Repair Methods
Hello TechNetters,
I have a question about acceptable BGA repair methods. I have always
required my EMS's to pre-apply solder paste to either the PWB or device
prior to placing and reflowing a BGA. Some of my EMS's pushed back
initially but always complied. IPC-7711/21B has two defined repair
methods which both require adding solder by either pre-applying solder
by using solder wire (section 5.7.1) or using solder paste (Section
5.7.2). No where does IPC-7711/21B state that you can get by with only
applying tacky flux. Here is where I feel there is a conflict. In
IPC-7095B Section 6.1.2 it states a couple places that you must "new
solder paste or flux". No where does it mention pre-applying wire
solder before applying the flux. IPC may want to review the wording
there.
I am not sure how you would control manually applying core (wire) solder
so I only consider pre-applying solder paste an acceptable method. I am
a firm believer that by not applying solder paste reduces the solder
volume, joint height and thereby reducing reliability of the solder
joint.
First, I would like to know if anyone knows of a published reliability
study of Flux only versus Pre-pasting a BGA repair.
Second, I would like to hear other opinions on this subject.
From a processing standpoint I understand the EMS's position that
applying tacky flux makes the BGA repair a whole lot easier and cheaper
as adding paste requires a mini-stencil and a trained operator. I am
all for easy if it does not but reliability at risk. I also have a
concern about the excessive flux that I find under a BGA that has been
attached with only tacky flux. They are no-clean fluxes but but excess
is never good or pretty. Thanks in advance.
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