Board Talk is presented by ITM Consulting
Phil Zarrow, ITM Consulting
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall, ITM Consulting
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.
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PhilAnd welcome to Board Talk
with Jim Hall and Phil Zarrow, the Assembly Brothers Pick and Place who by day
go as ITM Consulting.
Jim, today's subject
is graping. I didn't say griping, I said graping. We have two questions. The
first one is from AM, and no the second one isn't from FM. He says, we are
processing lead-free boards with large components that require high heat
temperatures and slow conveyor speeds in a reflow oven that does not use
nitrogen. Our smaller parts are showing the grape effect. What is the best
solution for this since we cannot increase the conveyor speed or lower
We have a second
question from RS, who says we have some PCB assemblies that show incomplete solder
flow due to signs of flux exhaustion. Is incomplete reflow of solder paste as
per 5.2.3 of IPC A610E a defect and would require rework, even If the fillet
appears to meet IPC standards?
The last question is
pertinent. The idea is how is this covered, in this case the graping effect on
the surfaces of solder joints. How is this reflected in IPC 610 standards?
The issue here is
these standards are released at a specific date. Although the IPC works really hard to keep
them up to date. They simply can't keep them covering all of the new specific
types of defects, such as graping.
As we all know graping
is a surface phenomenon due to, as RS identifies, flux exhaustion. Small
deposits as indicated in the first question.
Small parts have a small deposit of solder on the joint. A classic one
is a ultra-small chip capacitors and resistors. The deposit of solder paste for
those components is very small.
Likewise, when you put those components on a
board and run them in a reflow oven they heat up very quickly, much faster than
your larger components, as pointed out again in the first question by AM.
So what happens? Think
about you have this little bit of solder paste on a pad on a 0201 component. You
come into a hot slow profile, as described by AM, and you heat up very quickly.
The flux gets active. It does its job. It cleans all of the oxides off of the
three surfaces, the pad on the circuit board, the terminations on the chip
component, all of the little surfaces of the solder particles within the paste.
That happens very quickly because it is a small joint. It heats up rapidly. The
flux is activated so everything is clean and ready to go.
But because this is a
long slow profile, that solder joint specific deposit may sit in this perfectly
clean state for some time, until the entire board reaches reflow temperature
and the solder actually melts and coalesces.
As is indicated here, you are in an air environment. As soon as you
clean all of those surfaces, the air in the reflow environment starts to try to
So it is the job of the flux to try to prevent that. In these
very small deposits, you don't have a very large volume of flux because the
volume of the paste deposit is so small. It is not uncommon that flux can get
exhausted and the surfaces, typically the solder particles within the paste on
the outer surface of the deposit, become re-oxidized. So now you get to reach reflow.
All the solder particles melt, but those ones on the surface which have been re-oxidized
can't flow together and coalesce to give you a uniform shiny smooth solder
You get these little bumps or graping effect on the surface of the solder
joints. That's the effect. Is it acceptable? My experience is that it is not
well covered in the IP spec and that it is a subjective decision. Most
people feel it is only a surface phenomenon. That the solder particles in the
interior of the joint which weren't re-oxidized have coalesced. You have a nice
solder mass connecting the joint. So
everything but the surface is okay good solid solder and should give you good
reliable solder joint
Other people feel no
that as long as I see that on the surface that is not acceptable. Reworking
is highly questionable, particularly if you are down 01005. Doing rework on a
01005 is really highly questionable, at best.
Well, you know any
component because you are exposing it to another thermal excursion. But as Jim
says, the question is it a symptom or is it a defect? If facilities have access
to do that to determine whether if it is a strictly cosmetic issue or not. Would be things like strength testing, shear
pull strength and possibly looking at the internal joint. As Jim is saying, if it is sufficient looking
at it with x-ray.
Phil, there are
numerous pictures of actual destructive micro sectioning of solder joints
exhibiting graping and they all show that the interior of the solder joint is
totally uniform, solid mass. There is
little concern about reduced structural integrity of the joint.
There are some things
that AM and RS and everyone else could try, or consider trying, if it is something
that is bothering you. It is rare that
you will ever hear Jim and I say this, but if you have a nitrogen capable oven
you might try reflowing in nitrogen. As Jim said, discussing the way the oxygen
mechanism and the lack of surfactants are going there. But obviously if don't
have a nitrogen equipped oven. This isn't just something you blow a lot of
nitrogen in there. The other thing is the solder paste companies are all very
competitive. They are all working on some very interesting formulations
Many of the new formulations address graping by making the
fluxes more robust, so that they don't get exhausted. That they are more robust and they hang
around longer and they prevent this re-oxidation. That is probably the best
solution. But of course substituting a
new solder paste into an operation requires qualifications that can be
problematic and time consuming. I would use this as an opportunity to
promote evaluations of new solder pastes.
Graping is just one of a laundry list
of issues. Head and pillow is another on that is impacted by paste, voiding and
so forth. The solder paste companies have been working really hard to come up
with better formulations that reduce these tendencies for these defects.
are having these defects, evaluating these pastes now and for the future when
more new things are going to come in. You are going to have to deal with these
problems, even if you are not seeing them yet. Keeping up with what is
available from the paste manufacturers and having a system to evaluate and
qualify a new improved solder paste are really important in today's world of
ultra-miniaturization, which is driving a lot of these new defects.
So as we preach from
our one of our favorite soap boxes as Jim is saying, when was the last time you
did a solder paste evaluation, if ever?
So this might be just
the excuse you need, along with all the other reasons Jim gave. If it has been
a while since you have evaluated solder paste, what is out there, it is high
time you did. We almost guarantee if you do a good evaluation you are going to
find something better and you just might find a formulation to solve that graping
problem. Bring that up when you request samples that you will be evaluating
paste for a graping.
One more possible suggestion
that was shown somewhat effective when the ultra-miniature chips started to
appear and that is increasing the aperture size and actually overprinting the
paste a little bit to give you a larger volume of paste. Therefore with more
flux in it to resist that flux exhaustion and the re-oxidation causing graping.
Yeah, definitely worth
You have been
listening to Board Talk with Jim Hall and Phil Zarrow. We hope you got some
good ideas and directions to move on this. When you do get your solder joints
if you whether you have graping or you are just griping about your solder
joints, whatever you do, please don't solder like my brother.
And don't solder like
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