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After this issue with the Dark Pro 2 was put to rest, some questions about
our testing platform began nagging me:
- Is the degree of concavity in our i7-965E normal?
- Was it there before we started using it as a heatsink test plaform? Did
it become more concave as a result of the pressure it was subject to with
umpteen heatsinks that have slightly convex bases and high pressure mounting
- Do other CPUs exhibit a similar degree of concavity?
Checking the flatness of the test CPU was relatively easy. We lined up its
surface with several aluminum or steel straight edges in turn, and compared
visually. As suspected, the i7-965 Extreme exhibited a small degree of concavity.
We have no instruments capable of measuring such minute gaps, but suffice it
to say there was a visible gap. Without a straight edge to compare closely,
however, this concavity is hard to see with the naked eye. It's also difficult
to get a photo of this gap or concavity without a really good macro lens, although
we did managed to get photos
of various heatsink base flatness in a review of a couple of Scythe heatsinks
back in August 2010. Our point with those photos was that the best performing
heatsinks have bases that are flat or slightly convex, never concave.
Now, we've known that most CPUs equipped with an IHS all desktop models
for a decade are either flat or have slight concavity. We've discussed
this in the forums and in many of our heatsink reviews (like the one mentioned
Question #1 isn't easy to answer; we'd need a dozen or so unused and
used 1366 processors to compare. This is out of the question, as we have just
two 1366 models in total, and it is impossible for us to accumulate the large
number of new samples needed. Question #2 is also impossible to answer, as we
did not record the degree of concavity (or flatness) on our CPU before it was
put into service. Question #3 we could tackle somewhat, at least with the stable
of CPUs on hand in the SPCR lab. So... after examining some 30 CPUs, most of
them used with a heatsink for at least a few hours and a few that have never
been used, we can say that a small degree of concavity seems fairly normal in
most CPUs, but then again, a few of the never used CPUs seem to be just about
CONVEX BASES & DEFORMING PRESSURE
I mentioned these issues to a contact for another heatsink brand. His response
"Thermalrights use of convex bases is well documented. Our
product managers have seen coolers tested following a Thermalright
that lost up to 2~3 degrees Celsius of performance. [sic] Buying a
brand new CPU to redo the test will reveal the same pattern: A flat base CPU
cooler tested before and after a Thermalright cooler will show slight loss
of performance. As a result of this, we always make sure to test Thermalright
coolers and others that are also convex last.
"However, there are exceptions such as CPUs with smaller die/heat spreader,
which arent as easily deformed due to smaller surface area. If you look
around the web, reviewers with older platforms that have bigger CPUs such
as the LGA1366 you guys used that have gone through a lot of tests would likely
all have concave CPUs. Consequently Thermalright coolers (and those that are
convex) tend to dominate in these roundups."
Thermalright is named explicitly by this contact, but many other heatsinks
have slightly convex bases as well. They include models from Prolimatech, Phanteks,
Scythe and a few from Noctua. Some of these heatsinks feature mounting mechanisms
capable of very high pressure. All, not surprisingly, have had models that performed
very well on our 1366 test platform. Our LGA1366 CPU has taken them all on;
the implication is that it must be more concave than it was when new.
I decided to query a few reviewers and heatsink manufacturers about this issue.
My email to the select few:
"A sample supplier recently claimed that heatsinks that have a convex
base will reshape the CPU surface slightly after mounting them. The implication
is that it makes the IHS concave (or more concave) & makes cooling worse
with flat base heatsinks. Have you ever experienced or heard of this? Have
you considered it? What are your thoughts? I trust you use a stock non-lapped
CPU for testing? If the claim is true, then it could be argued that all HS
tested after tight mounting of a convex base heatsink must be biased from
that point onward for convex bases... and HS with non-convex bases will perform
worse than they would with a non-reshaped IHS CPU. It gives me a headache
to think about how to resolve this if the claim is true."
Some failed to reply, and not all the answers were useful. Matt, a fellow hardware
reviewer who's been at it for a long time, replied generously:
"Interesting topic. Ive always enjoyed SPCR cooling articles,
by the way.
"My first impression is that it seems like a pedestrian claim. Like
someone just heard they are convex, and is making a derogatory claim off the
cuff without any quantitative measurement or further supporting information.
Ive heard the convex claims before in the forum, though.
"So yes, some heatsinks have a slightly convex base but I havent
seen it actually measured, likely because it would be a hard thing to accurately
quantify - were talking millimeters and less here, and who knows what
their manufacturing tolerances are. It wasnt that long ago when we were
looking at heatsink bases to feel how bad ridges were with our thumbnail,
but at least smoothness seems to have improved over the years. Im not
sure Id have much confidence in how convex the bases are from one sample
to the next Im sure there would be some kind of range. Again
though, Ive never seen a reliable measurement on how convex a base is
though. JoeC did some pressure testing, but not specifically looking at what
were talking about here, and not in a controlled environment to really
analyze this: http://www.overclockers.com/heatsink-mounting-pressure-vs-performance/
"Taking this one step further, does a convex HS possess the ability to
warp the IHS? Maybe. Doubtful. And if were honest, Id dump this
into the same range as other things that are difficult to control or quantify
Like tinting. Every time you remount a cooler on the IHS, you
are reapplying thermal paste sure we clean it in-between, and it may
look visually as good as new, but on the microscopic level there is leftover
paste/particulate. It could be argued that over time, the IHS interface improves
due to better tinting of the IHS as the result of multiple paste applications
over time. Or it could be argued that it gets worse, from impurities working
their way into the interface and leftover cleaning material. Then theres
also the way the paste spreads Im not sure if you saw the article
Joe C. did on this, but a while back he did some evaluations with multiple
mountings and theres a lot of variance. All the factors like this, if
I get into them, will make my brain hurt on the amount of variance there could
be from poorly observed and difficult to quantify factors involved in the
testing we do on heatsinks
Yet our testing is fairly reliably reproducible.
I think there are a lot of factors that exist, but just dont matter
much for the resolution at which were measuring.
"You may already be familiar, but if you want to test it you can look
into pressure-x film from sensorprod.com. They give free samples, and if you
pick out the right stuff for sampling (I can help if needed as I figured out
which product version wed need in the past), it will probably be enough
to do a dozen or more tests without any cost. That should be the easiest way
to quantify the convex nature of a given base and how it affects mounting
pressure across multiple samples. You can analyze the results visually with
a color scale they send you, or if its worth it to you, you can send the samples
in and get some more scientific results ($$$). If you go the free route using
samples, the only cost is time and their sales rep following up with you a
dozen times or so."
Matt brought up many pertinent, relevant and informed points, but he lacked
enough experience or fine measurement tools (like us), to provide real clarity
on the particular issue of whether a convex base heatsink can deform a CPU IHS
to a more concave shape.
Another heatsink rep confirmed the claim, however:
"Sorry to give you headaches, Mike, but yes, this is a real issue
and we have reproduced it with heatsinks from Thermalright and other brands
that use convex bases.
"It's true that most Thermalright heatsinks have convex bases, but
this is extremely difficult to control in production, so they vary from almost
flat to extremely convex. For obvious reasons, the issue is more likely to
occur the more convex the base is and the higher the mounting pressure gets.
Some coolers (from TR and other vendors) feature mounting mechanisms that
apply pressure well beyond sane levels (or at least allow to do so). This
way, you're also more likely to run into said issue.
"By our testing, the implication is exactly as you've said, namely that
flat coolers will no longer produce good results on the CPU in question. Once
it is deformed, a flat base heatsink will struggle to keep up with a convex
base one even if it is thermally more efficient."
Both of the heatsink reps cited above are individuals I've known, worked with
and respected for many years, for their technical knowhow as well as their personal
integrity. I doubt greatly that they would feed me bad or misleading information
Sifting through their words and considering my own experience with heatsink
testing over the past dozen years, I came to accept that some level of IHS deformation
probably occurred during our extensive testing with convex base heatsinks, especially
the ones that provide a hefty screw over the center of the base to increase
contact pressure. How much deformation is hard to assess, but knowing that the
difference between a top performer and a middling performer is just a few degrees,
my educated guess was that the drop in performance for a flat base heatsink
(as a result of the extra IHS concavity) would be no more than about 2°C.
(The second rep's comment, "...no longer produce good results...,"
seems like an exaggeration, especially against the comment of the first rep,
"slight loss of performance.")
Now the issues were:
- Our CPU is probably more concave now than the typical CPU used by our audiences.
Most people don't subject their CPUs to multiple torture tests with 30+ heatsinks
over a 3+ year period. So if it is atypically concave, it might now be providing
worse results for all heatsinks that don't have convex bases.
- How much worse, for a flat base heatsink?
- The question of how much worse the concave IHS CPU is making concave or
wavy base heatsink perform doesn't matter; they're never going to perform
well without at least a flat base.
Could question #2 be answered accurately? Yes, there was one way that it could:
By testing some flat base heatsinks with our CPU as it was... and then again
after lapping our CPU to a higher level of flatness, or at least
so that it is no longer concave.
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