Revisiting STARS-922 and Other Stuff

There seems to be some skepticism and curiosity that surround STARS-922 thermal compound (aka "heat sink plaster") and some similar products.  I know I've wondered for a while myself and I've run across a handful of forum posts that question its purpose or properties.  Places like DealExtreme and Ebay sell lots of this product along side other more common zinc-oxide thermal pastes with limited product differentiation.  The poorly translated and dubious specifications and naming don't help to instill confidence that it is trustworthy of any particular application. On some occasions, customer reviews boast "strong" or "excellent conductivity" although i doubt either metric had been actually evaluated beyond the emotional satisfaction associated with the completion or failure of the parent project as determined by myriad other factors. I've found few satisfactorily detailed reviews, so I figured I'd add this bit of info to augment my halfass thermal testing from the prior post.

What the hell do you mean "plaster"?

Some of the questionable property specifications are:

• Thermal conductivity: > 1.2W/m-K
• Thermal Impedance: < 0.06 (what units?) 
• Clotting time: 3min (25 degree celsius) (all of it or just the exposed stuff?)
• Strength of connected buildings: 25Kg  (what?) 
• Temperature resistance: 200 degree celsius

Following some of my own experience and a few comments noting that it had little adhesive capability, I figured I would try to get a better feel for the mechanical properties of the product in a way that should at least lend intuition a better handle on what's appropriate to expect.

First off, this product is not a thermal grease.  It is much more viscous and does indeed congeal on exposure to air. The cured product is slightly rubbery, but not as elastic as one might expect from a conventional RTV silicone caulking.  The most common place I have seen this is in the bonding of the MCPCB to the radiator body of cheap LED retrofit lamps.  In such an application, temperatures are low and mechanical stress is small.  The MCPCB has only a small mass, and the aspect ratio ensures that it is difficult to produce a tension stress at the interface.  This is all overshadowed by the advantage of being enclosed.  Nothing can stress the bonded part except its own inertia.  This seems to be a rather forgiving application, mechanically speaking.  Will it work for larger SMT heat sinks or other things?  We need to have a bit more understanding about its actual strength in shear and tension.

In order to test shear and tension strength, I put together a setup in the lathe consisting of various repurposed fixturing and a 1000# load cell with a selectable-gain instrumentation amplifier.  Since I never finished the microcontroller DRO that was supposed to go with the amplifier I made for this load cell, I have to use the multimeter to take direct readings.  Despite the extra clumsiness of operations, the results would be the same.  To get the approximate peak value at break, I simply used the phone to record operations.  Accepting that the periodic multimeter update is the result of an averaging operation and may lag the actual value, and forgiving my inability to produce a smooth and monotonically increasing tension by handscrew, the captured values should be acceptable for the intended purpose if averaged across a few samples.

some assembled samples and spring clamp for alignment

The shear samples consist of aluminum plates cut so that they produce a lap joint with approximately one square inch of area.  The tension samples are old elevator bucket bolts that have had their head faces ground and scoured.  All samples are ground and scoured in degreaser as needed to prepare a fresh surface each time.  No particular assembly fixturing was used; no clamping was performed except to gently maintain alignment with adhesives that were flowable (e.g. molten hot glue).  Samples were tested with heat sink plaster, some Permatex RTV silicone products, as well as generic superglue and two unknown hot glue products.  One was a soft and flexible hobby-marketed product, and the other was a harder Arrow brand carpentry-marketed product, if I recall correctly.

In approximate terms, the heat sink plaster (HSP) is about on par with a low-strength RTV silicone.  As with the prior thermal resistance testing, I ran a few sample groups of Permatex Ultra Grey RTV and High Temp Red RTV gasketing products.  These represent the extremes of the product spectrum in terms of specified tensile strength and filler density.  It's a fair bet to assume that if you'd be comfortable with the strength of your favorite RTV silicone, the strength of HSP will be sufficient for your task.  There are a few things to consider, though.

One of the primary obstacles I ran into with this setup is that I'm testing air-cured materials in thin sections.  With nonporous sample substrate, the geometry of the test patch does end up becoming important.  Especially with attempts to cure products at room temperature, it was not uncommon to find samples that were only cured on an annulus around the perimeter of the bond patch.  Most of these partially-cured sample groups were discarded for various reasons, typically nonuniformity or large variance within a group.  In the case of some samples with uniform partial curing, I made my best attempt to measure the effective cured area using image processing after the break test.  While I note that my goal here is to measure strength of the cured adhesive fraction, I did not make any differentiation in the thermal testing process.  It may be worth knowing if the thermal resistance is much higher when the center of the sample patch has cured completely, but I can't exactly put the thermal test fixture in the oven with all its plastic bits.  I'm also far too lazy to do larger test groups over a longer natural curing interval of say a month. 

For what it's worth, the image processing used to calculate partial bond area is rather simple and goes to show that GIMP or Photoshop are useful for more than just meandering artfaggotry drawing and editing photos.  It goes like this:

• Remove uncured adhesive with solvent
• Take photos of both halves
• Transform image areas to geometric correspondence
• Perform threshold operation to isolate cured material
• Multiply images to get combined coverage
• Find average pixel value

The example shown is not a particularly well-cured sample (42% cured), but it was from a group that were uniform.  

In my attempts to expedite thorough curing of the samples, I tried several runs at elevated temperatures in the oven.  Some of these resulted in reduced and typically inconsistent strength.  In the case of the RTV, some of the extended cure times (96 hours at 70-80 °C) showed very low shear strength, and the bond patterns were not characteristic of radial curing.  My guess is that as the silicone cured, it becomes increasingly difficult to outgas any remaining volatile products and pressure produced tear channels in the material near which further curing can take place.

some bad rtv shear samples

Some of the samples of heat sink plaster which were cured at high temperatures (100 °C) became dry and friable, taking on a tan color.  These samples still exhibited normal shear strength, but had dramatically reduced tensile strength.  Whether this loss of pliability and tensile strength occurs at lower temperatures over a longer time frame is unknown, but could potentially be a limiting factor in reliability.  It also calls into direct question the product specifications.  I doubt that the dry state is an intended condition for the product; consequently, I doubt that the 200 °C rating is realistic.  The condition of samples cured at 70 °C was significantly better than those cured at only 30 °C more.

Overall, the product performance is more than adequate for small heat sinks, particularly low-height heat sinks.  The thermal performance is not exceptional, but for the same area, it has a similar thermal resistance compared to a grease, despite forming a relatively thick interface film.  I still would hesitate to refer to it as a self-shimming type TIM, as I doubt that it's intended to serve as an electrical insulator.  That said, It's important to note the limitations of these tests.

All of the RTV silicone and HSP samples are very fresh and really don't represent thoroughly cured material.  It may be reasonable to expect stronger bonds over a time span of a month or so, depending on the ability of the center of the bond patch to breathe.  Similarly, these tests do not indicate whether strength or thermal performance will be sustained over much longer time frames at elevated temperatures.  Longevity is still uncertain, and performance at temperatures beyond 100 °C is questionable. 

That said, there are always other products available that fill a similar role.  STARS-922 is simply the cheapest option.  There are other cheap options out there in the realm of silicone products, and there are also epoxies for use when greater strength is desired and disassembly isn't intended.  There are always the myriad products from Loctite and Dow for specific applications, though most of these are far too expensive for the hobbyist.

One other note as an aside: the hot glue samples were not applied with a glue gun.  These, like most cases where I use hot glue, were applied with a heat gun.  This is due to the simple fact that molten glue typically lacks the thermal capacity to heat a conductive substrate sufficiently to allow bonding to take place before the glue solidifies.  Disregard for this simple fact appears widespread, as it seems every time I encounter hot glue in assembled products, it simply pops off the surfaces cleanly.  So long as you can heat parts manually, hot glue does indeed have utility outside the realm of gluing googly eyes onto cotton balls.  Even the cheap hobby products are incredibly strong so long as the application temperatures stay low.

Finally, I'll leave you with this bit of Matlab kludge that I put together to create the bar graph above.  I always get so frustrated trying to use spreadsheet graphing utilities to do anyhing.  It's impossible to get half the features that are desired, and the figures come out with cartoonish inelegance and poor repeatability.  Granted, a configuration with proportionally-correspondent dual axes is a hassle even in Matlab, but at least Matlab provides a superabundance of flexibility.  One might ask why anyone would ever need dual x or y axes, but the simple answer seems like a terribly common necessity to me: depicting one set of data in both metric and imperial units. 

%% plot adhesive properties
clc; clf; clear all
format compact;

xt={'HT RTV 48h @ 70dC' 'HSP 24h @ 25dC' 'HSP 48h @ 70dC' 'UG RTV 48h @ 70dC' ...
    'Super Glue (generic)' 'Hot Glue (soft clear)' 'Hot Glue (hard amber)'};
ytkpa=[1299 1366 1860 2418 2552 3354 4758];
scalefactor=690/4758;
ytpsi=ytkpa*scalefactor;
scalelabels={'kPa' 'PSI'};
titlestring='Tensile Stress at Break';
ystretch=0; % stretches bar widths for better fit

% create first axis
hl1 = barh(1:1:length(xt),ytkpa);
ax1 = gca;
set(ax1,'XColor','r','YColor','k','box','off','color','none','YTickLabel',xt,...
    'XAxisLocation','bottom','YAxisLocation','left','XGrid','on',...
    'TickLength',[0.015 0],'ylim',[0+ystretch length(xt)+1-ystretch])

% this hackery removes the top and right tick marks from ax1
b = axes('Position',get(ax1,'Position'),'box','on','xtick',[],'ytick',[]);
axes(ax1); linkaxes([ax1 b]);

% create second axis
ax2 = axes('Position',get(ax1,'Position'),'XColor','b','YColor','k',...
    'XAxisLocation','top','YAxisLocation','right','Color','none',...
    'XGrid','on','Ytick',[],'TickLength',[0.02 0],...
    'xlim',get(ax1,'xlim')*scalefactor,'ylim',get(ax1,'ylim'));
hold(ax2,'on')
hl2 = barh(1:1:length(xt),ytpsi,'Parent',ax2);
set(hl2,'facecolor',[1 1 1]*0.5,'edgecolor',[1 1 1]*0);
set(get(hl2,'BaseLine'),'color','k')

% set and position the labels and titles
xl1=xlabel(ax1,scalelabels(1),'units','normalized','position',[-0.1 -0.02 0]);
xl2=xlabel(ax2,scalelabels(2),'units','normalized','position',[-0.1 1 0]);
t1=title(ax2,titlestring,'units','normalized','position',[0.5 1.09 0],'fontweight','bold');

% apply value labels on bars (not exactly robust geometry retention)
for n=1:1:length(xt)
    scalemax=max(get(ax2,'xlim'));    
    valb=text(ytpsi(n)-0.01*scalemax,n-0.0,['\color {red}' num2str(round(ytkpa(n))) ...
        ' \color {blue}' num2str(round(ytpsi(n)))], ...
        'horizontalalignment','right','fontweight','bold');
end

% this mess massages the plot to fit well within its container
marginoffset=[0.04 0.01];
margins=get(ax2,'tightinset'); h=[margins(1) margins(3)]; v=[margins(2) margins(4)];
set(ax1,'units','normalized','outerposition',[[h(1) v(1)]+marginoffset 1-sum(h) 1-sum(v)])
set(ax2,'units','normalized','outerposition',[[h(1) v(1)]+marginoffset 1-sum(h) 1-sum(v)])
set(b,'units','normalized','outerposition',[[h(1) v(1)]+marginoffset 1-sum(h) 1-sum(v)])

% updating axis sync without this is a bunch of shit
% similar to 'linkaxes()', but works with unequal axis limits (and 3 axes)
addlistener(ax1,'Position','PostSet',@(src,evt) updateAxis(ax1,ax2,b));

function updateAxes(a1,a2,a3)
    % callback function for updating internal plot geometry
    % to keep axis objects aligned if container geometry is altered
    xLim1=get(a2,'xLim');
    yLim1=get(a2,'yLim');

    set(a2,'position',get(a1,'position'));
    set(a3,'position',get(a1,'position'));
    set(a2,'xLim',xLim1);
    set(a2,'yLim',yLim1);
end