Glue sticks and guns

[akis]

I used my old and trusty glue gun to secure a thermistor onto a heatsink, but during testing at about 100C the glue melted and the thermistor became detached.

"hot" guns reach about 200C (400 F) but the glue sticks are another matter. Can someone recommend glue sticks that will only melt at these high temperatures?

[macboy]

Hot melt glue is the wrong choice as you discovered. A thermistor is delicate, so I wouldn't use something hard like epoxy; I'd fear the physical stresses when the heatsink thermally cycles. I'd try silicone caulking or a rubber cement like "Shoe Goo" or "Goop". All of those are heat resistant and flexible when cured.

[sleemanj]

Stick it on with a bit of kapton tape.

Sent from my HTC Dream using Tapatalk 2

[AndyC_772]

Thermally conductive adhesive is what you want. Loctite 315+7386 is excellent, though expensive.

[akis]

I bought that fancy tape, but has not arrived yet. Not sure I'd use tape however.

I also bought a high temperature glue gun and sticks (200 C)  http://www.amazon.co.uk/gp/product/B0001D1Q72/ref=oh_details_o02_s00_i01?ie=UTF8&psc=1. I will test it tomorrow.

However in the meantime I have mounted the thermistor mechanically using a custom made aluminium bracket with a flat half for mounting on the heatsink and a cup shape on the other side filled with thermal paste inside which sits the thermistor on the back of the heatsink.

[theatrus]

Mounting it mechanically is a good move for heat conduction.

[georges80]

I've used http://www.arcticsilver.com/arctic_alumina_thermal_adhesive.htm to attach thermistors to various heatsinks and devices. Works well and is not 'brittle' at medium/high temperatures.

I typically have it on hand for attaching high power LED stars to heatsinks, so it's I always have some tubes on hand.

cheers,
george.

[georges80]

PLA softens at around 60C (approximate glass transition point), it is quite hopeless as a hot melt glue... other than 3d printing stuff.

A thermal conductive epoxy is a better idea and a good solution. It provides a good thermal path for the thermistor to the DUT.

cheers,
george.

[rob77]

i'm using regular gel glue (patex repair extreme) - i'm usually using thermistors in TO-92 package gluing them to the heat-sink with the mentioned glue. in my last project i used a couple of thermistors  (mesuring temp on 2 differnt places) to trigger a cooling fan - i'm able to trigger the fan by touching the heatsink (takes only few seconds) so the termal resistance must be good for that glue.

[NiHaoMike]

Silicone glue is what's typically used to attach temperature sensors to heatsinks.

[T3sl4co1l]

Filled (nonconductive) epoxy.

What's bad about rigidity?  Thermistors are ceramic chips internally, nothing bad about it. Long as you have strain relief on the leads of course.

Performance is entirely described by the thermal time constant, and what error if any arises from dissipation off the outer surface of the sensor (or blob of goo it's inside), and how that stacks up against the thermal resistance of the "Blob-O-Goo" (as a professor I had would say), contact resistance, and wherever the sensor is in the midst of those temperature drops.

Heatsink goop is a good idea for mounted types (screw down / etc.), though not really necessary, the heat isn't going anywhere.  It'll probably improve the time constant and error a little, but I don't think I would bother if it were a production kind of thing.  If it's that precise, use an embedded thermocouple; you'll get better guaranteed precision and accuracy.

I wouldn't suggest rubbery goo for the most part, since the conductivity isn't going to be as great.  That said, as long as the sensor is at the bottom, close to the thing-to-measure, it needn't be too bad, and the insulation could even help reduce error.

For spot testing with thermistors, this is what I do:
- 0603 chip thermistor (1% 10k, R85/25 around 3500 -- a pretty average, highish tolerance, NTC thermistor)
- 3" pigtails: 37AWG enameled wire, tack soldered to the chip
- Hookup wire leads back to the DMM, resistance range; use lookup table to find temperature.  Or, set up your own temp measuring gizmo with an ADC (8 bits is marginal, 10 bits good enough, 12 bits plenty) and the lookup done in software.

Use: dip the thermistor in heatsink goo, then touch it to the DUT.

Time constant is quite rapid, and temperature pretty faithful.  It does "load down" small objects (like SMT transistors, which have about as much heat capacity as the thermistor chip), but works just fine on big things (heatsinks, etc.).  Makes a mess (bops of heatsink goo everywhere).  Beware of conductive objects shorting out the thermistor (maybe hold it lengthwise to the surface rather than broadside, so it doesn't get shorted out), or being shorted by it (don't use near device pins, etc.).  Not so great for very high temperatures (>100C), since the resistance is already very small by then.  If you have extended temp data, it'll be good out to 180C or so when the solder begins to melt (heatsink goo is quite robust, in and of itself, by the way).  Obviously, more than that and you have little choice but to use a thermocouple.

Tim

[rob77]

Filled (nonconductive) epoxy.

What's bad about rigidity?  Thermistors are ceramic chips internally, nothing bad about it. Long as you have strain relief on the leads of course.

Performance is entirely described by the thermal time constant, and what error if any arises from dissipation off the outer surface of the sensor (or blob of goo it's inside), and how that stacks up against the thermal resistance of the "Blob-O-Goo" (as a professor I had would say), contact resistance, and wherever the sensor is in the midst of those temperature drops.

Heatsink goop is a good idea for mounted types (screw down / etc.), though not really necessary, the heat isn't going anywhere.  It'll probably improve the time constant and error a little, but I don't think I would bother if it were a production kind of thing.  If it's that precise, use an embedded thermocouple; you'll get better guaranteed precision and accuracy.

I wouldn't suggest rubbery goo for the most part, since the conductivity isn't going to be as great.  That said, as long as the sensor is at the bottom, close to the thing-to-measure, it needn't be too bad, and the insulation could even help reduce error.

For spot testing with thermistors, this is what I do:
- 0603 chip thermistor (1% 10k, R85/25 around 3500 -- a pretty average, highish tolerance, NTC thermistor)
- 3" pigtails: 37AWG enameled wire, tack soldered to the chip
- Hookup wire leads back to the DMM, resistance range; use lookup table to find temperature.  Or, set up your own temp measuring gizmo with an ADC (8 bits is marginal, 10 bits good enough, 12 bits plenty) and the lookup done in software.

Use: dip the thermistor in heatsink goo, then touch it to the DUT.

Time constant is quite rapid, and temperature pretty faithful.  It does "load down" small objects (like SMT transistors, which have about as much heat capacity as the thermistor chip), but works just fine on big things (heatsinks, etc.).  Makes a mess (bops of heatsink goo everywhere).  Beware of conductive objects shorting out the thermistor (maybe hold it lengthwise to the surface rather than broadside, so it doesn't get shorted out), or being shorted by it (don't use near device pins, etc.).  Not so great for very high temperatures (>100C), since the resistance is already very small by then.  If you have extended temp data, it'll be good out to 180C or so when the solder begins to melt (heatsink goo is quite robust, in and of itself, by the way).  Obviously, more than that and you have little choice but to use a thermocouple.

Tim

there's nothing bad with epoxy and rigidity... but you know ... when you need to glue down a small thermistor only, then you will most likely look after other options rather than mix the epoxy and tolerate the stench. if you going to use epoxy for other things on your project, then of course the thermistor will get some as well

i was experimenting with many glues (just to avoid the epoxy because of that stench) and that gel glue i mentioned above turned out to be a good one. curing time comparable with the 1minute epoxy, doesn't smell too bad (almost not at all), good mechanical strength (i'm not able to rip off the thermistor off the heatsink by bare fingers)  and it has a good thermal conductivity (see my previous post).

[akis]

I purchased the Dremel GG11 glue sticks which are advertised as 165C-195C http://www.dremeleurope.com/gb/en/ocs/product/6116/432/multipurpose-glue-sticks-(7-+-11-mm)/dremel%C2%AE-11-mm-multipurpose-high-temp-glue-sticks;jsessionid=8029A0531D1A6DADDE4EC8F244E670F8.sl026-vm

However, once set, these go soft at 50C-60C and melt at 70C. I am not sure why this is, since they are advertised as "165C-195C".

I am searching now for *real" high temp glue sticks and I have also written to Dremel to ask them why they misadvertise their sticks.

I also have Araldite glue but it says, is only good to 60C. http://www.amazon.co.uk/gp/product/B0001OZH58/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1

The transistors in question are MJL1302/1382 and once they got so hot that the solder melted off the pins and the wires fell off! Now I have teperature controlled fan and auto shut down at 100C-110C.

It seems mechanical fastening is the best choice.

[akis]

I've used http://www.arcticsilver.com/arctic_alumina_thermal_adhesive.htm to attach thermistors to various heatsinks and devices. Works well and is not 'brittle' at medium/high temperatures.

I typically have it on hand for attaching high power LED stars to heatsinks, so it's I always have some tubes on hand.

cheers,
george.

Ah just seen it now, http://www.amazon.co.uk/Arctic-Silver-Alumina-ADHESIVE-Thermal/dp/B003D97V5W/ref=lh_ni_t?ie=UTF8&psc=1&smid=A1GHHA4GPDJQIJ

good price too. may be I should buy this and ignore the useless (for this purpose) glue gun.

[pickle9000]

When it comes to glue sticks high and low temp normally refers to the temp of the gun not the actual melting point of the glue.

Low temp sticks are designed for "hobby" use and require a low temp gun. Probably a marketing related gimmick.
High temp sticks are also known as normal sticks.

[rob77]

I purchased the Dremel GG11 glue sticks which are advertised as 165C-195C http://www.dremeleurope.com/gb/en/ocs/product/6116/432/multipurpose-glue-sticks-(7-+-11-mm)/dremel%C2%AE-11-mm-multipurpose-high-temp-glue-sticks;jsessionid=8029A0531D1A6DADDE4EC8F244E670F8.sl026-vm

However, once set, these go soft at 50C-60C and melt at 70C. I am not sure why this is, since they are advertised as "165C-195C".

I am searching now for *real" high temp glue sticks and I have also written to Dremel to ask them why they misadvertise their sticks.

I also have Araldite glue but it says, is only good to 60C. http://www.amazon.co.uk/gp/product/B0001OZH58/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1

The transistors in question are MJL1302/1382 and once they got so hot that the solder melted off the pins and the wires fell off! Now I have teperature controlled fan and auto shut down at 100C-110C.

It seems mechanical fastening is the best choice.

probably it just doesn't evaporate below 165C-195C 

but seriously - hot glue has "zero" strength once heated - it's good to glue down wires , or fix small boards to plastic cases, but should be avoided for mounting anything to heatsinks (especially if they going above 40C).

another problem with hot glue is the fact that it's flowing (very high viscosity, but still flowing) at relatively low temperatures (60-70C+).

[T3sl4co1l]

i was experimenting with many glues (just to avoid the epoxy because of that stench) and that gel glue i mentioned above turned out to be a good one. curing time comparable with the 1minute epoxy, doesn't smell too bad (almost not at all), good mechanical strength (i'm not able to rip off the thermistor off the heatsink by bare fingers)  and it has a good thermal conductivity (see my previous post).

Hmm, we don't have Patex over here, I don't think.  Not sure if we have a similar product.  Been a while since I perused the hardware store.  A thin layer of anything should do well enough -- superglue is a good candidate, under 100C or so anyway (cyanoacrylates depolymerize at high temperature -- keep that in mind if you're having trouble with some particularly tenacious Loktite'd screws!).

Tim