Heatsink adhesive foil failure - need tips for a quick fix.

[peteru]

I'm using some aluminium heatsinks that come with adhesive foil. These replaced much crappier heatsinks that were attached using an epoxy which eventually started decomposing, charring and corroding the PCB. The heatsinks are horizontally mounted to 3.3V and 1.2V LDO regulators in TO252 packaging.

Anyway, new heatsinks have been on for about 6 months and the adhesive foil is not doing an adequate job. While I was investigating some unrelated issues, I noticed that a heatsink was a bit wobbly. A gentle push on it dislodged it from place. The adhesive on the foil turned dark cream / milk coffee colour and showed signs of blistering. Clearly the adhesive foil is not up to the task. The other three heatsinks have the same problem.

I'm looking for some tips for a quick fix. I need to get the device working without having to wait weeks for delivery of purpose specific adhesive/epoxy. Ideally I would like to use something I can get from Woolworths, OfficeWorks or Bunnings. Double sided sticky tape? Superglue? Tarzan's grip?

The concerns I have:

• Potential for component or board damage.
  I don't want another instance of the adhesive decomposing and damaging the PCB further.
• Future repairability.
  Ideally it will be possible to remove the heatsinks if the regulators fail and it will be possible to do so without excessive forces on the board or components.
• Thermal conductivity.
  I don't want to use something that will trap the heat and defeat the purpose of the heatsinks. Clearly I'm not aiming for CPU/GPU levels of performance, but it still needs to do the job.
• Longevity.
  It would be nice to use something that will last longer than half a year.

Suggestions? I only need to reattach four 14mmx14mm heatsinks. The device is a PVR, so it does not get moved and the only source of mechanical stresses would be vibrations from a 3.5" HDD and a 40mm case fan running at low RPM.

[xavier60]

I have been using this product for years, https://www.mitre10.com.au/h-b-fuller-tool-box-adhesive-sealant?gclid=Cj0KCQjww4OMBhCUARIsAILndv7cdYCA4rWL39OC3vtgF2VxIAlddg6w2qHeJLyNvfCn5s1TRgpQT50aAn7REALw_wcB&gclsrc=aw.ds

As well as having good adhesion to most cleaned surfaces, it has greater thermal conductivity than the usual silicone sealant/adhesives.
Ill look for some data.

Thermal Conductivity ca. 0.5 W.m-1K-1 https://www.hbfuller.com.au/media/1325/toolbox_original_sealant_tds.pdf
Be certain to get the "Original" version only, not the clear stuff.

[max-bit]

There are two solutions:
- Mechanical mounting of the heatsink to the PCB (using pins)
- Use of good adhesives, and here there are two types: two-component epoxy or silicone-based (these are generally partially elastic).

If you want to glue, use only good adhesives, I will not advertise any company here, but these well-known companies do not produce rather bad adhesives and heat sinks last for years.

[Jeroen3]

Can you mount the heatsinks to the other side of the board, secured by a screw and heat transfer thermal vias*?

*vias filled with thermal epoxy during board fabrication.

[max-bit]

Rather, the heatsink has pins for soldering and then it is soldered into the PCB

[peteru]

The device I'm trying to repair (in a hurry) has a less than ideal thermal design for the LDOs. The original design has U45 & U47 in TO252 packages. These are regulators that convert from 5.0V to 3.3V and should dissipate around 1.5W each. In the attached pictures, these are the parts with the large copper mass on the board. Contrast that with U44 & U46, which are in D2pack form factor and have significantly less copper. These are 3.3V to 1.2V regulator and are expected to dissipate 4.2W each.

Here's what was hidden under the original heatsink:



Here's a picture after replacing the failed 1.2V LDO with a slightly larger TO252 package. This is from someone else's board, with much less PCB damage than mine had, but it illustrates the board design well enough.



There's not much scope for extensive modifications. There is minimal clearance between the bottom of the board and the metal chassis of the PVR. These regulators are positioned fairly close to four metal DVB-T tuner cans and those generate a fair amount of heat too.

tl;dr: Gluing aluminium heatsinks to the LDOs does an adequate job, however the pre-applied heatsink adhesive deteriorates over time. I suspect that these parts may operate at or above 100C on a 24x7 basis for years on end.

[peteru]

I have been using [ H B Fuller Toolbox Original Sealant ] product for years
...
As well as having good adhesion to most cleaned surfaces, it has greater thermal conductivity than the usual silicone sealant/adhesives.

Thanks for the tip. It looks good, except for the max temp rating of 100C. Unfortunately I think that this particular board design is problematic enough to require a product that can stretch past 100C. Perhaps not much more than 100C, but clearly the excessive heat has been problematic already.


I've had a good look through the various house maintenance products I have left over and found a generous amount of Waterbased Kwik Grip. It contains no solvents and it's rated to 130C. I might give it a go, unless someone can think of compelling reasons to stay away from it. 

Edit: It turns out I actually have Waterbased Kwik Grip Advanced, which is rated to 190C.