Looking for a high service temperature adhesive for use on PCBs - Page 1
Forum section: electronics / general

author: NE666
author_rank: Frequent Contributor
timestamp: August 31, 2025, 04:50:02 pm
content: Hello.I'm looking for a low-cost, readily available solution to the problem of adhering a small (~1mm) glass encapsulated NTC temperature sensor to the solder mask of a PCB. The finished bond needs to withstand heating to at least 250 Celsius on a regular basis, as said PCB forms the heating element of a (crude) surface mount pre-heater/hot plate.Would Kapton adhesive tape be suitable for this? I've seen some products described with quoted withstand temperatures in that range but, whilst I've never worked with it before, I'm skeptical that the adhesive used would last for very long even if the tape itself did.Another option I'm considering is high temperature silicone glue, akin to 'instant gasket':https://www.amazon.co.uk/TEMPERATURE-SILICONE-ADHESIVE-SEALANT-RESISTANT/dp/B006C68CMOHowever, that seems to come with potential problem that the tiny sensor elements gets encased in a comparatively large blob of silicone, and hence gets somewhat thermally 'isolated' from the PCB substrate itself.Does anyone have a recommendation? Since this is a probably a one-off repair, products which require specialist application equipment e.g. hot melt, aren't an option.

author: tooki
author_rank: Super Contributor
timestamp: August 31, 2025, 05:56:40 pm
content: Quote from: NE666 on August 31, 2025, 04:50:02 pmNo reason it shouldn’t, as long as it’s one rated for that temperature. However, I wouldn’t expect the adhesion to be quite as strong when hot. Probably not my first choice.Quote from: NE666 on August 31, 2025, 04:50:02 pmWhat you want is for the sensor to be in direct contact, with the silicone filling the gap.Silicone isn’t the best conductor of heat, but it’s much better than air, which is REALLY bad. So the silicone would likely be better than just taping it down. (Especially for a more or less spherical sensor like you show in the picture.) I would place a tiny drop of the silicone, press the sensor into the silicone so it’s firmly touching the board, and then tape the sensor down firmly by the legs so the bead is held in place until the silicone cures. Use a few dollops of silicone on the legs/wires to provide strain relief. If possible, I would very lightly sand/scuff the board beforehand to give the silicone something better to adhere to.Omega (the temperature sensor company) has special epoxy for this purpose:https://assets.omega.com/pdf/consumables/adhesives-and-paste/OB-100_OB-200_OT-200.pdfOB-200 would work. No idea where to buy it, as the Omega.co.uk website lists it as “temporarily unavailable” (aside from being mad expensive).There are, without doubt, other high-temp epoxies and thermally conductive adhesives.Another option is to use a mechanical fastener (like a spring clip of some sort) with thermal grease.Is there any way to use a flatter sensor? The spherical bead there is both thermally suboptimal as well as potentially fragile.

author: NE666
author_rank: Frequent Contributor
timestamp: August 31, 2025, 07:01:34 pm
content: Thanks Tooki, some good points.I didn't design this board or controller (it's a fairly common one right now, currently available at Aliexpress), and as you suggested, if I had I'd certainly have used a more suitable sensor shape and fastening method. Unfortunately, I don't think there's much scope for swapping the existing sensor out, partly since I'm not sure what has actually been used here and also since the temperature controller used doesn't appear to have any scope for (at least, easy) recalibration, if a substitution were to be made. In it's current state, with the glass bead of the sensor just touching the underside of the board (the leads have enough spring to them, I could bend them to cooperate in this way), it's actually not far out according to both my IR camera and a type K thermocouple.I've decided to give this a try:https://www.amazon.co.uk/JB-WELD-MAC8265-S-TEX-8265-S-Weld/dp/B0006O1ICEIt ticks the box for operating temperature, plus it claims to have some proportion of metal loading, so perhaps the thermal conductivity will be a little better than that of plain silicone adhesive.