SMD red glue - Page 1
Forum section: electronics / beginners

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: April 24, 2019, 05:41:55 am
content: I don't do a great amount of SMD work but have been successful with resistors, capacitors and the odd IC that I have replaced.LEDs need some form of thermal paste, and with the round COB LEDs such as in theatre spots I have put a blob of thermal grease onto the alloy disk on the back before soldering.Yesterday I showed a non-EE friend how to remove and replace the 5730 type LEDs in his reading lamp. I explained that in production the devices would be stuck down with a form of adhesive, and that it performed a thermal transfer role too. I didn't have any, so put a dot of thermal paste. This was not ideal as the component was slid around during the learning process and contaminated the flux etc.The red glue seems to be very expensive for a such tiny amount.Any ideas as to a source or alternatives for hobby use, and any tips?BT

author: Psi
author_rank: Super Contributor
timestamp: April 24, 2019, 06:20:50 am
content: There's some SMT glue for ~US$10 on Aliexpress if you're happy to wait the 2-4 weeks for china shipping.https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20190423221905&SearchText=SMT+glueNOTE: That some of these glues are epoxy and so you need to get part A and B.It looks like others are not epoxy. Your mileage may vary, this is not something i have ordered before.If i was ordering it i'd probably get a few types to cover all the options.

author: AndyC_772
author_rank: Super Contributor
timestamp: April 24, 2019, 06:35:43 am
content: I've never heard of thermal paste being used between any component and the PCB. Components which need to sink heat into the PCB have exposed copper pads which are simply soldered to the board, and the soldered joint provides heat transfer which is better than any thermal paste.The red glue is probably Loctite Chipbonder, which I've only ever seen used to secure components to stop them coming off the board during wave soldering, or possibly on the underside of a PCB during reflow. It's not used for thermal purposes.Can you post a picture of the PCB, and the LED data sheet?

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: April 24, 2019, 12:24:29 pm
content: Thanks both of you,I think that it will only be the LEDs that need some kind of heatsink to the PCB bonded to an alloy plate that they usually are mounted on.The larger types such as fitted to spotlights have an alloy back that presumably the die is bonded to. The solder connections are often tabs that project from the sides. The tiny (1206?) 0.5W SMD LEDs probably don't dissipate enough energy to need anything other than the thermal path through the solder 'end caps'.The type that I was helping my friend with had a tiny alloy plate in the back which I assumed would need to be in good contact with the PCB for heat transfer. The failed LEDs were glued down with red glue so I assumed that the purpose was for heat transfer as well as securing the device before soldering.I suspected that the appalling reliability of these LEDs in domestic lamps is due to being overdriven and poor heatsinking. This PCB/alloy back plate in this lamp had been covered in thermal compound then screwed to a further alloy plate that was part of the lamp head. This had a countersunk screw in its centre that was so proud of the surface that the PCB was bent over it and the compound had not touched anywhere !If I have to fit any more of these LEDs I will take care and put a spot of thermal grease on the back.BT

author: jeremy
author_rank: Super Contributor
timestamp: April 24, 2019, 12:32:02 pm
content: My experience is similar to Andy; I have only seen "red glue" (may be loctite, but I've seen other manufacturers make it too) for soldering/reflow purposes only.If you are looking for heat transfer, there are thermal transfer epoxies:https://www.electrolube.com/products/thermal-bonding/tbs/thermal-bonding-materials/although I doubt this would stand up to soldering temperatures.

author: tooki
author_rank: Super Contributor
timestamp: April 24, 2019, 03:24:18 pm
content: SMD LEDs that need heatsinking will have the thermal pad on the bottom, like yours. This pad is intended to be soldered, not used with thermal paste. Why not just solder them properly? Obviously these can’t be soldered with a soldering iron, you must reflow solder them (e.g. with hot air).

author: Psi
author_rank: Super Contributor
timestamp: April 25, 2019, 09:59:02 am
content: Quote from: BurningTantalum on April 24, 2019, 05:41:55 amThat could be a misinterpretation, the adhesive does play thermal role in that it prevents tombstoning if the part is glued to the board.

author: wraper
author_rank: Unknown
timestamp: April 25, 2019, 10:13:27 am
content: Glue for heat transfer purpose is a not a good idea. Improvement will be marginal at best. First of all, if large enough heat is produced by LED, solderable pads should already be designed to cope with that. Red glue is used in 2 cases:1. SMT wave soldering, together with TH components.2. Fixing heavy components when doing two side SMT assembly, so they don't fall off on second pass.