How soft is this gap pad? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: November 25, 2022, 07:06:41 am
content: Hi,We have a 300W SMPS board (24vin, 32vout, pushpull, 153khz FET) with SMD FETs mounted on the board bottom. Its mounted in an aluminium enclosure, and we need gap pad to thermally interface the FETs to the base. (to fill that gap).Do you know how soft this gap pad is?https://uk.farnell.com/electrolube/gp300s/thermal-gap-pad-200mm-x-200mm/dp/2777027?st=gap%20padIs it like "stiff rubber", or is it like "bread dough"or is it like "blu-tak".

author: coppercone2
author_rank: Super Contributor
timestamp: November 25, 2022, 10:06:55 am
content: Hardness (Shore C) : see datasheet

author: Faringdon
author_rank: Super Contributor
timestamp: November 26, 2022, 09:58:47 pm
content: Thanks , i saw it was rated at "shore C" for hardnesshttps://polymerdatabase.com/polymer%20physics/Shore%20Table.htmlUnfortunately, we dont know how hard that is?...if its too hard, and we push our FET boards into it...they may bend the PCB when we then screw the PCB down.However, if the "hardness" is like a "pastry" kind of thing, then we are ok.If its like a "leather belt", then we've had it.If its like a marshmallow, then again, thats ok.If its like a stick of wrigleys chewing gum then thats too hard.If its like bike tyre rubber, then its too hard.Gap pad costs a bomb and we cant afford to buy loads to compare.

author: Kean
author_rank: Unknown
timestamp: November 26, 2022, 10:34:03 pm
content: Look at this chart:https://plantech.com/convert-different-durometer-shore-types/The value in the datasheet is 40 on the Shore C scale. That chart should give you some idea.It is pretty firm, but keep looking as there are lots of other softer options.

author: Faringdon
author_rank: Super Contributor
timestamp: November 27, 2022, 11:16:57 am
content: Kean thankyou that is amazing...yes, shore C 40 is way too hard for our useage.Thanks.It would be good if they said if it restores its shape if pressed too.(thats what we dont need).We need it like pastry dough.

author: jpanhalt
author_rank: Super Contributor
timestamp: November 27, 2022, 11:51:07 am
content: So-called memory foam (https://www.mcmaster.com/polyurethane-foam/ultra-conformable-polyurethane-foam-sheets-and-strips/) is soft,but it seems you are asking for good thermal conductivity and low bulk density. That will be hard to find -- think of the foam sheeting used to insulate homes.You imply it needs to be removable. Why? You could cast a layer of thermally conductive epoxy* to give a flat, electrically insulating, and thermally conductive surface to mount against the case.*Just one example of several options.

author: Kean
author_rank: Unknown
timestamp: November 27, 2022, 09:43:42 pm
content: Check out Laird TFlex HR400 as a different alternative. e.g. Farnell O/C 2484738 for 0.5mm thick, but thicker material is available.I've used it and it is pretty soft - specified as 60 on the Shore OO scale (refer to previous chart).Note that it has a lower thermal conductivity (1.8W/m.K vs 3.0Wm.K) compared to the GP300S, and dialectric strength isn't specified (wasn't important in my application).