Looking for a rigid PCB glue that goes on thick and wont spread out when curing - Page 1
Forum section: electronics / general

author: Psi
author_rank: Super Contributor
timestamp: June 14, 2025, 04:56:41 am
content: I'm looking for a glue I can order to hard-fix some parts on a PCB for a run of 120 pcbs.My requirements are.- It must go on thick. This is critical. It can't become runny and spread out like epoxy does before setting. I need to be able to add 5mm high piles of it on the PCB and have them stay there and set in place. Something the consistency of silicone or toothpaste when applying but sets hard as epoxy, or close to it, would be great.- Needs to be quite rigid, silicone is almost definitely too soft. It doesn't have to set as hard as a rock. The hardness of potting compound or conformal coat would be fine.- This is for a small home lab manufacturer run, so something only available in a huge 20L container isn't going to work. Has to be available in under 1 or 2 litres.- Speed of setting doesnt matter, can take 24h to set.- Has to be a PCB safe adhesive obviously.Any ideas.I considered UV cure glues, but that has high setup costs so probably best to avoid that.An thick epoxy that does not go runny when setting would be fine if they exist?

author: jfiresto
author_rank: Frequent Contributor
timestamp: June 14, 2025, 07:00:32 am
content: Quote from: Psi on June 14, 2025, 04:56:41 amCould you hazard a guess as to the minimum acceptableShore hardness?

author: tooki
author_rank: Super Contributor
timestamp: June 14, 2025, 09:14:32 am
content: Quote from: Psi on June 14, 2025, 04:56:41 amPotting compounds, fully cured, can be as hard as rock or as soft as jello. There are just too many kinds for too many disparate purposes.Can you find a frame of reference that is a bit… narrower? As jfiresto said, a shore hardness would be ideal. (One example you can easily find at retail is the wheels for rollerblades and razor scooters, which are sold at sporting goods stores with specific shore hardness ratings printed right on them.)Also, don’t all epoxies get thicker as they cure? I.e. you could get one with a slow curing time. Right after mixing, while still thin, brush on a thin layer to act as a primer of sorts. Then let the stuff in the mixing pot thicken up, and apply it when it’s already thick.

author: jpanhalt
author_rank: Super Contributor
timestamp: June 14, 2025, 09:19:52 am
content: Filled epoxy can be made quite thick. Cabosil (fumed silica)* can be used to make fillets and can be modeled in free space. Faster curing might be had with polyester resins. Photocured may also work, but i am not sure how thick it can be.A non-catalyzed material like a hard setting hot-melt might work. 3M industrial would be where I looked for that.*Fillers such as glass or phenolic beads also thicken epoxy, but I prefer fumed silica when making a really thick paste.

author: Ian.M
author_rank: Super Contributor
timestamp: June 14, 2025, 09:43:21 am
content: Allowing epoxy to seriously thicken in the pot before use typically results in poor mechanical properties and very difficult application, as due to self-heating and resulting local temperature differences, it never thickens evenly.With enough colloidal silica (aka: fumed silca) to thicken it, you can make epoxy with the consistency of stiff putty, and if you keep the temperature down while it cures, it wont slump. Adhesion suffers slightly, unless you prime surfaces with a skim coat of the same epoxy, which rules out using any sort of 'rapid' epoxy. You probably want an un-thickened epoxy system as thorough 100% mixing is essential before adding the colloidal silica. Use a '24H' cure epoxy to get enough 'pot' life for production - depending on ambient temperature you should be able to get up to an hour of mixing and working time. The exact amount of colloidal silica to add is critically dependent on many factors including ambient temperature, so do some test mixes to get 'in the ballpark', then be prepared to adjust each batch to the desired consistency.It may be worth looking at commercially available two part epoxy putties . . .

author: Psi
author_rank: Super Contributor
timestamp: June 14, 2025, 10:41:01 am
content: Quote from: jfiresto on June 14, 2025, 07:00:32 amD50 or harder

author: jpanhalt
author_rank: Super Contributor
timestamp: June 14, 2025, 12:43:15 pm
content: Quote from: Ian.M on June 14, 2025, 09:43:21 amIt's my understanding that fumed silica and colloidal silica are not the same. I use fumed silica. Cabosil, which is often used generically by the public in the US, is fumed silica. A water suspension, if used, would not mix well with epoxy. Other suspending liquids that are miscible with epoxy would tend to thin and weaken the final glob.Source: Grok3https://grok.com/chat/09670b17-df69-41a0-9f70-69f8f7270135Quote

author: langwadt
author_rank: Super Contributor
timestamp: June 14, 2025, 01:01:10 pm
content: is all you need for UV cure the $2 UV flashlight that comes with the glue from aliexpress ?instead of make a pile can't you just glue on a small spacer of some sort?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 14, 2025, 01:09:19 pm
content: Quote from: Psi on June 14, 2025, 04:56:41 am3D print in TPU?Plenty of cheap options to cure UV glue.e.g.https://vi.aliexpress.com/item/1005007911305840.html

author: Ian.M
author_rank: Super Contributor
timestamp: June 14, 2025, 05:52:07 pm
content: @ jpanhalt,West Systems beg to differ:https://www.westsystem.com/products/406-colloidal-silica/It is of course bone dry and should be kept tightly sealed as moisture will cause it to clump, fail to mix properly and and also may cause curing problems.

author: jpanhalt
author_rank: Super Contributor
timestamp: June 14, 2025, 06:17:35 pm
content: @Ian.MWest Systems can call it hamburger, that doesn't make it hamburger.https://www.sciencedirect.com/topics/engineering/colloidal-silica#:~:text=Colloidal%20silica%20(CS)%20is%20an,against%20gelation%20with%20alkali%20solutions.https://en.wikipedia.org/wiki/Colloidal_silicahttps://pubs.acs.org/doi/10.1021/acs.iecr.6b01839(chart is ambiguous, but descripton is clearly consistent with Grok, Science Direct, and Wikipedia)Plus numerous other citations when searched for "scholarly articles on colloidal silica."I think someone who tries to be as precise as you do would want to be correct. Fumed silica and colloidal silica are used for different purposes and are made differently. If you believe they are "identical," that is not correct. I do enjoy reading your echos of other posts from time to time.

author: coppercone2
author_rank: Super Contributor
timestamp: June 14, 2025, 06:33:40 pm
content: For 3M products you want to get the NS suffix. NS means no sag. Its typically the same glue with a gelling agent added like silica or fumed silica or possibly other materials.Some other 3M glue available in the *NS form would probably work for you. The nice thing about a NS glue is that you are getting 100% testing and verification from 3M and for 100 quantity you can use it in a dual cartridge gun with mix tube, aka no screwup on assembly so long its not expired and you use a fresh nozzle when it needs a fresh nozzle. If you mix up your own additives you have no idea what the resulting parameters will be unless you do a test (and there is alot of tests and special test fixtures they use to vet their glue mixtures).For PCB I like DP270, but its flowy and takes 24 hours to cure. If you mix it with silica its some what less flowly, but that's why I like it for PCB it gets into holes and stuff good, for burn repairs + hole patching (I add some silica with a small spoon and mix it in on the PCB). But again this is for repairing old stuff without any specs.DP420NS = slow (2 hours)DP100NS = slower (15 min)very good glue for PCB fiberglassI forgot which ones are actually available in NS form. I think sometimes the colored ones have lower flow rate then clear ones (i.e. dp110 clear vs grey) but I am not sure if its a spec you can go by like the NS suffixBut I do mean sticking two fiber boards together. For parts to boards it can be totally crazy and 100% depends on what the part is and some parts need special treatment before glueing

author: Ian.M
author_rank: Super Contributor
timestamp: June 14, 2025, 06:43:35 pm
content: So why are West Systems, one of the leading epoxy suppliers to the marine market, and several of their competitors calling their silica additives 'colloidal silica'? Dried colloidal silica is commercially available, is used in several industries, and if produced as a very fine powder would be very difficult to distinguish from fumed silica except under high magnification. I'm not expert enough in epoxy to know how it compares as a thickening additive

author: coppercone2
author_rank: Super Contributor
timestamp: June 14, 2025, 06:47:20 pm
content: Dried colloidal silica sounds like fumed silica that is graded for grit.Kind of like if you make powders, you can mix it, sinter it solid (or crystalize it from solution if its a salt etc) and then crush it again and sift it. I think dried colloidal silica would be considered 'densified'. I think it might end up having less small particles in the mix this way, that perhaps electrostatistically and otherwise stick to the larger particles and bypass a sifter. and have preferred shapes. It could be that certain shapes of silica actually make the epoxy worse, and by doing some powder process to it you are ensuring they are not there in the final mix.I.e. cheap manufacturing process makes some kind of random powder from whatever process (blowing, rapid percipitation, etc + reaction).powder is densified (dissolve and slowly grow a big crystal, or press it with heat, or mixtures of thereove)smash what you made and sift it carefullyin the end it has more consistency. Some shapes and sizes are eliminated or reduced. It would be like making your own fake gravel from cement powder, or making sand stone of a particular grit). If you want sand stone pebbles and have loose sandto me cheap fumed silica sounds like random sand. anything more expensive is sandstone?

author: Psi
author_rank: Super Contributor
timestamp: June 14, 2025, 11:32:28 pm
content: Some good information here. Thanks.I am looking into the 3M NS stuff.Does anyone know of any PCB safe silicone that sets really hard, D50 or around there.All the silicone i've used has been way too soft, but presumably they do make hard stuff?

author: coppercone2
author_rank: Super Contributor
timestamp: June 14, 2025, 11:37:40 pm
content: you might find a platinum cure silicone of the correct durometer I believe it has no curing biproducts (it works like a bondo aka catalyst cure but with platinum instead of peroxide)No idea about adhesion. It sticks OK to holes in wood.

author: Psi
author_rank: Super Contributor
timestamp: June 15, 2025, 12:12:10 am
content: Currently also looking at this high-temp hot-melthttps://www.glueguru.co.nz/site/glueguruunleashed/images/Brands/Power%20Adhesives/TECBOND%207718%20TDS.pdfSeems to be compatible and available locally, so probably worth a test.It doesn't give a shore rating but does say it "sets very hard"Anyone able to confirm it's fine to apply hot-glue at a slightly lower temp to make it a bit more viscous during application?DS does say "Suggested" Application Temp 215C, so i'm guessing temp is not really critical other than to get it into a state where you can apply it.I have a mod'ed hi-temp glue gun with temp adjust so I should be able to lower the temp until I get the application viscosity I want.Unless there's some downside to this that I'm unaware of? I imagine it's adhesive power is reduced a little if applied at reduced temp due to not flowing into surface textures as well but that is easy testable. I'm not aware of any chemical reaction that needs to reach a set temp or anything that would affect the process. It's just melting glue afaik. The datasheet does say "polyamide" adhesive so not the normal EVA / polyolefin stuff i usually use.Anyone have any ideas for cheap gluing clamps that can open to 13mm wide, they dont need to provide much force.Clothes Pegs might work but I doubt they will open that wide.

author: jfiresto
author_rank: Frequent Contributor
timestamp: June 15, 2025, 06:52:50 am
content: Quote from: Psi on June 15, 2025, 12:12:10 amPerhaps you can use or adapt something like this (originally)one-hand woodworking clamp?

author: Psi
author_rank: Super Contributor
timestamp: June 15, 2025, 07:22:40 am
content: Quote from: jfiresto on June 15, 2025, 06:52:50 amReally small versions of those would work.They can be cheap and nasty and only need to be the size of a clothes peg or a bit smaller.They're correcting a slight bend in SIP DCDC module at edge of PCB so they stay against the PCB while gluing (SIP pins are hard at one end side of component, not centered, making the part annoying as they can easily bend over.

author: tooki
author_rank: Super Contributor
timestamp: June 15, 2025, 10:56:44 am
content: Quote from: Psi on June 15, 2025, 12:12:10 amI think it might not hold as well, as it won’t flow into the microscopic nooks and crannies as well, which are what hold it on. You’d have to experiment.

author: Bud
author_rank: Super Contributor
timestamp: June 15, 2025, 12:41:11 pm
content: Can you use epoxy putty? Like a 2-component epoxy putty stick. Cut a piece, kneal it with you hand to mix, make any shape you want.

author: wraper
author_rank: Unknown
timestamp: June 15, 2025, 12:49:33 pm
content: It needs to be hard for what reason? You may be tricking yourself by thinking it will be doing good because it's rigid while in practice it may be what leads to broken solder joints and component terminals

author: wraper
author_rank: Unknown
timestamp: June 15, 2025, 12:53:49 pm
content: Quote from: Psi on June 15, 2025, 12:12:10 amIf you're ok with it delaminating in a few years, especially at freezing temperatures, go for it. I've yet to see a hot melt glue that lasts. I would never consider for anything that needs to last longer than shipping stress until it's delivered to the customer.

author: Psi
author_rank: Super Contributor
timestamp: June 15, 2025, 01:13:04 pm
content: Quote from: wraper on June 15, 2025, 12:53:49 pmIt will be there partly to stop it getting knocked loose from handling at installation.The conformal coat holds it down but isn't strong enough to prevent handling bending the part sometimes. Once it's lifted/bent it becomes a vibration issue due to being a SIP.Next PCB version wont have this issue but still have to deal with existing ones

author: Poroit
author_rank: Frequent Contributor
timestamp: June 16, 2025, 09:03:06 am
content: G'day Psi,How about Sika 11FC ?It is great for sticking things in place.

author: Psi
author_rank: Super Contributor
timestamp: June 16, 2025, 09:41:44 am
content: Quote from: Poroit on June 16, 2025, 09:03:06 amWill have a look. Currently TECBOND 7718 is looking really good. I bought some today and did some tests

author: wraper
author_rank: Unknown
timestamp: June 16, 2025, 10:07:14 am
content: Quote from: Poroit on June 16, 2025, 09:03:06 amUsing it over RTV silicone is ridiculous. And it's softer than many if not most of the silicones. I would suggest 708 Chinese silicone for electronics (quite low viscosity that provides easy application but does not flow under it's own weight) as OP did not explain why it needs to be hard and probably does not realize that 'soft' silicones prevent component movement exceptionally well when applied properly. There is a reason why virtually nobody uses hard adhesives to hold components. Most of hot-melt adhesives are not that hard either, not to say their poor durability defeats any demanding use case anyway.

author: Psi
author_rank: Super Contributor
timestamp: June 16, 2025, 10:10:10 am
content: Quote from: wraper on June 16, 2025, 10:03:19 ami did, because it needs to protect from aggressive pcb handling, where a DCDC SIP gets bent over. (it's on the edge of pcb, which i will fix in next version but i have to deal with current stock atm.Soft silicone does nothing for this, doesnt hold it in place at all.If Sika 11FC is soft it probably wont work.

author: wraper
author_rank: Unknown
timestamp: June 16, 2025, 10:17:13 am
content: Quote from: Psi on June 16, 2025, 10:10:10 amIf there is any taller component nearby such as film or electrolytic capacitor, putting a blob of silicone between the parts will do the job very well. Or you could glue some rectangle or L shaped plastic support if there are no tall parts nearby.

author: tooki
author_rank: Super Contributor
timestamp: June 16, 2025, 06:21:27 pm
content: Quote from: wraper on June 15, 2025, 12:53:49 pmDo you have any experience with the professional hot melt glues, like the stuff inhttps://multimedia.3m.com/mws/media/44021O/3m-hot-melt-adhesive-systems.pdf?I have never used them since they only work in 3M’s big professional hot melt glue guns, not in the consumer hobbyist ones with the 7 or 11mm sticks.

author: wraper
author_rank: Unknown
timestamp: June 16, 2025, 06:54:41 pm
content: Quote from: tooki on June 16, 2025, 06:21:27 pmI have experience finding such glues in devices I repaired, usually glue is already loose. Even if hot melt glue is still attached, usually a little bit of prying should peel it off. If it's relatively fresh, putting in a freezer or applying IPA will do the job of detaching it. It will probably work well enough with wood as shown in that brochure but there is no such porosity on PCB it could fill.

author: tooki
author_rank: Super Contributor
timestamp: June 16, 2025, 08:05:16 pm
content: Well the brochure doesn’t just show wood and cardboard, it also lists one specifically made for circuit boards.

author: Juggle
author_rank: Newbie
timestamp: June 16, 2025, 08:50:00 pm
content: the key word is:thixotropic

author: MarkF
author_rank: Super Contributor
timestamp: June 16, 2025, 09:01:30 pm
content: Why does the glue have to be 5mm thick?Can't it be some plastic waffer/washer that's 4mm thick with glue on both sides(i.e. pseudo thick double sided tape like construction).

author: Psi
author_rank: Super Contributor
timestamp: June 17, 2025, 12:59:06 am
content: Quote from: MarkF on June 16, 2025, 09:01:30 pmyes, could do that, just more effort. It's an option.This is just temp solution before pcb redesign to solve the issue.

author: coppercone2
author_rank: Super Contributor
timestamp: June 17, 2025, 01:21:12 am
content: 5mm = puttywhy not just use epoxy puttyThe NS glue won't do 5mm. I missed that.

author: tooki
author_rank: Super Contributor
timestamp: June 17, 2025, 07:26:08 am
content: Quote from: Psi on June 17, 2025, 12:59:06 amActually, what is the issue that needs solving? Only the manhandling that occurs during assembly? Or other aspects too?

author: Psi
author_rank: Super Contributor
timestamp: June 17, 2025, 11:36:57 am
content: Quote from: tooki on June 17, 2025, 07:26:08 amHad some units with dry joints on the part that looked like it might be vibration damage.But from the look of it the part had been bent over first creating a susceptibility to vibration.The part is usually held on by the conformal coat, but if it gets bent over during handling that breaks the coating and stretches the pins allowing for vibration to move the part. Keep in mind these are SIP parts. The pins are in a line and much closer to one side of the case than the other. So it's quite easy to bend them to one side vs the other.I have tested TECBOND 7718 on many units and it seems to be working very well. It's quite a hard glue once set and bonds well to the side of the part. Well enough to create an anchor on the side of the part where the pins are closest. Meaning the part is much harder to bend over in that direction.Also easy to apply and sets fast, so im 99% sure im going to go with TECBOND 7718

author: 5U4GB
author_rank: Super Contributor
timestamp: June 17, 2025, 01:16:29 pm
content: It sounds like what you're after is more some type of putty than a liquid glue. For example there are a whole range of epoxy putties that stay in whatever form you apply them in, the exact type would depend on what other properties you want from it. You can buy those cheaply and/or in relatively small quantities.