|Elliott Sound Products||Yellow Glue|
'Yellow Glue' is a topic that's mentioned (and not in a good way) in a great many websites and forum pages. It's common in Asian made electronics products, and is often used in large quantities to hold components onto PCBs. This is either to hold them in place during assembly, or to prevent movement when the product is used. If parts can move or vibrate, it's inevitable that eventually the component's leads will break due to metal fatigue. While I've used the term 'yellow glue' in this article, it's not necessarily yellow. Various other colours seem to be available (including black), but the vile yellow stuff seems to be the most common.
In some products it's used with gay abandon, but there's a hidden side to it that is not well understood by some of the people who comment on it. The simple fact is that it's fundamentally evil. After a period of time (which is extremely variable and depends on temperature), it becomes brown 'stuff', that's brittle and partially conductive. Many otherwise perfectly repairable pieces of equipment have been converted into landfill by yellow glue, because it destroys the very product it's supposed to render 'safe' by preventing component movement.
When new (or if never subjected to any heat), it's usually fairly hard, but you can create a small indent with the tip of a screwdriver or a fingernail. When it 'goes off' and turns brown or black, it becomes something else altogether. It becomes hard and breaks easily (s small piece that just broke off is visible in the photo below).
Most people won't come across this inherently evil substance, unless they buy finished products and take the time to pull them apart. Service techs are another matter altogether, as they will be confronted by it (and the damage it can do) on a fairly regular basis. While I am hopeful that this article might help someone, it's actually doubtful. Asian manufacturers won't change what they've been doing, and perfectly good electronics will be consigned to the tip for no good reason (other than that they failed because this foul adhesive killed them).
It's probable that few people know the exact composition of this dreaded yellow glue. Some info found on a Chinese website made the following claims ...
The yellow glue used in the circuit board is a kind of water-based adhesive, with a pungent smell, is a kind of soft self-adhesive gel, has excellent insulation, moisture-proof, shock-proof and thermal conductivity, so that electronic components operate safely under harsh conditions.
It is prone to curing, curing speed and environmental temperature, humidity and wind speed: the higher the temperature, the lower the humidity, the higher the wind speed, the faster the curing speed, and vice versa. When the coated parts are placed in the air, there will be a phenomenon of skin forming slowly. Please note that the operation should be completed before the skin forming on the surface.
Main functions: fixed inductors, coils, transformers, electrolytic capacitors, receiving first-class electronic products, with the function of protecting and sealing electronic components, which can be used for filling and sealing electrical components, filling and sealing high-voltage components, moisture-proof coating of circuit boards, etc. [ 1 ]
The above is verbatim. No mention is made of the actual composition of the adhesive, and there are zero warnings about its long-term effects - especially if it is allowed to get hot in normal use. Many people who are confronted by it are at a loss as to what it is (which is understandable), but there are some completely false assumptions made. It's not silicone (in any form), and nor is it a rubber-based contact cement. When used, it's common to see it covering a large area of the PCB (including parts that don't require mechanical support.
One thing we can be fairly sure of is that it's cheap. Quite possibly it's the cheapest glue available, which explains why it's used so often, and in such large quantities. One theory is that it's degraded by heat, becomes hygroscopic (absorbs water), and then becomes both conductive and corrosive. Degraded yellow glue is easily seen, because it's not yellow any more. It typically turns brown, and by then it's probably already caused some damage to the PCB and/ or the parts mounted thereon. To say it's a scourge is being way too polite! It's far worse than that. When it's used around low-level circuitry, a common problem is crackling and other random noises. On high voltage circuits, the results cab be catastrophic.
Figure 1 - A PCB With Yellow Glue
There is not much that needs to be said about the photo shown. The glue has turned brown (some very dark), and it caused intermittent relay operation (top right-hand corner, relay removed) on the board pictured. Near the potted inductors (black devices along the bottom), it's completely changed. Measured with a multimeter, the resistance is around 2MΩ with the probes a couple of millimetres apart, but if subjected to higher voltage this will change dramatically. I've heard and read literally countless reports of this crap converting otherwise repairable products into e-waste, but for some unknown reason, Asian manufacturers seem to be blissfully unaware of the problems it causes.
Figure 2 - Another PCB With Yellow Glue
The second photo shows the same trend, but not as advanced - at least on the surface. The inductor at the front of the board quite obviously needs support, but it would have been far better to have used a little more PCB real estate so it could lie down, fastened to the board with cable ties to prevent movement. Perhaps a little rubber 'cement' could have been added to ensure it remains supported throughout the life of the product. In this case, the inductor had to be removed and re-wound, as the leads had been corroded by the glue. Lead damage is exactly the thing that the 'glue' was meant to prevent !
One possibility that's often claimed is that the manufacturers continue to use yellow glue because it pretty much guarantees that any product using it has a finite lifespan. However, this is a false argument, because if a purchaser buys product 'A' and it fails well before it should, they are more likely to buy product 'B' next time, so there is no reason to expect that there's some sort of conspiracy involved. Of course, I could be mistaken, but I really don't think that there's any (deliberate) malevolence involved.
In general, I discount conspiracy theories. I prefer to rely on the adage that 'one should never attribute to malice that which is adequately explained by stupidity'. In this case, the stupidity is undoubtedly helped along because this 'glue' is almost certainly cheap. No-one is going to use a neutral-cure silicone adhesive when there's something else that sets quickly (important for mass production) and is a great deal cheaper. Likewise, epoxy adhesives are far too expensive, and the fast-setting (5-minute) types tend to be thermophobic (they don't like heat). Hot-melt glue is marginal at best, as it generally falls off flat surfaces eventually. Using it to attach anything to metal (high thermal conductivity) almost always results in failure. It may take a while, but it eventually just lets go.
People have experimented with various solvents to try to remove yellow glue, but there isn't much evidence that any of them actually work (other than anecdotal stories which don't qualify as evidence). Most people chip away at it until they can get access to whatever is beneath, but if it's already turned brown, there could be some major repairs lurking below. I have seen a video showing a solvent containing acetone and a number of other (nasty) chemicals that looks like it works, but some PCBs and/or components may not be overjoyed by being subjected to very strong solvents.
If a PCB liberally coated with yellow glue has any surface mount devices (SMD) underneath the glue, it's probably a write-off. It's unlikely that you'll be able to remove the glue without also removing the SMD parts underneath. Any strong solvent that removes the glue will almost certainly damage some parts, and may even attack the PCB substrate. Any plastic used to enclose parts such as relays, or the potted inductors seen in Figure 1 will almost certainly either be dissolved or damaged by strong solvents such as acetone, MEK (methyl-ethyl ketone), toluene, or any other 'industrial grade' solvents.
I have no idea where you'd buy this evil yellow glue (not that you'd want any), despite some serious searching. About the closest I found is something called 'Higer Bond', which states that it is indeed yellow, and sells for around US$30 per kilogram. That's pretty cheap, but I would guess that the dreaded 'yellow glue' is a great deal cheaper. I have no idea if 'Higer Bond' is an example of the stuff we see so often, or if it's something else entirely. The seller claims it has UL certification and is flame retardant, and it's sold as 'PCB circuit board insulation glue' (sic). There is a limit on how much time I can spend looking for something like this, especially since no-one in China will take any notice whatsoever of complaints from me or anyone else.
Ideally, large (and otherwise well regarded) companies who subcontract PCB assembly to Asia should complain bitterly, and insist that their subcontractors do not use this product, but there is almost certainly a 'don't care' culture. It's been a problem for many years now, but there is no indication that the problem is diminishing in any way.
This problem has been with us for many years, and shows no sign of slowing down or being corrected in new builds. At one point (many years ago apparently) a similar glue was used by Sony, and for a while it was called 'Sony glue'. Unfortunately, I can't find the reference any more. It's worthwhile doing a web search for 'yellow glue', as you will find many complaints (mostly on forum sites). This is a problem that's most commonly experienced by service techs, and the hobbyist constructor or layman user probably has no idea that it even exists.
However, it is important. So much so that I decided to write this short article, in the (almost certainly futile) hope that someone will take notice. When manufacturers use the wrong material for years on end, and cause countless pieces of equipment to fail prematurely because they did use the wrong material, then people should know about it. There's a lot of equipment that is routinely discarded after a few years (sometimes less) because a 'new' model is available, but professional audio gear is expected to have a long life. Both the photos seen above are from supposedly 'pro' audio gear, which users expect to last. If the evil 'yellow glue' is slopped over everything (whether it needs it or not), then when hot components are in contact with it, it will go brown, become conductive. It's no surprise that it also becomes corrosive, because any conductive (and hygroscopic) material is exposed to DC it will cause electrolysis. The positive (anode) will generate oxygen, and that will eventually eat its way through any conductors.
It's quite likely that whoever makes the dreaded yellow glue has indicated that it's suitable for attaching parts to PCBs. Either through misunderstanding, disregard for any kine of testing, and lured by the price, this crap can be found in countless pieces of electronics. Provided nothing gets hot, it might be alright, but it should be apparent that a better material should be used as a matter of course. When Asian manufacturers are pushed to the limit on price, it's to be expected that they will cut corners, and/ or use the cheapest materials they can get. Unfortunately, we will probably never know the real reason that this glue is so common, or why no-one has noticed (or they have noticed but don't care) that it becomes toxic to the very electronics it's supposed to support.
At this point, I can say that I've probably done as much as I can to make people aware of the 'yellow glue' problem. I don't expect that this article will change anything, as the scourge of the yellow glue has been happening for at least 20 years, and quite possibly many more. At the very least, when PCB manufacture is subcontracted to Asia, the maker who's name appears on the panels should ensure that no 'yellow glue' is used during production. Alas, this would probably increase the price by a few cents, and the corporate bean-counters don't like that .
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