What is this black blob on PCB? - Page 1
Forum section: electronics / beginners

author: Fishplate42
author_rank: Contributor
timestamp: May 23, 2012, 04:08:11 pm
content: OK it is dumb question time...I have just taken apart a cheap USB hub (for the sockets and lead) and I now have the PCB left with a few SMDs and an LED bit what is the black blob of epoxy-looking stuff for? As you can see I have had a go at getting it off but it is stuck like you-know-what...Here is a picture of the blob!Thanks!Ralph.

author: Bored@Work
author_rank: Super Contributor
timestamp: May 23, 2012, 04:22:14 pm
content: Below is the IC that performs the hub's function.

author: Monkeh
author_rank: Super Contributor
timestamp: May 23, 2012, 04:24:29 pm
content: Big old lump of epoxy to stop the evil people who'll copy their reference implementation of a board for a bog standard chip which requires virtually no external components. Because clearly that saves them lots of money to cover the cost of the epoxy.

author: Fishplate42
author_rank: Contributor
timestamp: May 23, 2012, 04:36:18 pm
content: Quote from: Bored@Work on May 23, 2012, 04:22:14 pmAh! That makes scene - thank you! Is there any way of telling what the IC is? or is it just educated guess time. Would I be right in thinking it must be some kind of amplifier?I also have a - don't laugh - a plastic frog that came from the same pound shop and that has a sensor in its mouth plus a little PCB with a similar 'blob' on it there is a picture on my bloghttp://ralphsworkshop.blogspot.co.uk/. When the frog detects movement it crokes three times and stops. The blob is covering the IC controlling the 'croke' so what might that be? or is there no way of knowing.Thanks again,Ralph.

author: T4P
author_rank: Super Contributor
timestamp: May 23, 2012, 04:39:03 pm
content: It might be a bare silicon chip just epoxied to save costs, seen on many cheapo autoranging meters

author: Bored@Work
author_rank: Super Contributor
timestamp: May 23, 2012, 05:01:36 pm
content: Quote from: Fishplate42 on May 23, 2012, 04:36:18 pmNot easily in this condition. One could try a guess from the PCB layout. Or experts could resolve the blob and check the bare chip under a microscope.The easiest way would have been while the hub still worked. A USB hub, like all USB devices, can be asked for certain details, and this can sometimes reveal the chip, especially for no-name products.QuoteNo, USB hubs are not simple amplifiers. You could think of the chip more as a very specialized processor for processing USB data. If you want to learn what a hub must be capable of doing, then you could check the USB specification (lots of paper).QuoteSome specialized sound chip, maybe similar or even identical to those one can find in electronic greeting cards. Chinese manufacturers are very creative when it comes to use those chips. For example, there were flickering electric candles on the market. Some clever soul figured the flickering was not really random. When reverse engineering the candle people found out the manufacturer used a greeting card chip. Instead of having the chip driving a speaker it drove the candle's light, resulting in rhythmic flickering.The technique of using such a naked chip protect by a blob is called Chip on Board (CoB). CoB is not really used to protect design from reverse engineering, but because it is cheap if you produce very high volumes.

author: armandas
author_rank: Frequent Contributor
timestamp: May 23, 2012, 08:39:54 pm
content: Quote from: Bored@Work on May 23, 2012, 04:22:14 pmIf you're lucky and didn't get afake hub, that is.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 23, 2012, 11:35:26 pm
content: Quote from: armandas on May 23, 2012, 08:39:54 pmcontinuity testing might help?

author: codeboy2k
author_rank: Super Contributor
timestamp: May 24, 2012, 02:11:04 am
content: Quote from: armandas on May 23, 2012, 08:39:54 pmseems legit.

author: rsjsouza
author_rank: Super Contributor
timestamp: May 24, 2012, 03:37:09 am
content: QuoteJust to have an idea on what is involved on a USB HUB, check the datasheet of a "stock" device:http://www.ti.com/product/tusb2046bQuote from: DaveXRQ on May 23, 2012, 04:39:03 pmThis is known as "die on board" - an IC that is cut from the silicon wafer and glued to the board. If I am not mistaken its boom started with the japanese digital watches of the 80's. It saves board space and a lot of money due to packaging.The link below has some drawings that give a better idea:http://www.google.com/patents/US5959247

author: amyk
author_rank: Super Contributor
timestamp: May 24, 2012, 10:33:18 am
content: Quote from: armandas on May 23, 2012, 08:39:54 pmWow. At first when I read "didn't have any chip inside, only tracks" that wouldn't be a sign of anything wrong, since the actual die is tiny and would probably come off along with the bond wires, leaving only the pads behind. But the way the silkscreen contradicts the actual arrangement and soldering would've been my first sign of something suspicious, and the way the crystal is connected would never work.Even if there was a real die and everything in there, the track layout wouldn't make for any reliable communication at USB 2.0 speeds. There might've been a 1.1 hub with a similar design that they copied for making this "USB splitter".Here's a (despite the caption) real hub that's missing all components except the sockets:http://www.flickr.com/photos/drug123/5127906425/#Notice the pads for the bond wires are there.

author: CyberWalker
author_rank: Contributor
timestamp: May 24, 2012, 10:39:01 am
content: I've tried (on dead circuits) to see what is afterall under all these epoxies... and yes, most of the times they are just bare ICs!

author: T4P
author_rank: Super Contributor
timestamp: May 24, 2012, 10:40:51 am
content: Quote from: amyk on May 24, 2012, 10:33:18 amWow ... just wow

author: Fishplate42
author_rank: Contributor
timestamp: May 27, 2012, 04:05:50 pm
content: I now understand what the blob is and I am now experimenting with these pound-shop novelties. I know this may seem a bit trivial but it does give me something to experiment with and not spend a fortune. I have already learnt a lot from this post alone.Thanks for all your help - this really is a great place!Ralph.

author: Codemonkey
author_rank: Regular Contributor
timestamp: May 28, 2012, 08:42:48 am
content: For what its worth, its a chip packaging method known as "Glob Top".

author: metalphreak
author_rank: Frequent Contributor
timestamp: May 31, 2012, 11:10:13 am
content: Glob Top or Chip on Board (COB) packaging.Mass producing stuff on small PCBs like that, the marginal cost savings add up to quite a large amount. Rather than wirebonding the IC die to a plastic pin package, which is then soldered to the board, the die is wirebonded directly to the PCB and covered in epoxy for mechanical strength etc.See the second page of this:http://www.fkdelvotec.at/files/press/COB_E2.pdfIt's quite common in mass produced cheap products. Pretty much every character LCD i've seen has the lcd driver chips as a COB.