How NOT to Reflow a PCB - Page 1
Forum section: electronics / beginners

author: logictom
author_rank: Unknown
timestamp: February 17, 2012, 02:02:53 pm
content: I reflowed the PCB and all was working but one chip so I thought I would heat it up again because I didn't leave it in long the first time - I forgot about it and turned around to see the magic black goo escaping from the board....Won't be doing that again in a hurry, quite a bad stink in the room for the next few hours

author: grumpydoc
author_rank: Super Contributor
timestamp: February 17, 2012, 03:56:33 pm
content: Oops!

author: sacherjj
author_rank: Frequent Contributor
timestamp: February 17, 2012, 03:58:56 pm
content: A well done board indeed.

author: harnon
author_rank: Regular Contributor
timestamp: February 17, 2012, 07:09:30 pm
content: You win some, you lose some

author: george graves
author_rank: Super Contributor
timestamp: February 17, 2012, 10:32:06 pm
content: It would be funny if it still worked.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 18, 2012, 03:05:16 am
content: thats really gross. where did the black matter comes from? i suspect you over-temperature cook it.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 18, 2012, 05:40:37 am
content: Did you use an oxy-acetylene torch?Maybe not the cutting tip next time!

author: mobbarley
author_rank: Regular Contributor
timestamp: February 18, 2012, 05:54:39 am
content: Quote from: Mechatrommer on February 18, 2012, 03:05:16 amIt happens when you overheat fiberglass pcbs. I believe it is the epoxy that they are laminated with.

author: sacherjj
author_rank: Frequent Contributor
timestamp: February 19, 2012, 12:14:19 am
content: Quote from: george graves on February 17, 2012, 10:32:06 pmI remember back in the 90's, when I was in high school, I hooked up an LM555 wrong. I melted the plastic shell of the chip and the plastic of the breadboard. I was amazed that the chip still functioned once I hooked it up correctly.

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2012, 09:34:22 pm
content: That's definitely the advantage of old ICs such as the 555 timer, 741 and some old TTL ICs: there practically indestructible. Newer ICs are much more sensitive due to the smaller die size and use of MOSFETs which had a metal oxide junction that can be zapped at voltages as low as 20V. Older ICs are tougher because when hit with ESD BJTs can go into zener breakdown and not be harmed if the current is low enough and the larger die size has a longer thermal time constant so they can withstand more energy.

author: grumpydoc
author_rank: Super Contributor
timestamp: February 19, 2012, 09:44:10 pm
content: QuoteIndeed - the opposite is usually true, especially when youneedto get lucky..When I was about 17 I had a Z80 based computer that I'd build from a kit. A "Nascom 2" for those with long enough memories.Anyway I'd somehow been invited to a "personal computer demonstration" that the local council had organised. You have to remember this was in the late 70's so "personal computers" were pretty novel although the father of a friend of mine brought his Exidy Sorcerer - which we though extremely grown up and professional; I digress.One feature of the Nascom was that the RS232 interface was next to the keyboard interface - both anonymous headers, both the same number of pins and - of course - no keying or anything like that. The important difference was that the RS232 header had 12v on it - somewhat lethal to TTL.I was late and in my hurry guess what - you got it keyboard in the wrong place and one of the a 7400 on the PCB getting rather warm as it headed off to the keyboard in the sky. Sadly it wasnotas robust as the aforementioned 555 and was as dead as a dodo when the keyboard was plugged in correctly.In the end a sprint to Tandy(== Radio Shack in it's UK guise) and a borrowed soldering iron saved the day. That was when I learned that jamming a screwdriver under a 14 pin DIP and levering upward hard, then desoldering the individual pins was a valid technique when you're in a hurry!I was always a bit more careful after that (still managed my fair share of mis-pluggings though).

author: vk6zgo
author_rank: Super Contributor
timestamp: February 20, 2012, 12:55:55 am
content: As I drove under the carport at work,I heard a clink!,clink! clink! sound.The culprit was a 2N3055 which I had driven over,& which had become wedged in the tyre (luckily the oins weren't long enough to give me a flat tyre!)The poor old 2N3055 was in a horrendous condition,with the case squished.When I took inside & tested it,the thing worked!

author: dcel
author_rank: Regular Contributor
timestamp: February 20, 2012, 03:19:15 pm
content: Them 2n3055's are some tuff stuff, even abused electrically, they will survive. An old Meyers amp used nothing but 2n3055s and was used as a rental amp. It went out on a rental once with some dual 18" cabinets and came back with the 18" drivers having been set on fire! Sure that the amp had blown up, on to the work bench it goes, and to my supprise, worked perfectly. Man those things are tough. That amplifier took more customer abuse than should have been possible, the venerable 2n3055. Just about bullet proof.Chris

author: sacherjj
author_rank: Frequent Contributor
timestamp: February 21, 2012, 06:42:59 pm
content: Quote from: Dave.S on February 21, 2012, 05:56:23 pmIt takes but one mislabeled pin on a schematic to let out the secret smoke. However, sometimes all isn't lost if you are using a current limited supply.

author: metalphreak
author_rank: Frequent Contributor
timestamp: February 22, 2012, 12:51:53 am
content: Your standard 595 shift register will go up to ~11-12V still working before it burns a hole in itselfThey're only rated for 5.5-6v