Motherboard hot air rework, solder not melting - Page 1
Forum section: electronics / beginners

author: haggar
author_rank: Contributor
timestamp: March 29, 2016, 09:10:06 pm
content: Sorry if this is a dumb question, but I'm a super newbie!My hot air rework station arrived today, an 858D cheapie from ebay and I was keen to try it out so I took out an old broken PC motherboard for some practice. I picked a likely target, a small SMD capacitor and set the station to almost max air, 200c. The solder refused to melt, I've tried max air with between 200-300c, still not melting.As I'm new I don't really want to just jack up the temperature to crazy levels, but I'm guessing it's some kind of super solder.Any suggestions or tips would be greatly appreciated.Thanks

author: orbiter
author_rank: Frequent Contributor
timestamp: March 29, 2016, 09:24:35 pm
content: Nope not super solder.. Just many large copper layers within the PCB that's wicking the heat away real quick, thus preventing the solder from melting. For itemslike multi-layer motherboards you need quite a bit of heat, normally using a pre-heater to bring the board up to temperature first beforeusing the hot air to remove components.

author: haggar
author_rank: Contributor
timestamp: March 29, 2016, 09:29:51 pm
content: Ahh OK thanks for the info. In the absence of a pre-heater, any suggestions on a temperature and duration I should try?Thanks again.

author: mariush
author_rank: Super Contributor
timestamp: March 29, 2016, 09:32:11 pm
content: Like orbiter says, motherboards have several layers with traces of copper that absorb heat and spread it, it's not uncommon to have 6 layers or more.In addition, the part you're trying to desolder AND the fiberglass in the pcb also absorb some heat. The lead free solder used on pcb goes liquid when the temperature goes above 217 degrees Celsius, so your hot air gun has to be above 217c by a substantial amount, to fight against copper in the pcb layers absorbing heat - a small smd capacitor won't absorb much but if you were to try to desolder a mosfet or a larger chip, even the part itself would absorb a substantial amount of heat.You can use a hair dryer to heat the motherboard from the bottom to heat up the whole area below the part and when it's nice and warm you can bring your hot air gun (you don't want to blow hot air on components for minutes at a time, you'll damage them) and apply the extra heat required to go over that 217 degree Celsius threshold - 300 to 350c for about 5-10 seconds would do the job if the board is preheated.Also as a tip: it helps A LOT if you add a tiny amount of liquid flux to the joints.ps. lead based solders have a much lower melting temperature (around 180C for 60/40, 183C for 63/37) but most commercial stuff uses lead free solder which is supposed to melt at around 217C.ps2 i said hair dryer because it's something often easy to find in a home. I've seen lots of service repair guys useheat guns (for paint stripping or other uses)to quickly warm up boards , for example when you need to remove a single LED from a TV backlight, a led that's soldered to an aluminum based pcb which acts as a heatsink making it hard to desolder the led without burning the plastic around the led.See this video as an example, at around 3:00

author: Iwanushka
author_rank: Frequent Contributor
timestamp: March 29, 2016, 09:32:33 pm
content: hi,1) hot air temp depends on board's thermal design, the more copper the more temp you will need.2)don't trust that temp reading it's "random" temp depends on actual nozzle and distance from the component e.g. 1mm from nozzle you might have 400C and 3mm from nozzle you can have 200C.3) try adding some flux and increase temp to around 350C.P.S. from my experience that station is crap, tried to remove some dpak from 8 layer board, at max temp (480C)/air -> 30min nothing, then I thought okay my hands are crooked, but to make sure bought hakko station and guess what 300-350C with air set to 3-5 out of 9, you can remove everything in 5-10sec.. conclusion: my hand are still somewhat working and that cheepie is good for two layer boards without huge ground planes and of course heat-shrinking tubes

author: haggar
author_rank: Contributor
timestamp: March 29, 2016, 09:42:19 pm
content: Thanks for the info!I realise this thing isn't going to be great. Most of the stuff I'll be doing in reality should be on much thinner layer boards, I just thought I'd give the motherboard a go for a bit of practice.I'll give the hair dryer a go tomorrow, see how I get on!

author: MarvinTheMartian
author_rank: Frequent Contributor
timestamp: March 29, 2016, 11:11:19 pm
content: G'day Haggar, welcome to the forum.You might want to check out your Atten/WEP/etc 858D for safety issues to protect yourself!Seehttps://www.eevblog.com/forum/chat/deadly-wiring-fault-atten-858d-hot-air-rework-station/msg52447/#msg52447See also Dave's video

author: epiflow
author_rank: Contributor
timestamp: March 30, 2016, 12:07:56 am
content: Quote from: mariush on March 29, 2016, 09:32:11 pmI second the hairdryer. It's basically a poor man's heat gun. It is best used in conjunction with the hot air station on the surface to make removal easier. You need to rig it up so that the hot air from the blow dryer hits the bottom of the printed circuit board (PCB); directly under the part you are removing. You can also add a little more solder to the existing pads/joints via a soldering iron to help soften the old stuff.

author: haggar
author_rank: Contributor
timestamp: March 30, 2016, 12:32:48 am
content: Quote from: MarvinTheMartian on March 29, 2016, 11:11:19 pmHowdy!Thanks for the info, I'll have to check mine out.