Computer Power Supply Tear Down - Solder doesn't want to melt! - Page 1
Forum section: electronics / projects

author: DW1961
author_rank: Frequent Contributor
timestamp: April 22, 2023, 09:40:39 pm
content: I'm trying to remove all of the components from an old computer (Atenc) power supply I found and I'm having trouble desoldering the components from the PCB it. It's almost as if the solder they used is some sort of high heat never melt stuff. I have my iron set to 330C and it melts my Kester 63/37 on contact. I can drop some of the Kester on one of the PSU solder joints, and it does kinda melt, but just briefly and never really flows.Any ideas other than cranking up the station to fusion power levels and destroying my Hakko cartridges or 900 tips?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 22, 2023, 09:46:32 pm
content: Clean dust off the board, and then apply flux. Likely the solder is lead-free; add leaded solder to help soften it if you're still having problems.Your iron tip may also be crusted over. Make sure it's clean, shiny and wettable.Tim

author: langwadt
author_rank: Super Contributor
timestamp: April 22, 2023, 09:50:13 pm
content: 330 is on the low side especially if lead free

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 22, 2023, 09:56:38 pm
content: Old? How old? Capacitor plague old? I just finished re-capping a few boards where the electrolytic caps leaked and that corroded the copper, as well as doing something to the solder. It was difficult to melt.These were single-sided boards and the capacitor juice went directly down the unplated via to eat the copper and modify the solder. A power supply would also likely have single-sided boards?Check it out: you see that kinda blistery-looking solder mask around those solder joints? Cap on the other side leaked, those joints were harder to melt. But as stated above just plop on some flux and fresh solder.

author: wraper
author_rank: Unknown
timestamp: April 22, 2023, 10:20:23 pm
content: 330oC is really low for lead free, especially for solder joints with high thermal mass. Also you need relatively large tip for those.

author: DW1961
author_rank: Frequent Contributor
timestamp: April 22, 2023, 10:47:12 pm
content: Quote from: Alex Eisenhut on April 22, 2023, 09:56:38 pmNot seeing that at all, and I'm trying to melt on the opposite side of the board, which looks clean.

author: TimNJ
author_rank: Super Contributor
timestamp: April 22, 2023, 11:08:13 pm
content: I go close to 400C (750F) for stubborn joints with a lot of thermal mass behind them. Flux will help conduct heat from the iron to the board.

author: DimitriP
author_rank: Super Contributor
timestamp: April 22, 2023, 11:09:47 pm
content: Quote from: langwadt on April 22, 2023, 09:50:13 pmSince it seems it was missed the first couple of times around....."330 is on the low side especially if lead free"From the kester datasheet for Kester 24-9574-1402"Solder iron tip temperatures are most commonly between371 to 400 °C (700 to 750 °F) forlead-free alloys."

author: DW1961
author_rank: Frequent Contributor
timestamp: April 22, 2023, 11:26:58 pm
content: Quote from: DimitriP on April 22, 2023, 11:09:47 pmDamn that seems hot, since I have never used lead free. I think the first time that specific power supply was offered was 2010 and i don't think it is even available anymore. When I took it apart, it only had AWG 18 wire in it, even the 12V side. (750w Power Supply)Do you now if manufacturers were even using lead free back then?In any event, I'll crank it up and see what happens.

author: wraper
author_rank: Unknown
timestamp: April 22, 2023, 11:32:37 pm
content: Quote from: DW1961 on April 22, 2023, 11:26:58 pmDo you live in a cave? Most of electronics had gone lead-free almost a decade before that.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 22, 2023, 11:37:56 pm
content: When I repair ATX power supplies (usually only recapping) with my T12 soldering station (with a large tip) I rarely use temperatures higher than 330-350 C. I mean I need 350-360 C only for some really large 'copper-fields'. Typically just a large tip and 330 C (and a flux) 'fits it all'.Unsoldering electrolytes from motherboards usually is harder and more often needs 350-360 C. (I don't have a preheater table).P.s.: My soldering statiom may be is not calibrated really accurate, so I allow some +-20C error.

author: DW1961
author_rank: Frequent Contributor
timestamp: April 22, 2023, 11:56:36 pm
content: Quote from: wraper on April 22, 2023, 11:32:37 pmYou mean manufacturer stuff, not for hobby? And, no, I didn't know that manufacturers started using lead free in 2003. But, then again, i don't really keep up on the soldering industry.

author: DW1961
author_rank: Frequent Contributor
timestamp: April 23, 2023, 12:06:57 am
content: Quote from: Vovk_Z on April 22, 2023, 11:37:56 pmI probably need a larger tip for desoldering. What size do you use?

author: DW1961
author_rank: Frequent Contributor
timestamp: April 23, 2023, 12:08:25 am
content: Quote from: DimitriP on April 22, 2023, 11:09:47 pmIncreasing my station to 390 did it, but 400 would probably be even better. I think I may need a bigger tip than 2.4mm for de soldering of this type/size, also.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 23, 2023, 05:14:10 pm
content: Quote from: DW1961 on April 23, 2023, 12:06:57 amI don't remember the correct size, possibly T12-BCF3 or C4, but anything 3..4 mm should be fine: BC3, BCF3, C4, D4, etc. I prefer the 'BC' type for large tips but C or D possibly will work too.For example, T12-D24 is considered as a 'universal' tip and it is not big enough for this hard job.

author: mariush
author_rank: Super Contributor
timestamp: April 23, 2023, 05:46:33 pm
content: I found adding a drop of liquid flux helps a lot often. The existing solder is oxidized, there's a layer of oxides and crap over the existing solder, so even if you bring in the hot soldering iron, those oxides and crap act as a heat shield reducing the heat transfer.As others said, adding a bit of leaded solder will help as it lowers the melting temperature of existing temperature but also acts as a heat bridge between the old solder and the iron tip making heat flow faster.In power supplies there's also often wires soldered to such wide traces which can "suck" heat away, and there's often extra solder on top of traces to thicken them, so you do need a higher iron tip temperature. 350-400c is not uncommon for power supplies.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 23, 2023, 07:42:18 pm
content: Quote from: DW1961 on April 23, 2023, 12:08:25 amI lost one good D24 universal tip just doing that when I haven't had the required larger tip so I just increased the temperature. That puts more stress on a heating element and shortens its life. After losing a good tip I went to the shop and bought the needed ones sizes. So now I have three sizes: 1) small BC1.5 (??) for SMD, middle size universal D24, and 3) large BC3.

author: DW1961
author_rank: Frequent Contributor
timestamp: April 23, 2023, 07:47:37 pm
content: Quote from: Vovk_Z on April 23, 2023, 05:14:10 pmI have a Hakko 3mm BC tip for my 900 series. I can use that.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 23, 2023, 09:36:13 pm
content: Quote from: DW1961 on April 23, 2023, 07:47:37 pmI guess T12-D24 is superior to any 900 series clones.

author: DW1961
author_rank: Frequent Contributor
timestamp: April 24, 2023, 06:52:45 am
content: Quote from: Vovk_Z on April 23, 2023, 09:36:13 pmI have the 900 series and tip though so might as well use it for desoldering.

author: james_s
author_rank: Super Contributor
timestamp: April 24, 2023, 07:00:56 am
content: Quote from: DW1961 on April 22, 2023, 11:56:36 pmROHS legislation mandated lead-free solder for manufactured goods back in 2006. It was around long before then though, I think I first saw it in the late 90s. Never cared for it personally but it certainly isn't new.

author: Shock
author_rank: Super Contributor
timestamp: April 24, 2023, 11:32:59 pm
content: Along with a suitable temp, size matters. Here is a photo I took a while back to show ideal tip to pad size. Though you can get nice joints from heating with smaller tips, in the 80-120% range with a good contact area is the go to for effectively heating a pad.Specifically for removals - higher temp alloys, surface oxidization, old flux, solvents, contaminants and board coatings can make reflowing more challenging. Removing coatings and cleaning the joints along with using a higher activating (more aggressive) flux can help burn off the oxides/contaminants. Reflowing in a different alloy such as leaded solder can improve liquidity as it alters the existing alloys properties while refreshing with new flux.Oxides can quickly form on a dry iron tip. The tip should take solder easily over its tinned surface and appear shiny after cleaning, if not it requires further attention. When you reflow with fresh solder pulse feed it between the junction of the iron tip and pad to get a good thermal link established.