Ground planes and desoldering - Page 1
Forum section: electronics / beginners

author: banedon
author_rank: Regular Contributor
timestamp: March 28, 2015, 07:16:40 pm
content: Hi guysCan anyone give me some advice on how to desolder faulty components which are soldered to a large ground plane?I've got some old motherboards (Acorn A3000, Amiga 500 and a few others) which I'm trying to diagnose and I cannot seem to heat up the legs of those components enough to melt the solder.I've got a Weller WHS 40D solder station which I typically use at 360 deg C and have a screw driver tip. I do my best to use the flat of the tip, but it doesn't seem to help. I have no issues soldering and desoldering newer stuff.I'm fairly sure this is just down to the ground plane, but I see in youtube videos people desoldering from such ground planes successfully. I do have flux coated solder braid, solder pump, etc., but the solder itself just isn't melting and I end I marking the PCB with the soldering iron tip eventually.Am I using the right temperature and/or an incorrect method?Any help much appreciated.

author: Howardlong
author_rank: Super Contributor
timestamp: March 28, 2015, 07:33:34 pm
content: A 40W iron may not be enough, but can you share a picture of the board and the parts you're trying to desolder?If there are no thermals on the pads you're trying to desolder, or it's multilayer (>2 layers) board it will add to the fun.

author: Jeroen3
author_rank: Super Contributor
timestamp: March 28, 2015, 07:46:10 pm
content: Gently heat up (part of) the board with a pre-heater or hot-air. Then use your iron for the last bit.A higher powered iron might also work. 80 Watts maybe.

author: electr_peter
author_rank: Unknown
timestamp: March 28, 2015, 07:53:07 pm
content: Check if solder iron tip is clean and coated with solder. Other than that you will need 3 things - power, power and I forgot the third.Problem is that there is too much thermal mass, too much heat dissipation to environment and too high temperature difference required to solder. Use more powerful soldering station, add additional soldering iron or preheat whole board to 100-150C.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 28, 2015, 08:00:04 pm
content: The only way I know of is to pre-heat the PCB with hot air. I have both a re-work station and a regular heat gun. The process takes patience, but gently (slowly) heat it up to around 150 to 175C, and a regular soldering iron should work just fine. To prevent damage to the PCB, slowly bring it back to under a 100C over the course of about 3 minutes or so. Fast changes in temp can damage other components.Also, once hot, try adding some fresh solder to the joint. Sounds counter-intuitive, but it will help. Have some solder on the tip when you make contact with the pin to get a good thermal transfer. A dry iron tip is terrible at transferring heat.Good luck.

author: banedon
author_rank: Regular Contributor
timestamp: March 28, 2015, 08:15:26 pm
content: Thanks for the replies and advice - much appreciated.So... I need to possibly preheat the PCB and/or get a better soldering iron.With regarding to the latter, would this one be suitable?http://uk.farnell.com/weller/wsd-81i-uk/soldering-station-80w-uk/dp/2393555It's an 80W and Weller (which I've always associated with being good).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 28, 2015, 08:29:32 pm
content: Preheat the board.Use a powerful soldering iron (temp controlled units are usually >60W).Set the iron for a modest temperature. Not so high that it burns the board (and the solder, and the flux, and the tip), but probably above nominal (which should be around 350C for leaded solder).Tin the joint as well as you can.The last one seems to slip peoples' mind, especially when using a desoldering iron. Well, why would you want to add solder just to remove it again? Because you can't remove anything if you can't get it hot enough, and you need a clean, wet (maybe even saturated and globby) pool to conduct heat. And a properly tinned tip to swim in that pool. Whether you're using a regular pencil iron, or a hollow desoldering tip, or anything else, you must always have a shiny, sticky, wet, tinned surface.In the end, it may turn out that it's simply better to rip the leads off and solder onto the nubs, rather than joust with a multilayer board. It can be very difficult to get a clean hole -- for example, it's pretty bad if you have to preheat the board so much that chip components start falling off, and electrolytics swell and burst, and so on...Braid is probably less useful on a multilayer board, because the joint becomes less conductive as the solder gets sucked out. So it can be partially or fully frozen on the opposite side, the wick sucks up the melted region by the iron, and now you're left with a hollow, one-side-capped hole that doesn't carry enough heat through.Tim

author: KJDS
author_rank: Super Contributor
timestamp: March 28, 2015, 09:08:26 pm
content: There are a number of ways forwards.A decent iron, metcal, JBC, ERSA and possibly the addition of a preheater.Only use Chemtronics wick, anything else I've tried is a waste of time. Add fresh lead solder.If you've a lot to do, then invest in a desoldering station,here comes the advertshttp://www.ebay.co.uk/itm/351330722332?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1555.l2649http://www.ebay.co.uk/itm/351343587874?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1555.l2649PM me for a deal on either.

author: kjn4685
author_rank: Regular Contributor
timestamp: March 28, 2015, 09:53:32 pm
content: A lot of the time they use lead free solder. So I flood the joint with a lot of solder and suck off with a desolding station. I hope this helps you.

author: banedon
author_rank: Regular Contributor
timestamp: March 29, 2015, 03:00:44 pm
content: Thanks again, guys.Unfortunately, my wallet won't currently support any large purchases, but I will keep an eye on the above and when I have enough cash I'll be in contact (provided that the stations have not been solder) - many thanks for the offer!

author: miguelvp
author_rank: Super Contributor
timestamp: March 30, 2015, 03:29:04 am
content: I replaced my hakko 936 clone (made by tenma) tip with a genuine hakko 908-CK conversion kit, converting the 907 medium iron to a 908 large iron.Maybe it's overkill for most jobs having such a large thermal mass, and I do at times switch back to the medium iron tip but rarely.The thermal mass prevents the iron from cooling down too much when you are dealing with a large ground plane.However, I'm not sure if weller has such a conversion kit.Edit: I'm trying to find a video posted here, where someone used a huge soldering iron, I mean big as in it can probably solder railroad tracks.

author: Richard Head
author_rank: Frequent Contributor
timestamp: March 30, 2015, 06:47:40 am
content: With large components such as electrolytics that are connected directly to the groundplane with no thermal relief I generally use two decent sized irons. It's a bit fiddly but results in no stress to the PTH pads at all, and it's quick. Being an octopus helps!

author: BradC
author_rank: Super Contributor
timestamp: March 30, 2015, 08:35:04 am
content: Quote from: Richard Head on March 30, 2015, 06:47:40 amI have a pair of Aoyue SMD tweezers with short & fat tips that work really well for caps. I've done the two iron thing too, but I bought these for SMD and the use for caps turned out to be a happy coincidence.

author: nanofrog
author_rank: Super Contributor
timestamp: March 31, 2015, 06:05:00 am
content: Quote from: banedon on March 28, 2015, 08:15:26 pmThat's a decent iron IME (I use the same WSP80 iron that comes in that configuration).If you're after a decent soldering station, you might want to look up a member by the ID ofTopLoser. He sells quality gear at well below MSRP (i.e. might be a demo unit for example, but it works and if it's missing any parts, he'll tell you beforehand).Stations offered tend to be Weller or Ersa from what I recall of his For Sale threads (Buy/Sell/Wantedsection). Had it in my head he carried JBC as well, but only located Weller and Ersa in a search.Some examples:FS (UK) ERSA I-Con VARIO 4 complete rework stationFS (UK): Weller soldering equipmentFS: Weller soldering equipment (>50% off list price) all like newFS (UK) ERSA soldering equipmentPM might be well worth the tiny bit of effort involved.PS, if you go into your user profile and set your country, it's flag will display beneath your userID. Makes things much easier for everyone regarding links for purchasing goods/supplies.

author: Evil Lurker
author_rank: Regular Contributor
timestamp: March 31, 2015, 06:44:49 am
content: To remove caps from double sided motherboards you do not suck the solder out of the joint, you add solder to it. Apply flux, then reflow the joint and get a nice little blob on each lead. Then heat one blob and just as soon as the joint liquifies gently rock the cap over towards the other lead. You don't want to pull the cap's lead completely out, in fact you just want it to move maybe a millimeter or so. Then go to the other lead and do the same. Usually takes me ~3-5 "rocks" to get a cap off. I do it two handed, left hand on the top of the cap rocking it back and forth, iron in my right hand, board dangling precariously on it's edge upright. Usually takes me around 5 seconds or to get a cap off with a cheap 30W soldering iron and the caps barely even get warm, even those little 5x11mm ones. In fact I usually just let them fall into the palm of my hand and only slow down when my left hand is full of caps. No torn traces or via's ripped out either.When you are done removing a cap, scrap up your "blobs" with the tip of your iron and move on to the next cap. After the caps are off, you should have some sorta flat looking filled in joints. Get like a 21 or 18 gauge or whatever fits the hole best hypodermic needle like they give cows shots with, take a sharp tipped iron and touch the tip to the joint to liquify it, and shove the needle through the hole gently. Try to line up the hollow part of the needle with the tip of your iron, makes it easier to get it started in. The idea is that solder doesn't stick to the stainless steel the needs are made out of so all you gotta do is just take the tip of your iron off the joint and let the solder solidify and carefully pull the needle out. Leaves these funky looking hollow lead stalagmite looking things on the opposite side of the board. Just trim them off with some flush cutters or nail clippers, whatever you have handy, then you are ready to put a new one in.

author: FreddyVictor
author_rank: Regular Contributor
timestamp: March 31, 2015, 07:39:42 am
content: I have the same station as OP, its fine for normal soldering, but does struggle when it comes to ground planesI also have a circa 1970's 30w iron with dimensions in keeping with components/circuits of that era ie 'chunky' and it has no problem with ground planes, so would agree with miguelvp regarding size of bit making a difference

author: electr_peter
author_rank: Unknown
timestamp: March 31, 2015, 10:28:12 am
content: Quote from: 3roomlab on March 31, 2015, 08:15:39 amIt is about temperature sensor position relative to the very tip, heating element position and thermal resistance. If there is significant heat transfer (for example, to the ground planes), temperature difference across this thermal resistance is very significant. This has to do with available heating power as well. Big name brands (Metcal, Weller, etc.) have sensing almost at the tip.Such situation is similar to having battery with high internal resistance. With no load, voltage provided is high (= high temperature), with high current load voltage is dropped (= low temperature).

author: Ian.M
author_rank: Super Contributor
timestamp: March 31, 2015, 11:03:32 am
content: I favour a 100W Weller soldering gun for those b*****d jobs involving large ground panes or copper shielding cans where you just need to dump as much heat into the joint as you can to get it to melt at all. Due to the design of the gun, the heat is produced right at the tip where you need it.Of course there's no temperature control, but that isn't a major issue as the gun is trigger operated, so you can blip the trigger enough to keep the joint melted without cooking the board. It might be interesting to add a type K thermocouple in contact with the back side of the tip, a temperature controller and a solid state relay for a fully temperature controlled solution.You wouldn't want to be doing SMD rework with a 100W gun, but I've done a fair bit of 0.1" pitch through hole stuff that way.I also second the suggestion of a mounted steel needle for clearing holes. Ideally it should be stainless, but ordinary steel is OK if it has a good polish and is kept ever so slightly oiled. Once you have a through hole, you can usually clean up with flux and solderwick without much trouble.

author: ConnorGames
author_rank: Regular Contributor
timestamp: March 31, 2015, 01:52:04 pm
content: Also, while chemtronics solder wick is nice (I use it myself), even dirt cheap solder wick will work fairly well if you apply some flux to it with a flux pen right before you use it. Flux is magic.

author: Ian.M
author_rank: Super Contributor
timestamp: March 31, 2015, 02:13:54 pm
content: Quote from: ConnorGames on March 31, 2015, 01:52:04 pm+1Heck, fresh untarnished plain copper braid off RF coax cable, stretched and flattened works well enough with a good flux. It even works with DIY liquid flux - a concentrated solution of pale rosin in 95% or better Alcohol.

author: Evil Lurker
author_rank: Regular Contributor
timestamp: March 31, 2015, 09:12:03 pm
content: Quote from: Ian.M on March 31, 2015, 02:13:54 pmYou can even use the tin or whatever plated stuff off of any junk cables like VGA, parallel, whatever that would normally be headed to the recycling bin as long as the base metal is copper. Just push it together to expand it (think Chinese finger cuff toys kids used to play with back in the day), give it a quick soak in ferric chloride to strip the tin off of it, rinse it off good, stretch it back out tight, and whack it with a hammer. Expanding it gets the inside of the soon to be solder wick exposed to the ferric chloride so it will strip. Hammering it after stretching it mushes it back flat and exposes clean metal and assists in wicking action. Depending on how thick the wires are that make up the shielding wicking action and how much solder it can hold will vary but I'd say it's about 70-80% as good as commercial stuff for essentially a few coins worth of chemicals per meter and some time. Just make absolutely sure it is plated copper and not aluminum. Suffice to say the reaction of aluminum in ferric chloride is just a little bit vigorous.

author: Ian.M
author_rank: Super Contributor
timestamp: March 31, 2015, 10:13:41 pm
content: That would tarnish fairly rapidly and is more hassle than I care for. If the braid is tarnished, it isn't worth salvaging. If its tin plated, that doesn't need stripping as if anything it encourages the solder to wet and wick into the braid.One thing that is worth doing is dipping the salvaged braid into the DIY rosin flux and letting it air dry, preferably under tension wrapped round a non-porous cylindrical object to keep the braid as narrow and flat as possible. The thin coat of rosin virtually stops tarnishing for many months, helps the wick hold its shape in storage and provides flux when its used.I'm not saying anyone should stop buying Chem-Wick or similar high quality solder wick - especially for the smaller or more delicate joints, but for the heavier jobs where there is a lot of solder to mop up, pre-prepared DIY solderwick is a good substitute for heavy commercial solderwick.