Soldering potentiometer to PCB disaster

Discussion in 'General Electronics Chat' started by solderboy, Nov 12, 2014.

  1. solderboy

    Thread Starter New Member

    Nov 12, 2014
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    Hello, I'm new here.

    So I had a lose potentiometer on an electronic keyboard. Opened it up and got inside to the PCB of the potentiometer. The problem was it was wiggling and would mis dial by just touching it. So I figured I've soldered in the past, so I will just remove the current solder and solder it back afresh. This is when hell started to progressively break lose. First, the solder wouldn't melt from a 60 watt iron, except when pressing firmly against the joints, in which case only a spot right around the solder iron tip would melt, so there was no way of cleaning up the joint. What ended up happening was I placed the iron tip on top of a potentiometer's 'leg' on the opposite side of the pcb, and held some solder next to the leg, thinking the heat would transfer to it, but it didn't really, or it did but the solder curled up rather than flow. Thought I did everything right by tinning the iron tip and dipping it in flux, albeit some weird type of flux.

    I should mention that I had some flux, but it was hard as dry clay. The flux package reads "service welding flux". I figured flux is flux so I dipped the iron into it quickly, but am not sure if it did any good. It kinda looks like it got burnt on the iron. And solder wouldn't wrap around the tip, but not sure if that's because the solder wire was quiet thin, or if it's because of a wrong flux.

    So even after holding the iron on top of the leg for several seconds the solder next to the leg still wouldn't melt (forget about the old solder that seemed like something else), so I resorted to holding solder right to the iron tip hoping it would at least melt it and add some to the old joint. Well, the joints now look rough and spiky, for one. For some reason nothing I did would melt the solder smoothly. Cleaning the iron with a sponge didn't seem to help any, nothing I did helped, which is one part of the problem.

    So I thought that the PCB has some coating, which flux would normally remove, and had read that you can use some solvent to remove it. I sprayed some 70% isopropyl around the potentiometer's joints, which seemed to leave permanent spots on the PCB. I then went back to brute soldering just to get the darn potentiometer to at least stick. The final solder joint looked very ugly, but the joints did seem to hold the unit in place. So the isopropyl seemed to have done more harm than good since the old joints still wouldn't melt except right beneath the iron tip after a few seconds of pressing it against the joint.

    That is when the next problem happens. When I switched on the keyboard to test, the potentiometer didn't wiggle but didn't work at all.


    So my questions are:

    1. What kind of solder is that factory made kind? Is it the solder or a coating that prevents it from melting? How would you remove it?

    2. Since the potentiometer stopped working, would you say it's because the isopropyl damaged the circuit lines, or do you think the potentiometer got overheated when I held the iron on its legs for up to 10 seconds straight (since the heat just wouldn't transfer to and melt the solder), and that that is the more likely reason for it not working?

    I hope for the latter, since replacing a potentiometer is cheaper than a new PCB, if I can just figure out how to remove the old solder joints.
     
  2. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    Could it be that the legs were soldered to pads that were fairly large? Large pads are very hard to heat up sometimes because they draw the heat away form the iron so fast. So a larger iron might work, or hot air.

    They may have used some special solder but i cant see why, unless it was something other than solder. if you could remove it all perhaps you could add your new solder. If you melt some new solder onto the old joint it may help. If you cant do that then it sounds like the iron isnt hot enough or something really unusual is happening.
     
  3. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Lead-free solder melts higher than lead-containing solder. I agree that your soldering iron is probably too cold. 60W should be more than enough power for a typical, small potentiometer. Can you change its temperature? Do you have access to a Variac? If so, try running it at a little over 100%.

    As for the flux, a brief search for "service welding flux" found things mainly related to submerged arc welding. We can be almost certain your pot was not welded to the board using submerged arc. ;) Flux used for welding will probably not work. Not all flux is the same. You need flux for electrical soldering.

    As for the various coatings on your board...solder resist is the stuff (often green) that covers everything except were there is solder. It is usually an epoxy and is not soluble in any solvent. Then there can be a conformal coating. That will cover the board and soldered joints. It is sometimes an acrylic lacquer and is soluble in solvents that will dissolve lacquer.

    John
     
  4. solderboy

    Thread Starter New Member

    Nov 12, 2014
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    Well, I wondered too if the solder joints might be large, or the iron too small, (likely the latter, it's a pretty thin tip). While I could definitely try with a larger iron, the issue remains that the potentiometer doesn't work at all now. I'm wondering was it because I sprayed 70% isopropyl on the PCB and the joint which damaged circuites, or was it that I over heated the potentiometer by holding the iron tip to its leg for too long, thus burnt its internals? What's more likely?

    Not sure what they used as solder, but one thing I know it is that all the joints look very evenly shaped and sized, so at least they've used a robot, but now whether it's not solder or is solder is another question. Thought maybe it's a coating, but how could a coating withstand a solder iron's heat that well. Or it might be as you said that the iron is too small or the pad too large, may well be that since the iron tip wouldn't melt much solder wire at once at all, even though it's a thin solder wire. I did solder new solder to the existing joints but it ended up a spikey mess, like ice spikes, and wouldn't flow aat all, more like curl, even though I cleaned the tip in between, or so I thought. And the flux, don't know what that is it is hard and brittle, not creamy like usual. So two things: factory solder is very hard to remove, and potentiometer stopped working altogether after my solder attempt.
     
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Soldering iron power and temperature are two different things. A 60W soldering iron should be big enough. Are you sure it is 60W? If so, then it is probably the temperature or the tip of the iron is shot.

    As for the pot, it is possible that the alcohol dissolved something that got into the pot and ruined it. Once you get the pot off the board, you could test it. If it reads "open" or way off, it is probably ruined, but you could try dipping it in fresh isopropyl alcohol to see if it can be resurrected. Not all isopropyl alcohol is the same. What purity is yours? What color is it? (That may seem an odd question as IPA should be water white and clear, but I have seen rubbing alcohols at a pharmacy that were dyed green.)

    John
     
  6. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi again,

    Try soldering two pieces of solid #12 AWG wire together or at least #14 AWG wire. If your iron is 60w then it should work i think.

    Some pots have an element that is some sort of semi conductive plastic. Too much heat could damage it. As John says, once you get it out test it with an ohm meter and see if it is bad or not. Replace if necessary, maybe with a better iron.

    If you take some close up pics it might help us analyze what is really happening. They have to be clear pics though.
     
  7. solderboy

    Thread Starter New Member

    Nov 12, 2014
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    First thanks for the answers. I have some pics of the PCB but not the potentiometer itself. Keyboard's been reassembled, and the pot is under the lower PCB which takes a while to reach. The isopropyl was 75% btw, and clear liquid. I think I was supposed to use 90% but didn't have any handy so gave it a shot. It seems to have corroded something on the PCB, such as the coating or something, since there are spots where the alcohol touched which didn't come off with wiping.

    The iron is new. Not an expensive one. I wonder though whenever cleaning the tip with a sponge, it seems some oxidation remains, and sometimes even fibers from the sponge stick to it. Hopefully I tinned it enough after use, it didn't seem like it melted the solder wire very readily though. I sometimes wondered if it was actually hot enough. I did see smoke when heating the wire, it just seemed like the solder wouldn't melt around the tip very well, I had to melt wire spot by spot around the tip to cover it, it's a pretty thin solder wire and iron tip, doesn't say the thickness since it came as a sample with the iron.

    The pot I could probably get a new one. Do you know why this solder is so difficult to melt? Even when pressing the iron tip against the solder joints it took seconds to melt just a small dent, had to then press several times around the pot's leg but still didn't manage to melt the joint, so that meant that the solder sucker was of no use, even braid would've been problematic since melting just a small amount of the joint required pressing the iron tip against it for seconds at a time, and I'm worried about damaging the PCB. I had to press so hard the tip slid to the sides several times and scratched the board. What is this some solder joint from hell. I'll see if I can dig up a picture that shows at least the joints.
     
  8. Denesius

    Member

    Feb 5, 2014
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    Your description sounds like a classic corroded soldering iron tip problem. If its a cheapie, then get your hands on some sandpaper (from 100-220 grit, nothing fancy) and sand the sucker (cold state), until you see either solder/tinned/clean color metal or something resembling copper. Coat the tip with soldering flux, then power it up and when hot enough melt solder all the way around to coat the tip. Should be good to go, and sponge will be much more effective in the future. Some really cheap soldering irons (harbor freight comes to mind) use an iron tip which is worthless once it corrodes....

    MrAl's suggestion is a good idea, to make sure your soldering iron tip is getting hot (ie- not loose). If one of the legs of the pot is on a ground plane, you're going to have fun getting it off. Often it's better to cut the leg off the item, heat it out of the hole, then clean the hole by reflowing solder & blowing it open.

    I think what you're seeing with the alcohol stain is coating that has been dissolved and then resettled on the board. Usually looks ugly but harmless, as far as function
    My money says your pot is shot, with all the heating, solvent drenching, and previous symptoms!
     
  9. alfacliff

    Well-Known Member

    Dec 13, 2013
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    is the tip removable on your iron? sometimes corrosion gets in and blocks heat.
     
  10. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    I agree that it sounds like it could be just that the soldering iron is no good or the tip is bad or too long or even made out of the wrong metal and too long for that metal type. Try another iron or heat the tip up with a heat gun or something to see if it makes a big difference in soldering with it, at least for a few seconds before it cools back down.
     
  11. Metalmann

    Active Member

    Dec 8, 2012
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    "I should mention that I had some flux, but it was hard as dry clay. The flux package reads "service welding flux". I figured flux is flux so I dipped the iron into it quickly, but am not sure if it did any good. It kinda looks like it got burnt on the iron. And solder wouldn't wrap around the tip, but not sure if that's because the solder wire was quiet thin, or if it's because of a wrong flux."



    Sounds like you used flux for soldering copper pipes.

    Use rosin core solder for electronics.
     
    Last edited: Nov 13, 2014
  12. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    I think you mean "rosin" core solder.
     
    Metalmann likes this.
  13. solderboy

    Thread Starter New Member

    Nov 12, 2014
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    Hey thanks for the replies, feels good to learn directly from folks in the know.

    Btw went to get some flux at radioshack, but the location's stopped carrying it, so I might order online.

    I forgot to mention that another keyboard I had also had very stubborn solder spots, remember the repair guy struggled for a while to remove it up with braid. So it may not be just the iron, but something about the solder used at the factory. When I tried adding new solder to the pot's legs, my own solder melted on contact but the solder on the leg joints as I said took forever to just melt a hole the width of the iron's tip, the surrounding solder refused to melt. It's as if it wasn't solder but something else that just looks like solder.

    Here's one picture of the PCB where you can see the solder joints that were as good as impossible to melt. They look like solder but behaved more like steel. I only managed to melt dots right under the tip, and only after applying pressure and holding for between 5-15 secs. The picture was taken before my solder attempts btw, and the lower PCB on the bottom half was the one I worked on after removing the top one and then that. Anyone knows what's wrong with those solder spots, why they're so stubborn? Again my solder wire melted on contact with the iron tip, but not the joints on the PCB. Moreover the solder wire I melted curled up and was very difficult to apply to the leg joints and once the new solder stuck it had a very rough surface, due to all that is why I called it a disaster.

    pcbpic.JPG
     
  14. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    Where is the pot in that picture?

    There is another type of silver solder that melts at a higher than normal temperature so maybe they used that. I dont know why they would use that though for this application.

    We can guess all day long but in the end you'll just have to do the best you can. You may have to drill it out and use small wires to connect back to the PC board with regular solder. Maybe if you cut the pot out with cutters then you can solder the new pot to the old legs. Whatever it takes. You can usually scratch off the coating on the traces with a sharp razor knife so a few well chosen places may get the job done.
     
    Lundwall_Paul likes this.
  15. flat5

    Active Member

    Nov 13, 2008
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    Your new iron...is it a 220 volt iron running on 110 volts?
     
  16. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    One thing I have learned is Radio Shack has an EBay sith with the same parts as their web site, but many (most?) items come with free shipping. I needed a flux pen and they had a decent price that beat my normal place when combined with shipping. I think I ordered Sunday and got it Wednesday or Thursday. I'm good with the for free; my china stuff takes 2-3 weeks for a freebie.
     
  17. solderboy

    Thread Starter New Member

    Nov 12, 2014
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    Thanks for the replies.
    The iron reads 22o volts 60 W. As for the flux, radioshack only had one size, and way more than I need so I didn't get any. A tiny jar would be nice. I might check ebay or other online place if they have free shipping.

    I'm not sure the pot is in the picture, I mainly took pictures of screw locations and wiring. The unit has several pots, and the solder spots look similar to the smaller ones in the pic. I might re-check to see if there's room to scrape without damaging the traces.
     
  18. flat5

    Active Member

    Nov 13, 2008
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    Solderboy, it's good practice to specify your country in your board profile.
    It helps us to answer your questions.
    Does your power company supply 220vac to the wall outlets?
     
  19. solderboy

    Thread Starter New Member

    Nov 12, 2014
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    USA, but the product may have been shipped from China if I remember correctly.
     
  20. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    Try taking pics of your setup in order for us to better understand and diagnosis the problem..
     
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