Microsoldering (?) Help

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Coppercloud

Coppercloud

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  • #1
    Droped ye ol switch yesterday. The good news is the L buttond decided to cushion the fall so there was no major damage. Bad news is it sacrificed the solder joint of the button in the process.

    I can replace it with this but have a solder iron and I'd like to learn how to do some of this small soldering (is this considered "micro" soldering?). It's a cheap adjustable temp solder pen from Amazon, but, as you'll see, it gets hot. Normally I try to heat both parts to the same temp with a little flux and a dab of solder and voila! It's soldered. But with this tiny little board I don't see much in the way of a metal pad to solder to and I'm just melting the shit out of the button and globbing solder all over. Is this something that can be done? I'm probably going to be ordering a new ribbon/button part shortly and use that in the end, but I figured this might be good practice.Screenshot_20230719-130237-700.png
     
    Pont

    Pont

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  • #3
    Really small detail soldering is typically done with solder paste instead of a soldering iron.

    The PCB (flex PCB in this case) includes a solder mask. Soldering paste is applied to the exposed contacts, the parts-to-be-soldered are placed in position, and then heat is applied (heat gun, hot plate, etc.) until the solder paste melts enough to flow. The flowing solder avoids the solder mask and clumps up on the exposed contacts.

    But a tube of solder paste goes bad after it's opened (and must be kept refrigerated), so it's probably not worth it vs. that replacement part that is only $6 to begin with.
     
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    dmsilev

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  • #4
    Pont said:
    But a tube of solder paste goes bad after it's opened (and must be kept refrigerated), so it's probably not worth it vs. that replacement part that is only $6 to begin with.
    Click to expand...
    If you’re just using the paste for touch-up work and not spreading it across an entire board with a stencil and then into an oven, a tube can last for a lot longer than that. The failure mode in my experience is that the flux becomes harder to spread and the temperature to activate it becomes somewhat variable. But if you’re spreading it with a pointed wooden stick or similar, and heating it until you can visibly see melting and flow, the lack of large-scale consistency isn’t a deal-breaker.

    Definitely cap the tube when not using it though. Otherwise it will fully dry out and won’t be usable.
     
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    Coppercloud

    Coppercloud

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  • #5
    Lets say I wanted to use the solder paste. I know my picture isn't fantastic up there, but the only visibly metal sections I see are on the switch. I'll circle them in red. The blue section is part of the ribbon itself. So I don't even see a real surface to solder to. So a couple questions
    1689865016495.png

    1) looking at a video online no flux was used. Am I right to assume there is flux in the paste?
    2) looking at the same video they just spread the paste all over and it went right to the spots it needed to be (when heat was applied). Is this because the mask is still on the board? Can I expect that to be the case for repairs on components that aren't factory fresh?

    (video I was watching:
    View: https://www.youtube.com/watch?v=NxPWwHUJCqM&t=212s
    )
     
    Defenestrar

    Defenestrar

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  • #7
    Coppercloud said:
    1) looking at a video online no flux was used. Am I right to assume there is flux in the paste?
    Click to expand...
    Yes
    Coppercloud said:
    2) looking at the same video they just spread the paste all over and it went right to the spots it needed to be (when heat was applied). Is this because the mask is still on the board? Can I expect that to be the case for repairs on components that aren't factory fresh?
    Click to expand...
    Didn't watch the video cause I'm waiting in a doctor's office, but generally yes.

    Soldering to a ribbon is harder, especially if the eventual joint needs to be electrically and mechanically sound under flex conditions. But order the new part (just in case) and practice to your heart's content.
     
    Coppercloud

    Coppercloud

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  • #8
    Defenestrar said:
    order the new part (just in case) and practice to your heart's content.
    Click to expand...
    This is my plan exactly. Part is on order, but I want to learn/get the equipment to handle these things in the future. As far as this part is concerned - having already attempted with a pencil iron I started out very delicate and gentle and got more aggressive and that part is pretty banged up from heat at this point. I'm not sure its even good for practice at this point. I'll still try.

    Thanks all.
     
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    dmsilev

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  • #9
    Coppercloud said:
    2) looking at the same video they just spread the paste all over and it went right to the spots it needed to be (when heat was applied). Is this because the mask is still on the board? Can I expect that to be the case for repairs on components that aren't factory fresh?
    Click to expand...
    Generally yes. Watch out for scratches that cut through the solder mask though. If you see exposed copper, solder can stick to that, and if there's a scratch that uncovers two adjacent traces, you can get a bridge that shorts together those traces.
     
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    Coppercloud

    Coppercloud

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  • #10
    Alright, so I was being an idiot this whole time. I may have been able to do it with an iron, though paste and heat may have been best, but I would have had to expose something, anything, to solder to. It seems like this was actually a case of a ribbon being adhered to a board and the wire on the ribbon wasn't exposed anymore. I'm not sure how the process works as far as what material is put on when and where, but the end result is I'm guessing I ripped the pads off the board and ribbon all together along with the button, so I would have needed to scrape through some of the ribbon to expose metal to solder to.

    I thought life was weird here not seeing visible solder pads but figured they used to make contact and at the end of the day if I put em back where they were, added flux and a dab of solder, things would turn out. After realizing my mistake I did file some down, carefully to not bridge things, and was able to get some of the contacts to join. However, id damaged one side bad enough already it just wasn't going to work. In the end I melted the button apart in it's plastic housing!

    Still, I ordered the new part yesterday, this was just about learning, and I learned plenty!

    Here's a picture from the back as it makes it a little more obvious what it was originally like, in case anyone cares.
    IMG_20230720_122721358.jpg
    And here's a bad picture of the other side after scraping off some of the ribbon, having some success soldering before completely destroying the button itself, and fishing it out of the garbage to take a pic in case you wanted a reference.
    IMG_20230720_122739717.jpg
     
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    redleader

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  • #12
    I would not use paste for something like this. Paste is used with reflow ovens when the original board is assembled, but usually not for repair or rework. Instead since the joints are all exposed you can use flux and a simple soldering iron just like any other SMD part. You can scrape off the solder mask with an exacto knife or fine sandpaper (depending on consistency), add flux, and then put the part down over whats left of the pads or scaped off solder mask. Then tin your iron and touch the pads. The flux will do the rest.

    Theres tons of videos online of people reworking ripped off connectors if you need a reference.
     
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    redleader

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  • #16
    Coppercloud said:
    Isopropyl alcohol should work to, right?
    Click to expand...
    Yeah, most flux is soluble in alcohol. There's no reason to buy flux that requires cleaning though, good enough 'no clean' is a few dollars and even the high end stuff isnt too much more. Unless you're soldering to exotic metals you don't need to use anything that strong.

    Requisite SDG flux comparison:
    View: https://youtu.be/iKDAmY9Rdag
     
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