Do you really need a stencil to use solder paste?

Discussion in 'General Electronics Chat' started by spinnaker, Nov 26, 2011.

  1. spinnaker

    Thread Starter AAC Fanatic!

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    I might be doing some very light surface mount soldering. Probably just a SD socket or two for now,

    I realize I might be able to make do with regular solder but I would like to try some paste.

    1. Do you really need a stencil to use paste or is that just for reflow work?

    2. How long does the stuff last on the shelf? I read somewhere that it should be refrigerated?
     
  2. Wendy

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    Solder paste is just that, paste. It can be applied with a brush, it is sticky (among other things). It is cleaned a lot like paint.
     
  3. spinnaker

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    I have seen it dispensed in syringes too.

    So I can just drop a bit on my board and apply some heat?


    How long does the stuff last on the shelf?
     
  4. THE_RB

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    Yep. Provided everthing is pre-tinned (or your copper PCB is VERY clean).

    Paste keeps much better in the fridge. Be warned it is toxic so be careful near food.
     
  5. spinnaker

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  6. Smoke_Maker

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    I use the Chipquick SMD291AX, it comes in a 15 gram syringe. it's very easy to use and do SMT, you can use it with a iron or hot air. Put a little dab on each pad then drop the chip on the pads, it's OK to smear outside the pad because once you put heat to it it all draws up into a puddle on the pad. If you use the iron hold the chip down with a small pointed object so you don't knock the chip off the pads with the iron tip. Then watch for the :) , it's not as hard as it looks.

    I always keep a new tube on the shelf just in case the one I'm using drys out, but so far all I have seen is the paste drys out in the needle and the rest is OK, I just checked the tube I had on the shelf for a year and it's still good, the syringe comes with a cap on it and it is in a zip lock plastic bag.
     
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  7. CraigHB

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    I haven't done any reflow myself. I do my projects with SMD stuff on a PCB, but solder by hand with a magnifier and fine tip soldering pencil. I've been wanting to have a go with reflow. I've checked out some videos on it, there's a good number on YouTube. I've seen where people have just smeared a coating across the pads on the PCB and the stuff still pools up well enough to solder fine pitch ICs without issue. Seems like you can lay down paste pretty liberally.
     
  8. Wendy

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    Agreed, that is how I have done it in the past.
     
  9. GetDeviceInfo

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    getting the paste on isn't nearly as bad as getting the residue off. Go easy on the flux, it only takes a small amount. Although certainly not a requirement, stenciling helps control the volume of flux applied. For SMD I'll tin the pads then clean the residue. Then I'll apply the slightest amont of paste to the device pins, mount and touch solder, as mentioned, holding the device in place with a pick.

    I use a different approach for resistors/caps, etc. The reason is that if you tin the pads, solder one side, then the other, you can create large stresses in the device which can cause failure. In these cases, I tin one pad only, mount as before, then flux and solder the other pad.
     
    Last edited: Nov 28, 2011
  10. THE_RB

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    A couple of years, if the paste was fresh. When it goes "bad" usually the solder separates out from the flux so the solder part gets lumpy at the bottom and the clear flux is on top. Some people have re-mixed I believe.

    I put some old paste (that came in a syringe) in an incubator at about 60'C and it seemed to reconstitute quite well so heat might be able to fix old paste. Of course it depends on the age and type of the paste so your mileage may vary. ;)
     
  11. Jotto

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    Apr 1, 2011
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    To me the best part of solder paste is the ability to use lower temp on the solder iron. I have been using Chip Quik also. Clean up isn't that bad. I usually just use alcohol, but was just testing out Chemtronics Flux-Off. It works well and doesn't take a lot of fluid to clean up excess flux.

    Reballing is something that most don't have the proper equipment to do. A decent machine to accomplish this is about 60k.
     
  12. THE_RB

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    What's a reballing machine?
     
  13. ErnieM

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    Funny you mention reballing. I am the designated reballer where I work. Actually it's balling for us, as we're the device manufacturer so we're not removing balls from a failed reflow attempt.

    At first we tried a proprietary alignment fixture that held a paper preform over the device but the fixture had so much mass it affected our reflow oven profile. So I tossed the preform and the alignment fixture and just rolled the balls into their approximate position.

    Surface tension pulls the balls to their perfect centered position. Just takes tweezers, a microscope, and a semi-steady hand.

    Of course, I'm only rated for prototype quantities.
     
  14. Markd77

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  15. Jotto

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    Basically its a computer controlled device that allows you to remove BGA style chips. It can really be used on any type of surface mount chips, but its designed to remove the type chips that the pins are located under the chip itself.

    Computer controlled unit. It will remove the component and save the information for reinstalling the chip after you complete the reballing of the board. It can also be used in prototyping as stated before. Its a very nice machine to have if you are doing that type of work, hard to justify the cost if your not doing a lot of that type of work.

    http://www.circuitmedic.com/guides/9-4-1.shtml
     
    Last edited: Nov 30, 2011
  16. THE_RB

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    Thanks for the info. I know BGA chips but had not heard the term "reballing" for what sounds like a typical reflow machine.

    And as for why it costs $60k... :)
     
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