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  • EEVblog #346 – MLF/QFN SMD Reflow Soldering

    Posted on September 7th, 2012 EEVblog 13 comments


    Forum Topic HERE
    How to use solder paste and a hot air gun to reflow solder a surface mount SMD 0.5mm pitch 3mm x 3mm MLF (QFN) chip.

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    11 responses to “EEVblog #346 – MLF/QFN SMD Reflow Soldering” RSS icon

    • Nice video. I don’t have solder paste, so what I’ve done to mount these chips in the past is I tin the pcb pads a bit with my soldering iron, spread some flux on the pins under the ic, and then do the hot air reflow.

      • I’ve had success doing the same thing as Matt. I have solder paste but only use it if I have a stencil to apply it. Smearing paste can leave lots of little bits (actually tiny microscopic balls) everywhere even after reflow. When doing the pre-tin method I also pretin the chip but only leave enough solder to make it shiny. I will leave enough solder on the PCB pads to to the job. I also apply a thin film of solder paste to both chip and the PCB pads. I think the best solder paste is the No-Clean stuff from Chipquik.
        David, I’m concerned that the center pad (paddle) did not receive enough heat. It can be difficult especially in your situation where its is also a heat sink as the rest of the board will suck the heat away from that localized spot that you where heating. It may have recieved enough to make a spot contact here or there but the thermal conductivity may be compromised. Preheating the PCB would help this quite a bit.

    • The dessicant packaging and shelf life limit is mainly to prevent the epoxy potting from popcorning (exploding) in the reflow oven.

      • To elaborate – the epoxy absorbs some moisture from the environment, and when you reflow solder, it can cause the built-up moisture to turn to steam and pop the chip right off the board (the solder is molten so the chip’s only held in by surface tension and gravity). Not sure if you can damage the chip by having the package explode, but it’s usually used to prevent the package from popping off the board (which while amusing, means you have a dud board).

        And i’ve seen it with a lot of ICs – regardless of packaging. Not sure what’s so special about the epoxy that leads them to this precaution since others don’t have that requirement.

    • Dave, isnt’t pin “one” marked as the larger pad on the opposite corner?

    • you pulled the heat away from the chip right as it really started to be pulled into place. Should’ve left it for another couple seconds (or heat it up again).

    • Hi Dave.
      Have you tried using the Elmo Video Presenter for the SMT soldering?
      I purchesed one after seeing it on your tear-down epidsode. It can also do photos and videos and with the camera up so high above the work area, it is great to use.

      Denys in Shepparton, Vic.

    • I’ve found that applying solder paste on small pads is much easier if you use a sharp, dry toothpick.

      Put a little solder paste on a small square of wax paper, then pick up a tiny amount with the tip of the toothpick. Apply it onto the board with a steady hand, carefully rolling the toothpick against the direction you’re moving the tip on the pad, and the solder paste will nicely spread out on the pad.

    • I actually prefer the MLF/QFN etc, I find them so much quicker and easier to solder than things like TQFP, where I have to spend the next 5 minutes with solder wick removing all the shorts.
      If I don’t have a stencil I can use, I generally just tin/flux the pads and then reflow it.

    • What is the heat gun that david used?

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