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Hand solder of small SMT components

Discussion in 'General Electronics Discussion' started by peterlonz, Sep 22, 2010.

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  1. peterlonz

    peterlonz

    28
    2
    Feb 11, 2010
    I have just read a thread here on this very subject but then lost it.
    Anyway I wanted to ask more questions,
    The component I wish to solder to a prototype board is a 36 contact IC 5mm x 5mm.
    Linear LT3651 IUHE-4.2#PBF.
    The contacts are not visible from the top which surely presents a special problem.
    I just can't see myself accurately holding this in position, then soldering How exactly,
    If I allow solder to run under the IC no doubt I'll get contacts soldered ................ each to the rest!!
    Can't see how you can clean that up with wicking.
    Am I being negative, this is far smaller than other tasks I have tried & steadiness of hand is not one of my attributes.
    Which leads me to ask the obvious question:
    Why don't IC manufacturers make a larger base unit with flying leads into which the SMT IC can be safely & securely snapped into position. Or, offer a "mount to small GP board" service?
    Look forward to a spot of guidance.

    Rgds all
    Peter O
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,505
    2,849
    Jan 21, 2010
    Sounds like a BGA package.

    Basically, you place them on the board and heat everything so that the solder balls melt and solder the part to the board.

    They are probably the hardest thing to solder "at home", not least because you need to have some very fine tracks and probably a not insignificant number of vias on your board.

    I bought some small BGA packages to test my skill (marvellous what you can get for a couple of $$$ on ebay :)) being able to make a board at home with 0.1mm tracks is the sticking point at the moment. :D And these are only 14 "pin" devices.

    Why don't they make them larger? because they are designed for highly miniaturised devices. If you're working with them you should be able to afford to make your own prototype boards.
     
  3. Militoy

    Militoy

    180
    0
    Aug 24, 2010
    The LT3651 isn't in a ball grid array package - it's actually a 5x6 mm 36-lead plastic QFN. It can be hand-soldered - but there are two large inaccessable underside pads (37 GND and 38 SW) that need to be soldered to the pcb. In a prototype, it is possible to lay out a pattern with two plated-through holes in the center of the areas of 37 and 38 - then solder from below. In production - the part should be soldered with screened paste for more reliable thermal contact.
     
  4. NickS

    NickS

    367
    0
    Apr 6, 2010
    I put these type of packages down by hand all the time because our production dept has a really bad track record with them. I can tell you that the ones I use are in those packages because it helps at high frequencies to keep the parasitics down.

    Here are the steps that work for me nearly every time.
    Tools Needed:
    (1) Solder iron with fine tip
    (2) solder and liquid flux
    (3) Hot air(either pencil or gun)

    If these were BGA it would make your job slightly easier because you wouldn't have to add solder. But since they are not you want to make them like a BGA
    Steps
    (1) Apply solder to the part not the board. Flip the part over and apply a very small amount of solder to each exterior pad and a small amount to the slug. Resist the urge to put a lot of solder on the slug or your part will never sit down properly. Now hit the part with flux and then just quickly reflow each with the iron to ensure a nice even bump on each pad. note our production department insists on pre-soldering the board instead and their success rate is below 30%

    (2) Pre-heat your board with the hot air. I use a hot air pencil that gives me better precision but if all you have is a hot air gun then it will work. You just have to be more careful about not scorching your board. After the board is warmed up set down your part and carefully apply more heat until the solder reflows. At this point the part should have shifted slightly which is good. The part will automatically align itself with the pads. At this point I like to press down slightly on the part as it cools to ensure contact on all pads.

    (3) When the board is cool enough to handle lift it up one side at a time and inspect the corners between the part and board. Touch up any pads that don't look like they re-flowed nicely.

    That is what works for me.
     
    Last edited: Sep 24, 2010
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