Proton BASIC Compiler - Soldering Fine Pitched SMD Components


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  • Soldering Fine Pitched SMD Components

    For many people using these fine pitched ICs is a problem since their pin pitch has become so small you can barely see the pins properly unless you use a magnifying glass.

    It seems the larger and faster these processors become the smaller they get. Fortunately one do have a choice, in most cases, to choose between different packages for the same device



    Soldering these ICs is really no problem. You just haven't been showed how to do it yet. But there are a few conditions that must be met for you to be able to do this. Also, do not think automatic pick and place processes does not have problems with these chips.
    No wonder they charge what they do to put these on for you. Their component placing system is just less shaky than your hand and a wee bit faster.

    Firstly the PCB must be tinned and excess solder removed with a hot air gun. Most PCB manufacturers do this in any case, so this is what the PCB looks like - enlarged quite a bit hence the snowy picture but you can see the excess solder on the footprint clearly.



    Compared to the tracks beside the pads, there is a mountain of solder already on the pads, you just don't see it when you look at it without magnification. Liquid solder consist of small solder balls and flux. When looking under a microscope the balls are clearly visible. If the solder is dispensed onto a footprint with such a fine pitch, the IC pins are almost always shorted all over when the board is soldered. This is the biggest frustration, and it is difficult to remove such a relative large amount of solder.

    Read more on liquid solder here. http://en.wikipedia.org/wiki/Solder_paste

    Again with reference to the above picture, there is enough solder on each pad. The manufacturer has already done you the favor of dispensing the solder. What you need to do next is to wet the footprint with solder flux, but not submerge it. Just a thin wet layer will do. Solder flux is available in liquid form and some types need to be thinned. We use Alpha A83 flux, it is a brown flux that has to be thinned with about 4 parts lacquer thinners. You also get no-clean fluxes that is supposed not to require cleaning.

    Never ever use the flux used by plumbers, available in hardware stores !

    It is an acid based flux and will oxidize your tracks and your chip's pins.



    When applied on the footprint, the lacquer thinners will evaporate fairly rapid, leaving a thin layer of flux. The most difficult part is to place the IC on it's place. If you pick the chip up with your hand or a tweezer, you are almost sure to bend the pins and ruin in. Place the other SMD components first and leave these for last.

    You have to pick the chip up with vacuum. Keep in mind you have to place these small devices properly and fairly accurate. If placed while the flux is still wet, the tackiness of the flux will help to retain the chip in place.

    For the hobby guys vacuum is not such a problem either, just think the wife's vacuum cleaner and a piece of fish tank pipe... I didn't say that :-) You could of course just get our manual pick and place machine.

    Do get a decent magnifying glass. You need to align the chip on two sides, if the footprint was made correctly, all the pins should be on their pads, or about on their pads.



    Stacking two magnifying glasses makes quite a bit of difference as you can see here. These lenses are nice and big and you can use both eyes to look through them which is less tiring if you do production. You get the idea.

    Soldering is done the same way the other SMD components gets soldered - on a hot surface such as the Hot Plate. When the solder melts, you could LIGHTLY tap on the PCB to create a slight vibration. This vibration together with the surface tension forces between the pins and the solder should tend to pull the pins to the center of their pads, thus lining the IC up.

    If the IC does not 'swim' into position, you may have to help it a bit. This is easiest done with a small flat tip jeweler screw driver you press at an angle with the PCB next to the chip. When you rotate the screw driver slowly, the elevated side will swing towards the chip and can so push just the right amount to get the chip spot on in place. With a bit of practice, it becomes quite easy to do.

    Your footprint for these chips are also important.
    Note the pin width and the space between the pins.



    Drawing 1 is the ideal placement and soldering.
    The pads are wider and longer than the pins so that solder can form as you can see there.
    Drawing 2 is a skew placed chip, a bit exaggerated though. The IC can be slided to the correct position.
    This configuration with the pin wider than the gap between pads make proper chip positioning a lot easier.



    Drawing 3 is problem. The IC can not be slided to the correct position. The pins locate between the tracks and the chip has to be lifted up before it can be placed again. On a hot plate that can burn you and is difficult to work on it is less desirable.

    If in the event of a short happening, it is not a train smash either. Remove the short using a fine tip soldering iron and solder wick. The soldering iron tip lightly presses the wick flat against the pins. The solder melts and ends up in the solder wick. Try to use as little force as possible and apply heat for the shortest time to get the excess solder removed. Force or too much heat on those
    small pads could loosen the tracks, once they begin to lift and peel, the PCB is ruined .



    In short, apply flux on the solder pads. Place the IC('s), make sure pin 1 is where pin 1 should be.Place the PCB on the Hot Plate. When the solder melts, use the magnifying glass to make sure the pins are in position on top of the pads. If required move the chip into place using a small flat screw driver as described. When soldered properly, remove the PCB and allow to cool down.

    Inspect for shorts and remove if required.

    Courtesy of www.faze.co.za