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remove and re-solder Prop QFP? — Parallax Forums

remove and re-solder Prop QFP?

JavalinJavalin Posts: 892
edited 2011-04-14 18:13 in Propeller 1
Hiya

My prop QFP IC seems to have issues. In a previous post it was identified that cog6 has bad ram and I think other issues are there too.

So I ideally need to replace the IC - its on a PCB with other £££ components (GPS, etc) so I can't afford to distroy the PCB pads etc in the process.

So I guess what I asking is either:
(1) Is there a company in the UK that could remove and replace for sensible money, not keen on individual due to cost of board
(2) Any good tips on removing IC? I have soldering iron and hot air station.

Help??

James
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Comments

  • RaymanRayman Posts: 14,994
    edited 2011-04-11 03:44
    You'll want to use your hot air gun to avoid pad damage...

    It's pretty easy with a hot air gun, just heat it up until it's free...
  • JavalinJavalin Posts: 892
    edited 2011-04-11 03:46
    any temperature suggestions? Lead free solder melts at 250c (?) so set to 260c ????

    I have some old board's that I can practice on I suppose.
  • Andrey DemenevAndrey Demenev Posts: 377
    edited 2011-04-11 03:49
    Before I learned to use hot air gun and oven, I used to take my boards to mobile phone repair shops - they have all equipment and skills, and do not charge too much.
  • LeonLeon Posts: 7,620
    edited 2011-04-11 04:17
    Chip Quik works very well:

    http://www.chipquikinc.com/
  • Dr_AculaDr_Acula Posts: 5,484
    edited 2011-04-11 05:06
    @Leon, that is a really good find!
  • JavalinJavalin Posts: 892
    edited 2011-04-11 05:14
    awful website though! Not at all clear what it is, and how to use it.
  • Dr_AculaDr_Acula Posts: 5,484
    edited 2011-04-11 05:25
    I agree about the website. A bit retro 1996?

    What it does is hidden in one of the links - it is a low melting point alloy that makes the solder melt at a much lower temperature. Then it is possible to get all the pads molten at the same time and at a temperature well below cooking the chip.
  • max72max72 Posts: 1,155
    edited 2011-04-11 05:26
    Check you tube. There are video samples..
    http://www.youtube.com/watch?v=jaarYY2Lxiw
  • LeonLeon Posts: 7,620
    edited 2011-04-11 05:30
    Yes, it's a naff website.

    It's basically Wood's metal (a bismouth alloy), supplied in a kit with jelly flux and cleaning pads. Just apply flux to the leads, lay the stuff on them, and heat with a soldering iron to melt it. The chip will come off in a few seconds. If the soldering iron is adjustable, a low temperature can be used.
  • RaymanRayman Posts: 14,994
    edited 2011-04-11 05:58
    Javalin wrote: »
    any temperature suggestions? Lead free solder melts at 250c (?) so set to 260c ????

    I have some old board's that I can practice on I suppose.

    I usually set mine to 600..700F, but since the chip is bad anyway, it doesn't really matter...

    PS: When you're done use solder wick with an iron to clean up the pads for the new chip.

    Or, you could just use the hot air gun at the ~600F range to put in the new chip without cleaning the pads...
  • Dr_AculaDr_Acula Posts: 5,484
    edited 2011-04-11 06:38
    the link http://www.chipquikinc.com/alt_sold_new.htm says the alloy melts at 136F. Seems a very cunning solution. I might order some.
  • JavalinJavalin Posts: 892
    edited 2011-04-11 07:47
    yeah - i've ordered some chip quick stuff - £14 or so here. Worth it if I don't screw the PCB. I've some old boards to test on first.

    Thanks all.
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2011-04-11 08:45
    I have used chip quick and it does work very well, but you do not get much of it in the kit. For a couple of smaller chips it would be plenty though.

    As with all repairs it is imprtant to decide what is the valuable bit if the kit, if the chip is being harvested for other projects then the board is the sacrificial part and if the board is to be maintained the the chip can be written off. Hot air is ok as long as it is directed at the solder joints fairly evenly, usually the correct four sided air nozzle but even then any components nearby will become unstuck if left open to the air blast. There are gums for this that set to a rubber like protection layer.

    The air re-work stations are very expensive, if the chip is fairly exposed then a domestic paint stripper could be tried (with caution). Bring the whole board up to near temperature, frontand back and then give the part around the chip just that little bit more. That is what a lot of the high end IR stations do.

    You could thread some very fine wire, from some stripped back flex, around some diagonaly opposite pins, so that the instant that the solder become liquid then it could be gently lifted. I used to practice on old PCBs just so that mistakes didn't matter.

    Putting the new chip back is just a case of getting the chip into the correct position and the fixing down a couple of pins in opposite corners and the going around the rest. liquid flux helps a lot here.
  • JavalinJavalin Posts: 892
    edited 2011-04-11 08:52
    that should be fine, one IC to replace as its foo-bared. PCB is the important part as it has other components on it (gps etc).

    I have a re-work hot air station (£85) so will try that on 150c and see whata happens.

    I have some test boards to play with from previous versions.
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2011-04-11 09:12
    Lead free solder (spit) melts a lot higher up than that.
  • localrogerlocalroger Posts: 3,452
    edited 2011-04-11 16:59
    If you're not concerned with saving the chip you can simply cut all the pins off with a sharp tipped nippy pliers, razor knife, or dremel cutting tool. Then you can remove the cut pins from the pads with a normal soldering iron tip a few at a time.
  • davidsaundersdavidsaunders Posts: 1,559
    edited 2011-04-12 08:53
    Use your hot air gun (shield any nearby components, that would be damaged by the high temp, with aluminum foil), heat till the solder is liquid, remove the Prop, clean the pads, carefully apply a very small amount of solder paste to each pad, carefully place new Propeller on board (making sure of alignment), press down firmly (so the solder paste can hold it till soldered), use your hot air gun to melt the solder, remove aluminum foil shielding, and you are done. You will want to make sure that there are no bridges between pins after complete (though if you did not overdue the solder paste there should not be any), and that all connections to the pads are clean.
  • Graham StablerGraham Stabler Posts: 2,510
    edited 2011-04-12 11:02
    Last time I replace a QFP prop I used a scalpel, carefully press the tip on a leg close to the chip body and it cuts through with very little force, go around the whole chip and remove it, then mop up the legs with a soldering iron. I know it is not the best way but I find it very effective.

    Graham
  • Graham StablerGraham Stabler Posts: 2,510
    edited 2011-04-12 11:02
    Or what localroger said :)
  • JavalinJavalin Posts: 892
    edited 2011-04-12 11:05
    Hiya

    So I tried the chip-quik method, and its really good. Got the old IC off no problems, but screwed up soldering of the new one. Removed it too and now waiting for another IC....

    http://javalins.wordpress.com/2011/04/12/prop-ic-removal-part-1/

    James
  • davidsaundersdavidsaunders Posts: 1,559
    edited 2011-04-12 12:44
    For re-soldering, you probably want to use some solder paste and your air gun. Be careful not to use to much solder Paste on each pad, also be sure to line up the chip correctly before pressing it down.
  • RobotWorkshopRobotWorkshop Posts: 2,307
    edited 2011-04-12 13:19
    When re-soldering you may want to use the solder wick method. I use that all the time with excellent results. Make sure you use the solder wick to remove any excess solder already on the pads. Then just put a bit of solder on just one pad. Set the chip in place and heat that one pad to solder it in place. Check the orientation. If it isn't perfect just warm the solder connection and move the chip. Repeat as needed to get the alignment perfect. Then solder the remaining pads. Don't worry about solder bridges. Then use solder wick remove the excess solder. It usually leaves the perfect amount. I usually do one side at a time then let the chip cool before doing the next one.

    This is the method I used when building the modules here:

    http://forums.parallax.com/showthread.php?92127-NEW-40-pin-Propeller-prototype-module-Most-hackable-yet&highlight=bs2p40

    Robert
  • BigFootBigFoot Posts: 259
    edited 2011-04-12 19:18
    I carefully cut all of the propeller chip leads with an xacto knife and pull the chip off. Then you can use a small soldering iron
    and solder wick to remove the remaining leads from the board. The goal here is to protect the PC Board at all costs.
  • JavalinJavalin Posts: 892
    edited 2011-04-13 00:28
    yeah i've got some more solder-wick comming. Not sure what goes on, it used to work really well for me but tis time it wouldn't "wick". Ordered some more of different make to see if that works. The solder got in behind two pins - so gave up and removed the IC. Not worth screwing the PCB board for it.

    10 / 10 for the chip-quick though.

    J
  • RobotWorkshopRobotWorkshop Posts: 2,307
    edited 2011-04-13 07:17
    Javalin wrote: »
    yeah i've got some more solder-wick comming. Not sure what goes on, it used to work really well for me but tis time it wouldn't "wick". Ordered some more of different make to see if that works. The solder got in behind two pins - so gave up and removed the IC. Not worth screwing the PCB board for it.
    J

    I've run into issues with some off-brand solder wick. As long as you get a good quality solder wick the stuff works great. I'll see if I can get the brand/model of one I use now if that would help. I'm sure there are many good ones out there but there some out there that just don't work.

    Robert
  • JavalinJavalin Posts: 892
    edited 2011-04-13 07:51
    yeah that'd be interesting to know.

    Ordered : http://uk.farnell.com/jsp/search/productdetail.jsp?SKU=4754256 and http://uk.farnell.com/jsp/search/productdetail.jsp?SKU=860670

    have also seen this from Sparkfun : http://www.sparkfun.com/products/8775

    James
  • BigFootBigFoot Posts: 259
    edited 2011-04-13 18:43
    You have to get solder wick that has built in flux, and it is really important that your soldering iron
    has a clean small point. I usually blob a bunch of solder on all the pads on one side to clean off
    the remaining leads then clean off the pads with the solder wick for the next chip.
  • JavalinJavalin Posts: 892
    edited 2011-04-14 00:19
    yeah I think thats where i've been going wrong - soldering iron tip needs replaceing.... Funny how it creeps up on you.
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2011-04-14 01:17
    The solder wick is quite a heatsink for a small soldering iron bit, so it is a question of a small(ish) medium bit.

    I have used lots of the Chemtronics version (second, blue cased sort that you showed from Farnell) , mostly because an ex-workmate fell for the "I'll order 50 of those" and got 50 of the 10 packs. But they are running out now. The flux pens are useful with ordanary solder (full leaded of course).

    I have never got on with solder paste but then I found out that it has a shelf life, even whilst kept in ideal conditions.
  • RobotWorkshopRobotWorkshop Posts: 2,307
    edited 2011-04-14 18:13
    The spool of solder wick I've been using lately is:

    NTE # SW02-25 No-Clean Wick.

    Seems to work well.
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