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Question regarding Propeller Project Board USB regulator chip — Parallax Forums

Question regarding Propeller Project Board USB regulator chip

Hal AlbachHal Albach Posts: 747
edited 2013-12-30 10:02 in General Discussion
Merry Christmas, Everyone!

In the middle of some code modification on my current project my Prop Project Board USB became unresponsive. Power light went out and the Serial LCD stopped updating. Measuring the 3.3 bus read out at about half a volt. Removed all power sources and began looking for a possible short, finding only a 500+ ohm reading from 3.3 bus to ground. Using a procedure I learned from the Silabs web site for removing SMT parts I removed the regulator chip and the 3.3 to ground near-short disappered. I then carefully soldered a LM1086-3.3 to the Vin and SW pads using 0.01" wire and the board came back to life. Its not a pretty sight but it has me back on track.
I have found the regulator chip, an RT8250, available from Digi-Key at $0.93 ea plus a princely S & H fee. My question is since I don't have a reflow workstation and the chip has a grounded thermal pad on the bottom, could I get away with using a heatsink compound on the pad and just solder the 8 leads to their pads? Pin 4 of the chip is internally bonded to the pad and will provide ground reference. Any tips or suggestions would be most welcome.

Comments

  • PublisonPublison Posts: 12,366
    edited 2013-12-28 12:07
    Hal Albach wrote: »
    Merry Christmas, Everyone!

    In the middle of some code modification on my current project my Prop Project Board USB became unresponsive. Power light went out and the Serial LCD stopped updating. Measuring the 3.3 bus read out at about half a volt. Removed all power sources and began looking for a possible short, finding only a 500+ ohm reading from 3.3 bus to ground. Using a procedure I learned from the Silabs web site for removing SMT parts I removed the regulator chip and the 3.3 to ground near-short disappered. I then carefully soldered a LM1086-3.3 to the Vin and SW pads using 0.01" wire and the board came back to life. Its not a pretty sight but it has me back on track.
    I have found the regulator chip, an RT8250, available from Digi-Key at $0.93 ea plus a princely S & H fee. My question is since I don't have a reflow workstation and the chip has a grounded thermal pad on the bottom, could I get away with using a heatsink compound on the pad and just solder the 8 leads to their pads? Pin 4 of the chip is internally bonded to the pad and will provide ground reference. Any tips or suggestions would be most welcome.

    Hal, Did you get this resolved. I seem to have the same issue. I changed the FTDI and Propeller chip, and no joy. I ordered a regulator chip.

    After all I spent, I could have bought a new board. :)

    Just wanted to see how you handled the thermal pad.

    Jim
  • Hal AlbachHal Albach Posts: 747
    edited 2013-12-28 13:25
    Good Afternoon, Jim;
    I am still waiting for my parts to arrive from DigiKey. Hopefully they will arrive early next week. My current strategy is going to be to thoroughly clean the pads with solder wick and then apply a small layer of solder on the pads with my iron, wipe a tiny amount of flux to the thermal pad on the chip and solder the chip in place. Then I'm going to apply hot air from my heat gun for 20-30 seconds and hope the air flow won't blow the very tiny caps and resistors off the board. I think as long as I don't draw excessive current from the regulator (it's rated for 3A) it should work out. If not, I'll go back to the LM1086 and make it look as neat as possible. I'm in uncharted territory (for me) and if you or anyone else thinks the length of time for the hot air is excessive or insufficient don't be afraid to set me straight. The gun says it will produce 560 degrees on low so I will back it away a bit.

    Hal
  • PublisonPublison Posts: 12,366
    edited 2013-12-28 13:36
    I guess my question is, how did you deal with getting the chip off if it was soldered to the center thermal pad.

    Do you have a link to : "Silabs web site for removing SMT parts" ?

    Thanks,

    Jim
  • Hal AlbachHal Albach Posts: 747
    edited 2013-12-28 14:10
    http://www.silabs.com/Support Documents/TechnicalDocs/an114.pdf

    I     didn't do all the steps in the app note, but just used the solder wick to remove as much as possible and then used the 30 ga wire threaded behind the legs, anchored one end and gently pulled on the other and each leg popped up. As for the thermal pad I just gently pried the chip loose. It popped off rather easily and undamaged. In the attached picture you can see the pad was only attached at three small points.

    Hal


    DSCF0005.jpg
    DSCF0005.jpg
    1024 x 764 - 27K
  • PublisonPublison Posts: 12,366
    edited 2013-12-29 10:44
    Hal Albach wrote: »
    http://www.silabs.com/Support Documents/TechnicalDocs/an114.pdf

    I     didn't do all the steps in the app note, but just used the solder wick to remove as much as possible and then used the 30 ga wire threaded behind the legs, anchored one end and gently pulled on the other and each leg popped up. As for the thermal pad I just gently pried the chip loose. It popped off rather easily and undamaged. In the attached picture you can see the pad was only attached at three small points.

    Hal


    DSCF0005.jpg

    Thanks for the update Hal.

    I'll try and yank mine off the board later today. I prefer to use ChipQuik for taking chips off of boards. It's great. Leon turned us on to this stuff a couple of years ago.

    It's available a Fry's if you have on nearby.

    http://www.chipquikinc.com/
  • PublisonPublison Posts: 12,366
    edited 2013-12-29 12:28
    Managed to get the chip off Chipqick don't work as the chip was still held by the thermal pad.

    Solderwick did the trick, and with a little leverage with a tiny screwdriver under the chip to pop it off as you said.

    Now to wait for my new regulator. I may try you temporary regulator fix in the mean time.

    Jim
  • Hal AlbachHal Albach Posts: 747
    edited 2013-12-29 13:19
    That Chipquick looks like it may be great for removing SMD's that do not have that thermal pad, going to have to give it a try. I think Fry's is a West Coast retailer, don't think any are here in Florida. However, the product is available through Mouser & Digikey.
    Just checked my shipping status and my regulators are at the local Post Office for tomorrow delivery. I'll update you when I get the little guy replaced.

    Hal
  • PublisonPublison Posts: 12,366
    edited 2013-12-29 13:27
    I'm thinking that I might use a little Chipqik on the thermal pad under the regulator when I replace it. It stays liquid for a while, and that might give me time to solder the eight pins. Hopefully some thermal transfer.

    Worth a try. Let you know.

    Jim
  • Hal AlbachHal Albach Posts: 747
    edited 2013-12-30 10:02
    The regulators arrived, cleaned up the board with the solder wick and alcohol wipes. Placed a tiny dab of Arctic Ice on the thermal pad, carefully aligned the new chip and soldered it in place. Again cleaned the area with alcohol, let it dry real good. Holding my breath I applied power to the board while holding my meter on the 3.3 bus and was rewarded with a reading of 3.31 and a cheerful green glow from the boards power led. OK, breathing again. Hooked up the project I was working on and the program loaded and ran from the EEPROM, not skipping a beat. Placing my finger on the regulator did not brand the chip ID into my finger, and actually felt cool. My hats off to whoever designed this board because with all the heat I poured into those tiny pads, they never moved or lifted.

    Good luck with the Chipqik, and use a tiny amount, very little space between the thermal pads. Wouldn't want it oozing out between the leads.

    Hal
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