Enough solder on pads here?

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  • spleenharvester
    Badcaps Veteran
    • Mar 2010
    • 882
    • UK
    #1

    Enough solder on pads here?

    Hey guys.

    This is a QFN32 package. The chip attached, a TPS51120, was damaged.

    When removing it took a bit of solder off with it but not a huge amount. I'm planning on heatgunning the replacement on with flux. Is there enough left on the pads?

    http://i.imgur.com/bPIZ8.jpg

    http://i.imgur.com/mIHIb.jpg

    The black stuff was left by the chip so try ignore it, it's been rubbed off since.

    Also please excuse the picture quality, my camera broke and I had to use this ancient 2MP thing lol.

    Cheers
    Last edited by spleenharvester; 10-31-2012, 04:12 PM.
    Dell E7450 | i5-5300U | 16GB DDR3 | 256GB SSD
    Tags: None
    • tom66
      EVs Rule
      • Apr 2011
      • 32560
      • UK
      #2
      Re: Enough solder on pads here?

      I would recommend using new solder.
      Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
      For service manual, schematic, boardview (board view), datasheet, cad - use our search.

        Comment

        • spleenharvester
          Badcaps Veteran
          • Mar 2010
          • 882
          • UK
          #3
          Re: Enough solder on pads here?

          Is there any chance I can use solder paste or similar? Problem is my soldering iron is way overpowered for the job, and I have a tremor, which doesn't mix too well.

          Something like this, thinly spread over the pads? http://www.ebay.co.uk/itm/Solder-Pas...item3cc7dceec2

          Only wishing to bring out the soldering iron as a last port of call.
          Last edited by spleenharvester; 10-31-2012, 05:09 PM.
          Dell E7450 | i5-5300U | 16GB DDR3 | 256GB SSD

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            • Agent24
              I see dead caps
              • Oct 2007
              • 4913
              • New Zealand
              #4
              Re: Enough solder on pads here?

              QFN should be done with solder paste and hot air as far as I know. Don't think you can really do them with a soldering iron.

              Unless you're confusing it with QFP?

              If it was QFP you could do drag soldering...
              "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
              -David VanHorn

                Comment

                • spleenharvester
                  Badcaps Veteran
                  • Mar 2010
                  • 882
                  • UK
                  #5
                  Re: Enough solder on pads here?

                  Nah it's deffo QFN. Apparently it's possible to use heavy amounts of flux and sort of drift the iron along to retin the pads. But I'd much rather not bring the iron out :p

                  Think I might just have to get some solder paste then. Do I need a stencil for it?
                  Dell E7450 | i5-5300U | 16GB DDR3 | 256GB SSD

                    Comment

                    • Agent24
                      I see dead caps
                      • Oct 2007
                      • 4913
                      • New Zealand
                      #6
                      Re: Enough solder on pads here?

                      Originally posted by spleenharvester
                      Think I might just have to get some solder paste then. Do I need a stencil for it?
                      You shouldn't really need either, if you do it this way: http://www.youtube.com/watch?&v=c_Qt5CtUlqY

                      Might pay to practice first on something else I guess
                      "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
                      -David VanHorn

                        Comment

                        • spleenharvester
                          Badcaps Veteran
                          • Mar 2010
                          • 882
                          • UK
                          #7
                          Re: Enough solder on pads here?

                          Found half of an old motherboard (lol) out in the garage with a QFN chip on it. Removed the chip and tried to tin the pads. All that happened was:

                          a) All the solder ended up in a big ball in the middle

                          b) My soldering iron was way too big/hot

                          c) Two of the pads burnt off

                          This is the problem I have, my iron is just way too terrible for anything involving precision.

                          Surprisingly, I tinned the pads of some other slightly bigger QFP chip using drag soldering, and it worked out okay.
                          Dell E7450 | i5-5300U | 16GB DDR3 | 256GB SSD

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                            • spleenharvester
                              Badcaps Veteran
                              • Mar 2010
                              • 882
                              • UK
                              #8
                              Re: Enough solder on pads here?

                              Just went out and bought myself a nice new Antex C15. Updates soon to follow. :p
                              Dell E7450 | i5-5300U | 16GB DDR3 | 256GB SSD

                                Comment

                                • spleenharvester
                                  Badcaps Veteran
                                  • Mar 2010
                                  • 882
                                  • UK
                                  #9
                                  Re: Enough solder on pads here?

                                  Okay, just did some more practice on dead boards out in the garage. Replaced several QFP chips (the big SMSC things you see on laptop motherboards all the time) without much trouble.

                                  Tried two QFN32 chips, knackered some pads on the one trying to get it tinned, the other went fine apart from 3 leads being untinned.

                                  TL;DR - is there any way of getting the QFN pads tinned without all the solder just going to the middle pad? Am I going to have to Kapton tape it off?
                                  Dell E7450 | i5-5300U | 16GB DDR3 | 256GB SSD

                                    Comment

                                    • spleenharvester
                                      Badcaps Veteran
                                      • Mar 2010
                                      • 882
                                      • UK
                                      #10
                                      Re: Enough solder on pads here?

                                      Guys, just found something that's worrying the hell out of me.

                                      The orientation of the chip is supposedly the same in both depictions, yet...

                                      The motherboard's schematic suggests that C462 and R458 are to the left (circled): http://i.imgur.com/Nmnk7.png

                                      Yet in this picture I took of the chip in question, C462 and R458 are at the bottom: http://i.imgur.com/GxnTM.jpg

                                      What the heck! Should I follow the schematic or the picture? I'm now freaked that the (original) chip may have had its orientation markings printed wrong or something.
                                      Dell E7450 | i5-5300U | 16GB DDR3 | 256GB SSD

                                        Comment

                                        • Agent24
                                          I see dead caps
                                          • Oct 2007
                                          • 4913
                                          • New Zealand
                                          #11
                                          Re: Enough solder on pads here?

                                          Did you try tinning the pads with the method as shown in the video I posted?

                                          I've never done it myself though, but what they did there seemed to work just fine.

                                          It seems you need barely any solder, you just want a lightly tinned tip and some flux on the board, then just touch the pads, enough to get some on, you shouldn't be adding any solder in addition to that already on the tip at all.


                                          As for the orientation, go by the physical chip. The orientation of any part on a schematic diagram does not have to follow actual component placement at all. The pins for the IC in the schematic need not even be in the same order as the real device.

                                          Schematics often rotate or otherwise move components or their pins around to make reading the functionality of the circuit easier.

                                          IC markings are extremely unlikely to be wrong.
                                          "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
                                          -David VanHorn

                                            Comment

                                            • tom66
                                              EVs Rule
                                              • Apr 2011
                                              • 32560
                                              • UK
                                              #12
                                              Re: Enough solder on pads here?

                                              Originally posted by Agent24
                                              QFN should be done with solder paste and hot air as far as I know. Don't think you can really do them with a soldering iron.
                                              You're not a real man until you've done QFN32 with 0.5mm pitch by hand. Just use lots, and lots of flux.

                                              I did it under supervision from an electronics tech in my internship, he really knew his stuff. But I would do it by solder paste any other day, it's far easier.
                                              Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
                                              For service manual, schematic, boardview (board view), datasheet, cad - use our search.

                                                Comment

                                                • spleenharvester
                                                  Badcaps Veteran
                                                  • Mar 2010
                                                  • 882
                                                  • UK
                                                  #13
                                                  Re: Enough solder on pads here?

                                                  Gonna mount it the way it was originally then. Seen another board and it's the same. Thankyou!

                                                  Did it like in the video yeah, I think I need a really fine tipped iron (my current one is 2.3mm and is a tad wider than the QFN pads themselves, hence just coating the middle pad).

                                                  Surprisingly fun doing QFP chips though haha.

                                                  Also, just another question while I'm here. One of these is also on the board, a 2N7002DW Dual N Field Transistor: http://www.ebay.co.uk/itm/2x-2N7002D...item19d00ecd44

                                                  One side seems okay. But the left side, S2 gets nada on the DMM, unless connected to D1. Does this mean the part's shorted?

                                                  Also, the laptop would shut down if the multimeter touched D1.
                                                  Last edited by spleenharvester; 11-04-2012, 02:47 PM.
                                                  Dell E7450 | i5-5300U | 16GB DDR3 | 256GB SSD

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