Forum Discussion

Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
15 years ago

exposed pad in EQFP package

Hi,

I have a problem with the exposed pad in the EQFP 144pin package.

What is the correct way the connect the exposed pad to the PCB ?

I cannot simple solder it because it has height and it is in the center of the chip.

also putting many Via's in the PCB wont help because of the height issue /

mabe I need to manufacture the PCB with an hole for the exposed pad and then to solder it throw the hole to the print ?

Please advise.

Amir

7 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    Hi,

    I'm afraid, but I think the exposed Pad makes manual soldering of the EQFP impossible, this chip package requires either reflow soldering or vapour phase soldering (or a professional rework center). Soldering the exposed pad requires the package itself and the PCB to be heated up to the solder liquid temperature by a profile not exposing the die too long by too high temperatures. I did a lot soldering on the "good old" TQFP myself and thus I'm not really amused on the exposed Pad..

    I did rework on smaller packages of switch-mode regulator with "pad on botttom" with heatgun, this physically soldered the device but I'm quite sure I shorted this device's lifetime by this temperature treatment...

    CU, Carlhermann
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    Thanks Carlhermann.

    What about the Via's Idea ? (putting many Via's and filling them up with solder)

    Or The hole idea ?

    Do you know about someone who done it ?

    Amir
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    Exposed pad packages can be most easily soldered with solder paste on a hot plate or other reflow techniques used in production. When using hot air from the top, the board should be preheated. Also you have to avoid local package overheating with too small nozzles.

    For prototype boards, designing one large (3-4 mm drill) through-plated via in the pad center allows to solder the pad with an iron tip.

    I don't understand about the "height issue".
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    Thanks FvM.

    The height Issue :

    Because the exposed pad,

    when placing the FPGA in place,

    has height from the PCB - you need to put a lot amount of solder in order to connect the pad in the PCB and the exposed pad with solder.

    FvM,

    Is this your preffered techniqe or you recomend to do it in an other way ?

    thanks,

    Amir
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    I see, that the height difference is specified with min. 0.3 mm in the package data sheet. I must confess, that I didn't yet use this particular EP3C package, but similar enhanced pad TQFP from other vendors. The assembly service providers didn't yet complain about it and in prototype assembly, you have no problem to apply sufficient solder paste.

    In my opinion, solder paste and hot plate are optimal for single sided prototype SMD assembly (up to a certain board size).

    P.S.: Sorry, I looked at the wrong table. The height is specified 0.05 mm min, 0.15 mm max, sounds more reasonable. There should be no problem to get the solder paste in contact with the IC pad, even if the finished solder joint will not cover the pad uniformly.
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    This exposed pad being tied to ground and soldered properly is no joke, they really mean it. we spent a lot of time troubleshooting crazy errors, mostly jtag issues , due to a faulty ground on the pad.

    We used something like a 25mil via to solder to it
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    I have done this before on the same chip using vias. I forgot how large they were... > 20 for sure. To solder it correctly, flood the vias with flux and then add solder until it sinks into the vias. As mentioned above, if you do not connect that pad, you will have serious issues - if your chip runs flaky until you press hard on it, your pad is not connected. The height did not seem to matter in my case. The better way to do it is of course through solder paste...