High Currents + Thermals = Bad?

Discussion in 'General Electronics Chat' started by joe426, Sep 5, 2012.

  1. joe426

    Thread Starter Active Member

    Aug 11, 2009

    Above is a motor controller with 3oz copper. The maximum amount of current the motor will draw is 25A which enters the PCB through a connector, then through a fuse, and then is distributed to the rest of the board. The fuse is surface mount and (as you can see in the attached image) has thermals on its pads. I believe the 25A going through the small gaps in the pad would destroy the small gaps entirely? Should I remove the thermals on these pads all together?

    Additionally, if you look at the connector in the image which has plated slots, they also have these thermals. Should I remove the thermals here as well?
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    SMD 25A fuse...?
    Are u mental ?
  3. joe426

    Thread Starter Active Member

    Aug 11, 2009
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    So you have a 40A fuse to protect a 25A line... that sounds good.

    Thermals are there so you can heat a local area for manual soldering with an iron. They are there to help localize the heat at the terminal, but as thermal energy and electrical current both flow the same paths you are correctly noting parameters that are in conflict.

    If I was laying in thermals I would do this only on the outside of the pads, basically extending the tabs their entire width by their width (adding a new isolated square). That gives a defined area for the thermal.

    Littlefuse recommends a .135" pad height. If I plug that into some of the online trace width calculators I get a rise of 45 to 70°C above ambient. This is a max value never obtained as the rest of the land would act as a heatsink.

    Additionally, your open isolated area may be too small, especially when plating up to 3 oz: small openings tend to close up during plating. So give yourself a bit more open room and your PCB manufacturer will love you for it (and perhaps drop the price to manufacture).

    If you are going to do an oven reflow thermals may be irrelevant as the board, tracks, and components should all be heated (somewhat) equally, though I have no test data to support this supposition.
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    Thermal spokes should be designed so that there is sufficient spoke width to avoid excessive localized heating but narrow enough to prevent cold solder joints.. Fuses by nature run HOT. Don't make it worse.. Personally I would avoid the thermals on something like that.
    Thermals make a HUGE difference in soldering through hole parts mounted to large copper pads but like Ernie I don't have much information on them with reflow SMT stuff and would tend to think they do more harm than good in this application.