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Green solder mask and heatsinking

Discussion in 'Electronic Design' started by pimpom, Nov 1, 2008.

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  1. pimpom

    pimpom Guest

    Does anyone know something or point me to a web resource about
    the heat radiating/insulating property of green solder mask?

    I'm making two PCBs for a custom-built project and want the
    finished boards to have a professional look. I've done the screen
    printing and etching. One of the PCBs has a large (~8 sq.in.)
    plain copper area to supplement the separate finned heatsink. All
    soldering will be done by hand and from experience, I find it
    difficult to get a nice finish covering even a couple of sq.in.
    with a thin film of solder by hand (for corrosion protection).

    The alternative would be to cover the heatsink area with green
    solder mask along with the rest of the PCB. But I would like to
    be able to estimate the loss of heat radiation. I'm not really
    expecting precise figures as there are many variables, but I'll
    be grateful for some indication other than a wild guess.

    (Note: Without going into details at this time, let me assure you
    that I don't have the alternative of ordering the PCBs from a
    manufacturer. I'm doing everything myself).

    Thanks in advance for any help.
     
  2. Guest

    Loss of radiation will be miniscule. I always cover my gound planes/
    heat blocks with solder mask.
     
  3. pimpom

    pimpom Guest

    Thanks for the info and for the quick reply.
     
  4. pimpom

    pimpom Guest

    Thanks for the confirmation. In that case, I wonder why many
    manufacturers leave solder mask off ground planes that double as
    heatsinks even when there's neither a clutter of solder points
    nor a need to reinforce conductor cross-section with solder.

    (The HS is for two 7812 regulators that together will dissipate
    about 10W max. The copper plane will be thermally coupled to the
    ICs via their ground leads and to a blackened extruded Al
    heatsink via mounting bolts. I considered using a switched-mode
    PSU but decided against it).
     
  5. pimpom

    pimpom Guest

    The regulators are mounted on the Al heatsink. The ground plane
    is also thermally coupled to the heatsink by the bolts simply
    because 1) it is mechanically convenient and 2) it supplements
    the IC leads as a heat conduction path to the copper.
     
  6. NoSPAM

    NoSPAM Guest

    Large areas of unetched copper is considered bad practice, especially with
    Mil-Spec, because of the differing thermal expansion between the copper
    laminate and the (typically fiberglass/epoxy) substrate. When large areas
    are needed because of shielding needs or a ground plane for a stripline, the
    etching pattern normally used is a grid or parallel traces which allow for
    thermal expansion without the copper lifting from the board during reflow.
    Unfortunately this means the loss of surface area for heat convection.

    Dr. Barry L. Ornitz WA4VZQ
     
  7. krw

    krw Guest

    Interesting. Large pours are "normal" in some neighborhoods (I've
    never done it) and certainly solid internal planes are the norm.
    How do they get away with the differential thermal expansion
    problem?
     
  8. Guest


    CONFORMAL COATING WORKS FINE
     
  9. NoSPAM

    NoSPAM Guest

    It depends on what you consider "large" is! I am generally talking about
    areas around 35 x 35 mm, or about 2 square inches and up. It also depends
    on just how reliable you want the board to be and whether rework or repair
    is ever necessary. I suspect the modern designers are used to very fine
    lines and more modern copper bonding technologies allowing them to ignore
    differential thermal expansion. I do remember that most of the PCB CAD
    programs I once used had the option of "cross hatching" large pours.

    Microscopically, the copper foil contacting the substrate today is covered
    with little "mushrooms" of copper grown electrolytically on the foil. When
    used with a partially cured substrate (pre-preg), the high pressure curing
    process locks the foil to the substrate quite tenaciously such that you
    rarely see the copper lift from the substrate like you did 50 years ago.

    I still have some old printed circuit boards in the junque box where every
    through-hole is a small pad surrounded with a ring with only two small
    traces to connect the center pad and the ring. These are quite old and I
    suspect they were hand soldered.

    Dr. Barry L. Ornitz
     
  10. Get gold plating for your entire board. Many PCB fabs don't even charge you
    extra for that.
    Green solder mask is passe, there are many other colours that are much
    better looking. Gloss black is nice.
    A good cheap place to get your boards made is www.pcbcart.com

    Dave.
     
  11. dalai lamah

    dalai lamah Guest

    Un bel giorno pimpom digitò:
    For the typical PCB temperatures, heat radiation gives a very little
    contribution anyway, it's almost all about transmission and convection. For
    the heatsinks in free convection, there is a difference extimation in
    thermal resistance less than 10% between the best radiation case (heatsink
    painted in black) and the worst case (bare metal). In forced convection,
    the difference is practically zero.

    Probably if you use a dark solder mask you will get a very small, almost
    unnoticeable improvement.
     
  12. krw

    krw Guest

    In the case of planes, the size of a panel is fairly normal.
    That's tiny to be a hard restriction.
    How is "cross hatching" any different? I'd think you still be
    pulling at the corners of the "hatch". In any case, planes aren't
    "hatched". I *have* seen boards that are somewhat balanced on
    top/bottom (equal number of planes, etc) to reduce warping.
    50 years? No, I didn't see much of that 50 years ago either. ;-)
    A *lot* of things have changed in 50 years.
    Wasn't that to prevent delamination?
     
  13. Guest

    Well, it seems to me that talking about how good of a
    heatsink soldermask makes is like talking about how
    good of a parachute an umbrella makes.

    If you have heat to dispose of, and the heatsink is
    adequate, that's all there is to it. You should not be
    depending on soldermask to do the job. The only
    time we use on-board copper as a heatsink is in
    cases where no heatsink is used and the device
    generates only very minimal heat.

    In those cases, the soldermask is opened, to allow
    a direct metal-to-metal contact.

    Jean

    www.pcb-now.com
     
  14. AFAIR even pcbcart charges extra for non-green boards.

    Philipp
     
  15. neon

    neon

    1,325
    0
    Oct 21, 2006
    Ridiculous you worry about corrosion on a copper square but not about IC pins. believe me they go first.
     
  16. krw

    krw Guest

    The manufacturing manager at my CPoE decided to change the solder
    mask each board revision to make them easy to spot. White is
    *ugly*. I think he gave up after that bad idea because they all
    seem to be (not quite as ugly) green again.
     
  17. I think solder mask over bare metal would be an improvement over bare
    metal. So far in my experience, heat transfer from very warm bare metal
    to the ambient is improved by having the metal covered by solder mask,
    paint or tape. Bare metal radiates longwave infrared poorly, and coating
    the metal with something else achieves a radiation improvement that
    outweighs the thermal resistance of the coating.

    - Don Klipstein ()
     
  18. Gold does indeed have less emissivity than copper, Gold is more
    reflective of IR than copper - gold is used for IR mirrors, including
    ones for thermal IR.
    I have found it is often significant - especially in cramped areas where
    the PCB faces something other than bare metal.

    - Don Klipstein ()
     
  19. In <>,
    I would agree with having the heat source touch the copper without
    solder mask in between. But copper or tinned copper that is getting rid
    of heat does that better with solder mask over it.

    - Don Klipstein ()
     
  20. And when the PCB is in a cramped area where convection is poor and not
    surrounded by bare metal that reflects the radiated heat back, the
    difference gets to be more than 10%.

    - Don Klipstein ()
     
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