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FR4 time*temperature discoloration

Discussion in 'Electronic Design' started by John S, Dec 22, 2013.

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  1. John S

    John S Guest

    A component hot enough will cause FR4 to discolor after a few years. An
    end user will suspect there is a problem with the board (even if there
    is none) because of the discoloration.

    Does anybody know how hot a component on the board can be and still not
    cause discoloration eventually?

    I have searched, but I only found information regarding short-term
    temperature extremes.


    John S
  2. If you are concern, there is also FR5 to use. that is a higher
    temperature material and also higher voltage handling.

    It does cost more, obviously..

  3. Phil Hobbs

    Phil Hobbs Guest

    Use black solder mask.


    Phil Hobbs

    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510

    hobbs at electrooptical dot net
  4. John S

    John S Guest

    Excellent suggestion! Thanks.
  5. John S

    John S Guest

    A good way of hiding it, but what if forced to use the typical green color?

    John S
  6. Den søndag den 22. december 2013 23.41.16 UTC+1 skrev John S:
    remove the solder mask underneath the hot part?

  7. My experience is that solder ages slightly faster than FR4, and copper
    traces age almost as fast. Your customers are likely right to call
    boards defective when they have dark spots.

    It was common in 1980s home theater systems to have extremely hot
    voltage regulator transistors that eventually destroyed their solder
    joints. You could probably find one and measure the temperature. I'd
    guess nearly 200C based on what they do to your finger tip.
  8. I looked into this some years ago. The only information I could find was submitted by Isola, referring to UL746 which has figures for the lifetime versus temperature:

    It does not however state the point at which the discoleration occurs

    The rule we used for FR4 temperatue was max 95 degrees, which would be on the conservative IMHO.


  9. John S

    John S Guest

    Many thanks. This is a number to keep in mind. Every degree lower is
    better, I know.

  10. John S

    John S Guest

    Thanks to all who contributed. I think I have a goal now.

    Merry Christmas.

    John S
  11. Den mandag den 23. december 2013 09.26.20 UTC+1 skrev Klaus Kragelund:
    there is something about the resin if FR4 starting to change state at
    ~130C causing a big expansion in the Z direction stressing vias etc.

  12. The glass transition temperature (Tg) of FR4 is 130°C, give or take
    10°C or so. Indeed, it's like a phase change.

    The expansion of the resin would be the same in all directions, except
    the glass fibers limit the dimensional change in the X & Y directions
    due to the layup.

    One rule of thumb to prevent delamination is to keep the operation
    temperature of the laminate at least 25°C below the Tg.

    You can specify materials with a Tg of 170°C or even higher (for a
    premium, of course).

    Best regards,
    Spehro Pefhany
  13. John S

    John S Guest

    Wow! Thanks. Best info yet! First I need to find a chicken. Then I need
    to find out if my cleaning lady understands to clean up what's left
    after... er, what do I do with the chicken?

    Heating a chicken is no problem. There are a number of ways. But, don't
    tell the ASPCA.
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