Rotten solder flux?

Discussion in 'General Electronics Chat' started by jpanhalt, Jun 28, 2009.

  1. jpanhalt

    Thread Starter AAC Fanatic!

    Jan 18, 2008
    Does solder flux go bad with time?

    I have a 2-ounce container of Kester Rosin Paste Flux SP-44. It is at least 10 and possibly 15 years old. My usual practice after etching a board is to apply a very thin layer of flux over the entire board on the assumption that the flux protects the board during assembly.

    Never had a problem until my last 3 boards. I started getting ugly tarnished spots on the boards. A swab used to apply the flux developed light green spots too.

    My thoughts are:

    1) The flux has oxidized to a point that it is no longer just "mildly" activated, but is now quite acidic. In support of that possibility, the flux does form a dark red-brown color on its surface (oxidation?). Beneath the surface, the flux is more applesauce in color. The tin is now mostly darker.

    2) At the same time as this problem started, I switched from ferric chloride to cupric chloride for etching. Cupric chloride is known to undercut more. It may , therefore, be more difficult to remove completely. Incomplete removal followed by reaction with flux could also be contributing.

    I can address #2 by getting some ferric chloride and testing it. Before doing that, I thought I would ask about others' experiences with old flux. As a flux, it seems to still work well, but then zinc chloride would work too. :D

  2. SgtWookie


    Jul 17, 2007
    I have some rosin paste flux that is around 40 years old. It's still good as long as it hasn't been contaminated.

    You may have contaminated your paste flux by transferring "gunk" from the board with the brush.

    Sounds to me like you still had some etchant hiding under the edges of some of the traces, and then smeared it around with the brush. Better board cleaning procedures may help you with that.
  3. jpanhalt

    Thread Starter AAC Fanatic!

    Jan 18, 2008
    Thanks. #2 was my best guess also. Since I am using a photoresist, which I remove with alkali, I suspect a tiny bit of residual copper chloride is worse than the same amount of iron chloride.

  4. KL7AJ

    Senior Member

    Nov 4, 2008
    Golly, I thought I was unusual. :) I have a bar of solder from my Dad's collection that must be 60 years old (he built HO trains from SCRATCH) and an old soldering "mace" you used to heat up on the gas stove. :)