Folks,
A few days ago, I promised more from Tim Jensen on Halide-free soldering. Here it is:
DR: There is a significant push in the electronics industry toward
halogen-free. How does that affect solder suppliers?
Tim: There is a strong environmental push to produce "halogen-free
electronics" and take the brominated flame retardants out of the PCB.
Halogenated flame retardants tend to create a toxic smoke when burned.
Although the halogen-free push was based on the flame retardants in the
circuit boards, the halides in fluxes also came under this same scrutiny.
In fact, halide-bearing fluxes don't produce the same toxic gas as the flame
retardants do.
With respect to solders and fluxes, halides (halogenated compounds,
typically bromides or chlorides, found in fluxes) have been used for years
to reduce oxides. In the past, there was concern that ionic halide residues
left on the PCB could cause corrosion or dendritic growth in the solder
joint, so the solder industry began using covalently bonded halides, which
are very stable.
Halide-free fluxes are available, but there is a significant
misunderstanding surrounding them. First, halide-free fluxes may not be
more reliable than halide-bearing fluxes. A halide-free activator is not as
effective at oxide removal. Therefore, to obtain good wetting with
halide-free fluxes, more total activator is needed (no halides, but
chemically aggressive). Any activators, halide or halide-free, have the
potential to produce corrosion if not processed properly. The more
activator present in the flux, the greater the risk. Since halide-free
fluxes typically have more activator, the reliability risk is greater.
The second important point regards measuring the halides in fluxes. The
standard test methods currently in use, titration and ion chromatography,
are effective at detecting only ionic, not covalently bonded, halides.
Therefore, the statement, "halide-free by titration" simply means that there
are no ionic halides. If you encounter this claim, remember, the flux might
contain a high quantity of covalently bonded halides. The best method for
true halide detection is an oxygen bomb test followed by ion chromatography.
The oxygen bomb burns off all organic content, breaks the halide bonds, and
leaves behind an ash consisting of the halides and other inorganic content.
Testing this ash gives the true halide content of the flux.
Although the amount of halide allowed in "halogen-free" is still up for
debate, it will probably end up somewhere between 300-900ppm. To show the
insignificance of fluxes on these criteria, assume that a circuit board
weighs 300g. On that board, about 1g of solder paste is printed, of which
only 50% is the flux. Therefore, the board has 0.5g of flux. A
halide-contained flux must have less than 5,000ppm of halide, which, even at
its maximum, would be about 0.0025g of halide. This results in a halide
contribution of about 8.3ppm to the 300g PCB (below the detection level of
most equipment).
Despite this kind of analysis, the industry still appears to be moving
toward totally halide-free flux usage. I expect the interest in this topic
will continue to grow.
PS; The Photo is of Tim at the Great Wall in China