First, you would not put the phase old technique in quotation marks. The rule is you can say so-called, or you can use quotation marks to indicate so-called, but you don't use both. As one Grammar page says:
Use quotation marks to denote so-called or to show that a word is not being used in its literal sense. Do not use the words so-called AND use quotation marks – that is tautology. [emphasis added]
Second, you would only use so-called if there was something about the phrase old technique that was not literally true. So, if one technique was developed two years ago and the other just two weeks ago, you could argue that the word "old" is misleading, because the technique is still relatively new. However, I'd say that in one of these two ways:
We propose a new technique under which fault analysis becomes more tractable than the so-called old technique.
or (and I like this one even better):
We propose a new technique under which fault analysis becomes more tractable than the "old" technique.
Here, old is put in quotation marks to warn the reader that the technique is not really all that old; it's more like comparatively old.
If that's not what you're trying to say, though, then widely accepted may be better.