How long does the overlap have to be to reliably join two DNA strands with overlap PCR?

[cirosantilli]

https://biology.stackexchange.com/questions/89186/how-long-does-the-overlap-have-to-be-to-reliably-join-two-dna-strands-with-overl

I was having a look at how Overlap extension PCR as part of a de-novo DNA synthesis mechanism.

I was thinking that first smaller pieces would be synthesized somehow with overlaps, and then joined with OE-PCR.

The key parameter is then how long does the overlap have to be between the smaller sequences, so that we can keep the initial fragments as small as possible?

The page on Gibson assembly says that a ~20-40 base pair overlap is needed for that technique, but I'm not sure if that is the most efficient way to join DNA strings that we have currently.