Sorry, I wasn’t clear. I meant a to compare a hypothetical CERSOIC to the LCC. Clearly the CERDIP has already been done.
As you say, carefully forming the CERDIP legs to gull-wing is probably a good way to go.
Edit: the thought of a flex PCB interposer is interesting. I guess the question would be how the polyimide compares to FR4 in terms of matching the LCC package. Maybe it could be slit in creative ways.
Anyhow, good to see someone else offering the LCC packaging.
A quick search shows that polyimide does take up water as well. But I would hazard a guess that the polyimide flex pcb is well a lot more
flexible.
The polyimide has to have a degree of "stretchiness" to conform to the forces bending it - my guess would be that as a consequence of that forces on the LCC package would be a lot lower.
Another point where geometry comes into play is: how to arrange the soldered contact between rigid and flex so that the stresses are not only decoupled by stretch but reduced by geometry.
Imagine a flex pcb with LCC + say a decoupling cap. It is now in one case soldered "all around" with contacts on 4 sides. Stresses would only be reduced by stretching of flex.
But when having all contacts to one sides of the flex, stresses from rigid to flex are completely decoupled in one direction and concentrated in the area of the solder joints in the other direction.
Further shenanigans could be played by having a "bend" in the flex pcb, but mechanical assembly would be more cumbersome.