Hi,
By 'assemblers', do you mean 'contract manufacturers or CM'?
There has been a trend in the last few years where CMs have been developing design services. These are called 'Original Design Manufacturers, ODM'. The idea is that will add more value.
This is a trend away from traditional CM activities, where the CM built the products using the drawings and BOMs supplied by the customer.
I have concerns with 'IP leakage', where even with NDAs you have to educate the ODM on how to design for your market, then they could sell their services to your competitor.
In these situations I like a very simple NDA:
If you f**k with our business, we will f**k with yours.
This work while there is a strong business relationship with the ODM or CM. But there is still IP leakage.
Regards,
Jay_Diddy_B