Imprinting and bonding are actually two different things-
Imprint:
The imprint happens within the 1st three days from hatching. Ducklings will imprint on a species, NOT an individual (and typically only on living/moving objects that are larger than they are, so they don't really imprint on each other). So if they see humans during their first days of life, they will imprint on the species. If they see ducks, they will imprint on ducks. In Asia, ducklings are shown a moving big stick so they imprint on it- then when they go to graze in rice paddies they'll follow and stay near the stick. Typically, ducklings that imprint on humans make for friendlier pets, since they typically don't fear humans. However, the imprint is not failproof.
Bonding:
bonding is really what makes a duck people friendly or not. Bonding typically happens when a duckling spends a LOT of time with one individual in particular. It's even stronger when there are no other ducklings/chicks around. The duckling will form a strong bond to the individual, and will see them as "mama". While ducklings imprint on a species, learning they are ducks, they bond to their mama. A duckling can imprint on humans, but never form a bond with a human. Ducklings can also bond to each other. Ducks can bond to more than one individual at different times (like my ducks are super bonded to me, but they also have a bond between them they developed after their initial bond to me).
For the friendliest outdoor/backyard ducks, having them imprint on humans is the way to go. Then to help establish some sort of bond, they should be handled/picked up/petted very often, but they should not be separated from other ducklings.
Hope that clears it up! I have two wonderful house ducks who are imprinted and bonded seperately to me, and each other. Because of our bond, my ducks are house pets and will not live outside with other ducks (in fact my own is completely convinced she is a person, not a duck).