How to etch finely-packed SMD boards?
In my experience, homemade boards become very difficult with traces thinner than 16 mil (approx. 0.4 mm). Besides the need for an etching process that works uniformly over the entire board, the biggest problem is the photo transfer film and process.

Structures down to 16 mil only worked for me when I used a good laser printer with transparent paper. Printing on plastic sheets (even the ones designed for laser printers) didn't work well because the toner would not stick uniformly. Any case sizes down to SO-8, -14, -16 and the like for ICs and 0805 for passives were o.k. Maybe 0603 would work, too.

New and tiny SMD ICs often have a pitch of 0.4 mm, i.e. a pad/trace approx. 0.2 mm wide and a space approx. 0.2 mm wide. This is just about half as big as the smallest structures that have worked well with my DIY PCBs.

It helps to have a temperature-controlled etching bath where your boards are inserted upright, and where tiny air bubbles keep the solution moving around your board - similar to the air outlet on the bottom of a fishtank.