Thanks to Hackvana, I’m making teeny tiny circuit boards!
No matter how good a board house’s process, there are some fundamental limits to how small things a board and the design on it can be.

A PCB is much easier to understand with a silkscreen! However, you can’t draw silkscreen lines narrower than a certain width, often about 5 mils. (Or 4 mils if you’re really lucky) So you’d better make sure none of your silkscreen is smaller, or lines will be spotty and text will be unreadable.

(Huh? What’s a mil? / One thousandth of an inch. / Okay. What’s an inch? / …)

With Eagle able to display line widths in mils, it’s not too difficult to check your lines.
What about text?
(How do I get mils? / Click on the Grid button in the upper left, and change the grid units to mils. You can change it back later)

Bad Text

Text Settings: Size: 20, Ratio: 5%. NO.

Here is the text “ISP”, and the text attributes dialog.

• Note that font has been set to vector. Keep your text in vector mode, it will ensure that the Gerber files you output are likely to match what you see inside Eagle.
• The grid size in these examples has been set to 10 mils
• There are two important settings – Size and Ratio.

Size is the height of one row of text.
Ratio is the text’s line width, as a percentage of size.

• For the above image, the line width is 5% of 20mils – only 1mil! That definitely won’t work!
• Remember; SIZE x RATIO > 5 mils

 But I Want Small Text!

That’s fine. You’ll just need to make it thicker.

Size: 20, Ratio: 10% : 2 mil thickness. NO.

Size: 20, Ratio: 20% : 4 mil thickness. NO.

These are still a bit small…

Size: 20, Ratio: 25% : 5 mil thickness. JUST…

This one is probably juuust thick enough. But the text is only half a millimetre high!

Okay, I’ll be sensible…

Good. 50 mils (1.27mm) or larger text is probably more sensible. If size is 50, and we need at least 5 mil silkscreen, then the ratio should be higher than 10%. Easy!

Size: 50, Ratio: 5% : 2.5 mil thickness. NO.

Size: 50, Ratio: 10% : 5 mil thickness. JUST.

Size: 50, Ratio: 20% : 10 mil thickness. GREAT! And you still need reading glasses!

One more time, graphically

WIDTH = SIZE x RATIO

In Conclusion

A board house will often tell you their dimension limits, for traces, isolation, and silkscreen.
For best results, avoid the limits.

If you can make your text 10 mil thick, and the board house can manage down to 5 mil, you should have no problems.