NOTE: I've put the steps here first for convenience. For those wanting
more info, read beyond for the how's and why's. I originally wrote this
with the description first and the steps at the very end. I don't know
now how this will read.
========================================================================
1. Create the slot in the Milling(46) layer. It is best to draw this
with the longest edges straight, and the shortest edges radiused to a
size larger than your board house's smallest end-mill or drill. Think oval.
2. Add an SMD pad to either the Top(1) or Bottom(16) layer.
3. Draw a polygon of the same shape as the SMD pad, and place it on the
opposite layer.
4. When you CAM-up your .brd file add an extra section, call it MILLING,
set the extension of the output to something unused like .mll, select
the Milling(46) layer and process the entire job with this new section.
5. Tell your board house that the .mll file has information on milling
that needs to be done before metallization.
========================================================================
I am posting this to describe a viable procedure for creating slotted
pads. Slotted pads are often used on current sense devices because they
have leads of a high aspect ratio (one side is much greater in length
than another) which would require a very large diameter circular pad to
accommodate. This over-sized pad would require LOTS of solder and heat
to properly mount since these are typically higher-current devices and
therefore have great heat-sinking capabilities, not to mention the board
itself will likely be a great sink as well because it will most likely
be of a higher weight of copper. These holes also unnecessarily waste
valuable board real estate.
The solution I propose is better, cleaner, and only has the drawback of
putting in the extra effort in both library construction for these types
of devices, and board CAM processing for any boards containing these
device types.
I have searched and read every post on both the User and Support forum
relating to "slots" and this also seems to be inline with what most have
said, although no other post describes anything as thoroughly, or as well.
First, please understand how EAGLE, the CAM processor, and the Board
House all play into turning what you see on your screen into what is in
your hands at the end of the process:
EAGLE draws things in layers only to distinguish different aspects of a
board for itself. A pad and all your wire on the top layer will end up
on the same layer at the board house. A layer in EAGLE, a physical
layer on your board, and processing layers at the board house are three
entirely different concepts, however intertwined. I will discuss no
further. ALWAYS keep this in mind.
Some have said that the dimension layer is useless, and that is a false.
Any layer can be used to do anything you want. It's how you process
your layers, and how you instruct your board house to use your output
files that determines what the board will turn out like.
If you separate the dimension and milling layers and process them
properly then you can use the to make the difference between plated and
non-plated slots. Simply tell your board house that anything in the
output file created by the Dimension(20) layer is to be used AFTER
metalization, and anything in the output file created by the Milling(46)
layer is to be used BEFORE metalization.
Without further ado, to have a board created with Plated Slot hole for
high aspect ration pins follow these simple instructions:
1. Create the slot in the Milling(46) layer. This slot can be drawn as
a rectangle or by air wires. Do NOT use a polygon. Draw it exactly how
you want the slot cut-out, but keep in mind the board house will more
than likely use an end-mill to route this slot. End mills are ROUND,
you CAN NOT have a 90-degrees edge. You can draw it this way, but I
guarantee your board house will call you and ask if you want the edge of
the radius to match the edge of the rectangle or if you want the center
of the end-mill to go the edge of the rectangle. It would be best to
just go ahead and make shorter edge a radius (the longer edge should be
straight), but make sure it's radius is at least as large are their
smallest end-mill or drill size.
2. Add an SMD pad to either the Top(1) or Bottom(16) layer.
3. Draw a polygon of the same shape as the SMD pad, and place it on the
opposite layer.
4. When you CAM-up your .brd file add an extra section, call it MILLING,
set the extension of the output to something unused like .mll, select
the Milling(46) layer and process the entire job with this new section.
5. Tell your board house that the .mll file has information on milling
that needs to be done before metallization.
-jh