Homebrew PCBs

On Wed, 10 Aug 2011 09:59:52 -0400, you wrote:

I'll weigh in on this....

>I wanted to rekindle the discussion about using CNC for pcb fabrication. I
>love the toner transfer method, but I hate the chemicals and the drilling. I
>think a CNC would alleviate this.

It can...... <note dots....>
>
>What I have learned from this group and CNCzone forums is that CNC is a
>whole other rabbit hole, but can be worth the time and effort. My questions
>are:
>

Cost of equipment, maintenance, accuracy, and some inherent
limitations.

>1. Can I reasonably expect to get enough resolution to create designs that
>use such things as SOIC and even TQFP IC packages from a CNC kit?

Yes, but as I understand it, within limits.  The main problems with
CNC go to the resolution and repeatability of the basic setup and the
wear and tear and resolution of the cutting bit.  

Cutting bits seem to be about 5 dollars each minimum, how much linear
cutting they will do seems to be (as I have once  heard, about 300 to
600 inches).  That may be a single complicated board, or many lesser
ones.  Commercially available bits are in the 35 dollar range.  

My recollection is that they will do (perhaps) pins with a 1.0 mm
spacing, but you do have to be careful with it.


>2. Is the fact that the FR4 is not flat (has peaks and valleys) going to
>make it impossible to perform the CNC process of creating boards?


Yes.  Methods are either a vacuum hold down or a foot on the milling
head.  It is possible to probe the board and compensate with that.
Currently, there is an experiment/development project on the PCB-Gcode
newsgroup for adding probe capability to PCB Gcode.  This is a program
which works with EAGLE, and produces milling files from it.



>
>
>What do big board houses do? Do they etch chemically or use CNC techniques?
>I would think they would load a 4 foot by 8 foot sheet of stock and etch it
>all at once.

Chemical etch, including plated through holes.  Drill each layer,
assemble, plate through, resist, etch, solder mask, silk screen.

>
>It would be really neat to just load in some double-side copper cladded FR4
>and have the cad drawing etched and drilled. I remember someone saying that
>small board houses were run out of business years ago because of
>environmental concerns, but I am not sure this has anything to do with it.

It doesn't.  Mechanical etch is not profitable for boards in general,
but because it involves no chemicals, and can be done at home (with
varying results), can be a decent technique when minimal runs are
needed and time (to make) is all automated.  

The EPA got most of the smaller board houses, and the Chinese low cost
market got most of the rest.  (IMHO).

I've used photoetch (quality absolutely dependent on negative density
and registration), and toner transfer (quality is more variable, but
essentially cheaper for materials).

Currently doing toner transfer.  Typical run can be 4 to 14 board
sides.

Harvey


>
>
>Chris
>
>
>[Non-text portions of this message have been removed]
>
>
>
>------------------------------------
>
>Be sure to visit the group home and check for new Links, Files, and Photos:
>http://groups.yahoo.com/group/Homebrew_PCBsYahoo! Groups Links
>
>
>