Homebrew PCBs

Robert Hedan wrote:
> "The solder paste could be dispensed via screw-driven syringe
> substituting for the drill/mill head."
> 
> EXCELLENT idea!  I was racking my brains for a consistent technique of
> applying paste.  I kept thinking motor and couldn't get out of that
> 'groove', a screw is perfect for a slow regular flow control.

The syringe should be as rigid as possible so the
post-drive flow is minimized.  I'd choose something
like a stainless tube and plunger.  The seal on the
plunger can simply be an o-ring.

Then again a minimal amount of post-drive dribble can
be handled in the control code by backing off a tad
on the screw drive at the end of a dispensing run and
pre-loading by the same amount when the next run begins.

Some thought is needed in how the syringe is loaded
with solder paste to avoid inclusion of air pockets.
Loading via reverse vacuum through the dispensing tip
might work depending upon the viscosity of the paste
but may also be slow.  If another means of creating a
larger orifice to load the paste is provided it will be
necessary to initially purge the air from the
dispensing path.  A few bubbles may be output during
the dispensing process but you will notice this and
be able to correct such.

> "Component placement could be made by a vacuum head able
> to articulate at least +/-180* normal to the board."
> 
> And another great idea!  I had figured the vacuum part, I was missing the
> rotating head.  I already have a Pick and Place file exported from QCAD that
> is just sitting there.  The rotation is already determined in the file, the
> data is all there waiting to be used.

I don't know how much a pick and place file will buy
here as the operation is going to be manual unless
component pickup is from an automated dispenser (eg:
tape and reel).  Even then commercial solutions include
machine vision to align the component on the vacuum head.

I'm just looking for something to pick up a 0603
capacitor out of a dish and give me an accurate means
to manually locate it on the board.  There is some
tolerance for placement inaccuracy in reflow soldering
as the surface tension of the solder paste upon
liquefaction will pull the component into alignment
with the pad.

> "Getting this far would be cause for celebration but
> a useful addition which quickly comes to mind would
> be the addition of a small CMOS camera to the head."
> 
> Now that one I already had covered, in fact, I'm going even one step further
> with this one.  I have 2 USB cameras waiting by my desk (read pile of junk).
> I was initially going to place only 1, but now I intend to place 1 camera on
> each axis for a perfect view of alignment.  Small lamps will come handy,
> maybe high intensity LEDs.

I'm unsure how much usable resolution you may get
with USB cameras.  I'm also uncertain if standard
NTSC resolutions will be enough though I haven't yet
experimented.

Low power microscope cameras are available which
render NTSC which is both encouraging and a possible
solution as well.  10x seems to be more than enough
for 0603 work and likely 0402 as well.

> Check EBay for USB miniature cameras.  The resolution might not be
> excellent, but I've bought several magnifying devices through EBay and I'm
> going to test adding a DIY mag lense on the camera.

Try a jewelers loupe.  They are both inexpensive and
available in suitable magnification powers.

If actually available I'd prefer the greater resolution
and use an NTSC frame grabber if I needed to get image
data into a computer.  The advantage of NTSC is also
that you don't need a PC to decompress the video and
can simply use a standard analog monitor.

-- 
uhmgawa@...        www.gnu.org