Homebrew PCBs

--- In Homebrew_PCBs@yahoogroups.com, Stefan Trethan 
<stefan_trethan@g...> wrote:
> thanks Dave.....
> 
> i don't like the "couple of weeks" part,
> an the "fast programmer".


Compare to a newbie making a PCB with plated thru holes.....


 
> i could of course edit the librarys so that there is a short line 
on the 
> place
> where the paste has to go. 

If you can add a goto line then that is half the battle.

With the X/Y location, your table moves to those coordinates and 
stops.  The goto command would send the software to the paste 
dispensing section.


Of the top of my head, I see it as 
state a rapid to some 20 mills above the PCB.
slow feed to close to the part while also slow feeding a drop.
then slow retracting to allow the drop to remain on the board.
then a fast retract to normal Z height
(then retrun to g-code for move to next posisiton.)

Of course, if one were to have 5 drops along a SOIC pad, either the X 
or Y would have to move while dispensing, OR each of the 5 
coordinates would be needed to be a place the machine stopped and 
called the paste function.

Since there are only so many specific pad sizes, it is quite possible 
to make a routine that would so a whole 8 pin SOIC chip or a routine 
that would do specific pad sizes.

I have a variable for feed rate so that I can change that one point 
at the beginning of the program and run the part with that value.  
Basically on my machine tool, it is for drill depth as I change 
drills and find it is easier to measure the new tool and put in an 
offset than to try to get a tool to go back into exactly the same 
height as the last tool.

Dave









> Is there a really simple solution to modify these "z axis" action 
to get a 
> dispenser
> to "mill the paste above the pad"?
> I mean imagine you have a 2d drawing of a part you want to mill, 
how do 
> you add the
> z axis? how do you tell how fast the dispenser goes down, and how 
far it 
> moves?


YES.

as I laid out above, a simple goto in the existing files can send the 
software off to a sub-routine that handles the feed and dispense 
section.  That leaves your drill file very readable and puts all of 
the feed/dispense stuff into one small section.


> 
> Maybe the "pump paste now" could be hardware engaged - whenever the 
nozzle 
> is down fire up..
> one could use adjustable delays and "suck back" as soon as the 
nozzle is 
> raised.

I would look at a CNC stepper motor with a leadscrew.  imagine a 20 
turn per inch leadscrew and a 400 step per inch motor. and a quarter 
step motor driver circuit.

that is 20x400x4 or 32,000 steps per inch.  each step is 0.00003125 
inces per step.  If there are 40 mils per mm, then add a zero for 
steps per mm.

As you can see, a very tiny step will push on the syringe a very tiny 
amount.  As I said a long time ago in this thread, accuracy will go 
way up with doing CNC.

Also, if you wanted, you can get an 8 step driver that will multiply 
motor steps by 8. or a 10 step driver.  I have seen one that had up 
to 250 microsteps per motor step.

Bottom line on the dispenser it that it is the easy part.
the CNC is the easy part.

the file handling between generating your Gerber files to getting the 
paster file is (currently) the hard part.

Dave







> 
> ST