soldering paste stencil on cheapo supplies, LQFP144 - success!

[nuclearcat]

CO2 laser 40W(JL-K3020) on 16.5%, 100 micron OHP film, LQFP144 (0.5mm pitch), 5mm/s vector cut over K40 Whisperer, took for testing a bit dense CIAA NXP board from github, converted in kicad solder paste layer to DXF
Film is cheapo one from nearest shop, important note - i put film between wet sheets of usual A4 paper (otherwise laser burn edges, and melt thin dividers on small pitch).
And watch out, if paper not enough wet - it will ignite film and paper

[texaspyro]

I laser cut stencils using a 405 nm, half watt, single mode laser diode chucked into a small CNC mill. 

 I use red "vellum" sheet from the craft store that is around 0.003 inches thick.  I get less than a 10 micron kerf width.  The laser shines down a tube with a nozzle on the end.  A small pump provides "assist" air that greatly improves the quality of the cut.

Mylar, projector transparencies, etc are not good for making stencils.  The resulting melty edges of the cuts are no good.

[nuclearcat]

I laser cut stencils using a 405 nm, half watt, single mode laser diode chucked into a small CNC mill. 

 I use red "vellum" sheet from the craft store that is around 0.003 inches thick.  I get less than a 10 micron kerf width.  The laser shines down a tube with a nozzle on the end.  A small pump provides "assist" air that greatly improves the quality of the cut.

Mylar, projector transparencies, etc are not good for making stencils.  The resulting melty edges of the cuts are no good.

Wow, thanks for the suggestions.
My machine is seems old model, as i dont have air assist, my guess if i will be able to add it, it will improve my stencils quality.
Also i am not sure about laser focus, it is not adjustable on this type of machine, but should be very close to it for thin sheets.
Right now issue with melting i solved "somehow" by wet paper, but air assist might do job as well.
Do you have any reference what kind of vellum sheets you are using?
Is it something like this: https://www.amazon.com/Strathmore-59-854-Vellum-Inkjet-Sheets/dp/B005JJVKUQ ?

[phil from seattle]

Can you PWM your laser to reduce the power?  I bet that would help cut down the melting.

[nuclearcat]

Can you PWM your laser to reduce the power?  I bet that would help cut down the melting.

It is 16.5%, but it is set statically on CNC panel before each print, and i guess it is PWM (as on low settings i can heart PWM specific humming when laser is working).
Probably i should make some test pattern and try different levels of laser power, but thats going to take a while.
I am considering to get "Cohesion3D Mini Laser Upgrade Bundle" board from tindie, as drop in replacement, it supports laser PWM dynamically during print, but not sure it will improve things a lot, and even might complicate (as i can't use K40 whipsperer anymore).
And another thing, on very fine pitch, maybe surface tension can get solder on pins automagically? As i will do 1-3 prototypes at once, not even small series, for now.

[texaspyro]

Do you have any reference what kind of vellum sheets you are using?

I get the vellum from either Michaels or Hobby Lobby.  The sheets are around 12x12".  The red ones cut best with a blue laser but the gold-ish yellow ones also work.

The air assist helps to make sure the cuts are clean and don't have any whiskers keeping the "chad" attached to the stencil. When cutting thinks like wood (balsa and basswood) that the edges don't burn.

CO2 lasers tend to have a larger beam width than the 405 nm laser diode that I use.

One way to help focus a laser is to use it to draw a line on a tilted block of wood, etc.  Look at the line with a magnifier and see where it is the smallest.  Use the distance that point was up from the base to adjust the laser height.

[nuclearcat]

Quick update, not sure if it is interesting for anybody.
Before i was able to make stencils, but using 2 sheets of wet paper covering film and some efforts and tests required for each sheet, i didnt liked it, so decided to make process more predictable.
Preferably without wet paper and long process of finding right power.
What is done:
1)Upgraded my CO2 controller to smoothieware board (Cohesion 3D). Now it can eat g-codes, and not some useless for my case proprietary software and protocol from chinese vendor.
Also it has PWM control for laser power, sure within limits that CW CO2 tube can run.
2)Did slight modifications to Smoothieware, default firmware doesn't fire laser precisely (by checking code i can say offset might be +-0.05 to 0.01mm on 50mm/sec moving rate), i dont blame them at all, their software is great, but it is mostly for engraving and cutting wood and acrylic, while i need _precise_ operation for very thin film.
3)After endlessly trying various software intended to engrave and cut much more thick materials, wrote my own, crude g-code generator that swallows svg generated by kicad, generate g-code, that does stencil cutting by firing 0.1mm dots like matrix printer, and avoiding continuously (over)heating film in same place. Improving WiP.

I can't say result is everywhere better, but it is MUCH more easily reproducible and no mess with water and paper cooling, also on previous one thin separators was burned and its easy to break them while "stenciling".
Result, and 1mm ruler for comparison

Also visualization for stencil parse result(g-code will cut yellow parts), where i keep in mind laser kerf width, still heatmap visualization not done yet.