LS1 Solder Reflow Oven

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Re: LS1 Solder Reflow Oven

Postby TLHarrell » Mon Jan 30, 2012 6:43 pm

I have seen laser cut stencils made from overhead film. Worked fine. Forgot where I saw it though, so I can't drop you a link.
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Re: LS1 Solder Reflow Oven

Postby Enraged » Mon Jan 30, 2012 6:48 pm

Any reason that you couldn't use a similar setup for the heater on a vacuum forming machine? How about a Makerslide Vacuum Former? er... MakerForm?
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Re: LS1 Solder Reflow Oven

Postby lasersafe1 » Tue Jan 31, 2012 2:04 am

I found the pololu "low power CO2 laser cut mylar stencils". Is transparency film "mylar"? I didn't think it was, but then again the film that needs to go through a laser printer needs to handle some serious heat to melt the toner. I remember in the earlier years putting ink jet film through a laser copier and it turned into a melted mess and jammed up the copier at work. oops..

I guess I going to have to do some X'spermenting
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Re: LS1 Solder Reflow Oven

Postby lasersafe1 » Tue Jan 31, 2012 2:14 am

Answer = yes. Christopher Wong and Daniel Waldorf at Cal Poly did some extensive testing of different methods and found ordinary transparency film to be quite adequate.
Thats going to save me several hundred dollars over paying for several stainless stencils.

http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1054&context=ime_fac

They conclude in their paper:
After completion of this project, it can be concluded that:
1. Plastic copier transparency film is a successful alternative material for stencil manufacture.
2. Solder paste technology has advanced greatly and its use on plastic has no adverse effects.
3. The application of a laser cut plastic solder paste stencil is a resourceful process that may used in electronic
manufacturing, particularly for rapid prototyping of SMT boards or short run production.

Recommendations for the implementation and use of a plastic solder paste: stencil include:
1. Verify the proper aperture ratio for printability of solder paste through stencil.
2. The orientation of the stencil should be aligned with the squeegee print direction stroking along the axis of the
majority of longer pad dimensions and perpendicular to smaller pad widths.
3. To print results, similar to a metal stencil, use a light pressure and slow speed squeegee stroke leaving a thin
film of paste over the stencil. Angling the squeegee towards the print direction may also aid in print quality and paste roll.
4. Peel away plastic solder paste stencil rather than lifting straight off to prevent scooping and shifting of printed paste.
5. If printing repetitiously, or for more than one stroke through the stencil, be sure the underside of the stencil is
clean fiom solder paste, otherwise bleeding may result.
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Re: LS1 Solder Reflow Oven

Postby lasersafe1 » Tue Jan 31, 2012 3:18 am

And so the learning continues. I'm ready to take my board design and isolate the pads for the stencil. I use Eagle PCB, so I press the CAM button and it launches the gerb274x.com. That is my setup, so you might have to do a file-open-job, and select gerb274x. Next I hit the "ADD" button and it copies the section displayed into a new section. This new section can be renamed under the Job - Section to be called "solder paste stencil". I then go to the Output section and for Device I select "PS". The file naming area should also be %N.ps
Next I go to the layers and deselect anything that was in the selection. We don't want traces and silkscreens or dimensions, we only want the pads that are related to surface mount components, so I select layer 31 tCream. This is the layer used to create the solder paste stencil. Hit the "process section" button and you will get your "filename.ps" in the same directory as your board.

I could probably load the post script file directly into the LightObject DSP, (not sure), but I have CorelDraw, and I absolutely know I can load it in CorelDraw and then launch my laser right from there. This is SWEET! Only took a couple minutes to figure out.
snap.jpg
A simple pcb that was panelized
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Re: LS1 Solder Reflow Oven

Postby lasersafe1 » Tue Jan 31, 2012 6:30 pm

While I'm giving away all my secrets, I will also tell you that I like to purchase solder paste from Stanley Supply & Services. They have the best price anywhere for Kester Bar Solder and Paste.

Sometimes I solder my boards by hand when I have small orders to fill, but when I get into larger production I use the reflow oven for the surface mount stuff, then I put in all the thru-hole items and I dip solder the bottom.

Dip soldering is much like wave soldering, but you do it by hand. First you spray a fine mist of flux on the bottom of the board (old pump hairspray bottle), then you use tongs to hold the board just above the liquid for a few seconds to pre-heat and activate the flux. Then you dip the board so it is "floating" on the solder for about 1 second. Then you lift out at an angle. Done. A whole board of connectors and thru-hole parts done in 10 seconds when it would have taken minutes to solder by hand. I bought my CM201 lead-free bath from ebay and I have had great success with the Kester K100LD Low Cost Lead-Free Bar solder.
pot.jpg
Titanium alloy lead-free solder pot
pot.jpg (14.88 KiB) Viewed 22960 times

I suppose if you wanted to be adventurous, you could build a CNC controller to hold the board and do the dipping using makerslide components.

Here is an example of a rush job hand soldered. Yes, I know I didn't make perfect filets, but they all were wetted.
hand soldered.jpg
quick hand soldering - not great


And here is another example of a board that I dip soldered. Textbook perfect! And no-clean flux!
dip soldered.jpg
dip soldering
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Re: LS1 Solder Reflow Oven

Postby lasersafe1 » Wed Feb 01, 2012 1:41 am

Tonight I have made my first stencil with an overhead transparency. My first attempt was vector cut, but I found that the edges would melt even at moderately high cut speeds (80mm/s) and my laser turned down to 5% power. I then converted the post script to a 1 bit bitmap at a resolution of 256pix. I then tried raster engraving at various speeds and I settled on full power (30W) at 300mm/sec, 0.1mm interval with x-swing and unprocess overstriking with a whisper of air assist. (these are all LightObject DSP settings in case you are wondering).

I think it turned out just fine and it just has a little smoke haze, but the plastic was vaporized and left perfect pads. My smallest chip on this board is SOIC, but it could probably handle a slightly smaller pitch and still make a good stencil. I'm sure with very careful focus, X-unilateral, and .05mm stepover, it could get quite fine indeed.

I finally worked this project to the point where it actually uses a CO2 laser! It belongs here after all! Just making 5 of my own stencils saved me enough to pay for the LO DSP!

stencil.jpg
Solder paste stencil from copier transparency
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Re: LS1 Solder Reflow Oven

Postby bill.french » Wed Feb 01, 2012 3:14 pm

Very awesome; what's the no clean flux you're using?
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Re: LS1 Solder Reflow Oven

Postby lasersafe1 » Wed Feb 01, 2012 5:42 pm

Kester 951 No-Clean Flux. 1 gallon is $48. At my usage rate, the gallon should last 10 years.

Compliant Specifications: Bellcore Issue 1 GR-78-CORE & ANSI/J-STD-004 Flux designator ORL0
Container Size: 1 gal.
Flux Characteristics: Very low solids, rosin free, foam and spray application flux. Virtually no residue after the soldering process.
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Re: LS1 Solder Reflow Oven

Postby lasersafe1 » Fri Feb 03, 2012 9:00 pm

Today I printed a few boards using my new transparency stencil. Works great! I used lead-free paste that expired in 2009 and it still works fine (been in refrigerator all this time). I also just bought a 600 gram cartridge of the newer Kester Easy Profile 256 (EP256). This is a 63/37 lead paste, so not useful to those on the other side of the pond that have the rediculous RoHS rules. Wow! It IS easy and reflows real nice. The flux rises to the surface and hardens into a nice clear protective shell that insulates and protects the solder joint.
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