The test run of stencils are here! They work well… with a little practice. My first attempt at applying paste took around 6 tries to get it right. The next took two. The next two went without a hitch.
What I do is stick the star down to a table with a small piece of double sided tape… smaller than the star so the tape does not stick out past the edge and snag the stencil. I stick my finger on the tape a few times so the top is less sticky than the bottom. Keeps the tape on the table when you remove the star.
I put a couple of strips of masking tape across the stencil, align the pattern to the star, and stick the stencil down to the table with the masking tape. It takes a bit of practice to get the tension right so the the pattern stays aligned. A little stabilization with one hand can help keep the stencil aligned.
I put a little dab of paste on the edge of a strip of credit card material (You say you are a wise man that doesn’t have credit cards? Go to Kinkos/Fedex store and get a free payment card from the machine). Hold the strip of card at a 45 degree angle and swipe it across the stencil.
Carefully lift the stencil. If all went well you will have the pads covered properly with no bleed between them. You will be amazed at just how little paste is used. If you got bleed, misalignment, etc, wipe off the star, clean the stencil with alcohol, try again.
Lift the star off the double sided tape. You may have to scrape the tape off of the star.
Drop the LED onto the solder paste. I was able to test the LED with a meter before reflowing the star. There was no shorts between the power connections and the thermal pad and the LED lit up.
Reflow the star… I use a small butane torch. One thing is that I got no excess solder oozing onto the overflows to indicate that the solder flowed. Also the LED was perfectly aligned to start with and did not do the “floaty rotatey dance”. I looked for a tiny amount of flux bubbling out of the edges and a shiny edge of solder at the edge of the LED.
Ok, XRE’s and (probably) MTGs are real easy… you can do them blindfolded. The XRE stars that I used were DX non-Sinkpad style stars. All the others were Noctigons. I don’t have any MTGs to play with (frankly I don’t see much good in those things).
XPG’s are a bit of a pain. The apertures are quite a bit smaller. It took me 5 tries to get a clean print. Should improve with practice, but I only had one Nichia 219 to test with. The first try left one pad dry. I could have used the second through fourth prints if I took a resistor lead and cleaned up the slight bleed I was getting between one of the pads and the thermal pad, but I wanted to see what it took to do it with just the stencil. I should have cut a smaller width piece of Kinko-card for use with XPGs. The piece that I was using was too wide.
Also, I think that just holding the stencil in place with one hand works as well as trying to tape it down. The double-sided tape holding down the star works very well.
A little bleed through won't hurt anything.. The solder will snap back onto the pads as soon as it reflows - especially if its tin-lead.
One trick that really helps with mylar/kapton stencils is to set a few of the same-thickness stars around the one you're pasting. It prevents the stencil from wanting to flop over the edges of the tiny star while you're squeegeeing the paste.
I know that the bleed doesn’t usually matter, but I was testing the star/led before I reflowed it (if it tests good you know there are no opens/shorts). If both pads had bled to the thermal pad, I couldn’t test it that way.
Supporting the stencil with other stars should work really well. With the stencil flopping over the edge, it wants to bow the stencil upwards and you get more bleed/misalignment.
Also, I was using some pretty “low tack” masking tape to hold the stencil in place. If your tape is too aggressive you might damage the stencil when removing the tape.
I was thinking that blue low-stick painter’s tape would be about right. Blank FR4 PCBs are about the same thickness (~1.65mm) as the star. I will take a spare one, trace around the star onto it and cut out a same-diameter hole. Tape the board down, set the star in the hole, and lay the stencil over it; no sagging, and the star will stay put.
I’ve got an XP-G2 waiting for a star swap for Ezarc. I think I’ll wait until this is ready. The few XML’s I’ve done were nerve racking enough. It looks so horrible when you put the solder paste on that you think there is no way that will work but somehow they all have.
OK, I have 18 stencils available from the first run. Cost is $3 each shipped in the US. International add $1 to the total amount for postage.
They will be shipped in a standard letter envelope, no tracking… you pays your money, you takes your chances. If you need tracking, cost goes up to stupid levels… I think that the cheapest postage rate with international tracking is over $10, plus it draws the attention of nosey, sticky fingered customs agents…
First come, first served… I’d like to get some feedback on these before doing another run.
PM me for my paypal address… say how many you want so I can track the quantities and include your address.
