Intro: Easy BGA Alignment With Laser-Cut Kapton
I recently enjoyed the experience of hand-mounting a 484-ball BGA, and found a great trick to make it stupid easy to get these things perfectly aligned and solder pasted in about 20 minutes. The chip was a Xilinx FPGA, namely a Spartan6 LX45T, and the board was the Spartan 6 PCIexpress card from Christophe Carpentier on Opencores.
Christophe was kind enough to sell me a board will all the components soldered except for the DDR and FPGA. All I needed to do was slap the FPGA and DDR on there, and was good to go! Unfortunately, I ended up getting a bad FPGA, so I was soldering it and desoldering (and reballing) it over and over trying to figure out what the problem was.
My best guess was that slight misalignment was causing a short way down in the forest of BGA pins, but without X-ray I just couldn't be sure I had gotten the alignment right.
During an intense head-scratching session, I realized that with a little sticky Kapton film and a PDF of the footprint, I could just raster out a stay-in-place stencil (like those expensive ones you see online) and my alignment would be guaranteed perfect!
By cutting out the pattern of the BGA footprint in Kapton and placing it on the PCB, the chip will drop into the holes cut in the Kapton as you slide it around. This makes perfect alignment very easy, and gives you a nice tactile feedback as to when the chip is aligned properly with the PCB pads.
Step 1: Find a Properly-scaled Footprint for Your Device
You can use a CAD tool to dump out the paste layer ala Adafruit's tutorial, but since I like to tweak the line sizing and other stuff a PDF was fine for me. The PDF method also works well for odd footprints where an eagle library is not easy to find.
Typically, the mechanical drawings in datasheets are accurate even if the scale is wrong. That's easy to fix.
First, websearch the FPGA's package type (FGG484). This search brought up the package data as the first result.
Grab the PDF and fire up CorelDraw.
Step 2: Open the PDF in CorelDraw and Start Hackin'
My Chinese laser cutter uses the "CorelLaser" setup, where an extra toolbar is installed to your existing CorelDraw with commands for sending to the laser cutter. So I fired up Corel and opened the PDF.
All we really want is the footprint, so it's a simple matter of selecting and deleting the other junk that comes in the mechanical drawing.
I deleted the crosshairs, row/column designators, arrows, and a few other little line bits that I found in some of the ball grid. It really only took a minute or two.
The first picture shows the original PDF, the second picture shows the "stripped" image in Corel, and the last picture shows just the ball grid itself after scaling the whole shebang to the right size.
This BGA has a 1mm pitch, so it was pretty simple to measure the distance between the centers of the 1st and the 20th ball. That distance should be 20mm. It was not, so I scaled by 50-something percent to give a final result of 19.999mm. 1 micron off? Good enough for me - the kapton is a bit stretchy, you'll never notice.
Step 3: Zap It
Once you've got your footprint squared away, simply send to the laser cutter as a raster operation. This operation scans back and forth like a printer, pulsing the laser to zap away the shapes you made in Corel.
My suggestion is to stick the film on something metal, and raster it. The metal will conduct away excess heat and prevent the Kapton film from melting and deforming. Both of those are problems when dealing with high-density laser cutting as we are using for this project. Here, I'm using a thin piece of sheet metal from a big-box hardware store, and just stuck a piece of Kapton film right in the middle.
Step 4: Clean It
The Kapton that is removed by the laser cutter leaves an ugly residue of char and ash. It's easy to clean with water or alcohol. A bit of light scrubbing with a cloth or paper towel will remove nearly all of the crud.
Some of the crud may have adhered to the metal plate and make the stencil look dirty even though it's not. Take a close look at the edges of the cut holes to see if they're pretty much clean, then stop. You don't want to overdo it, as the swiss-cheese of Kapton is a bit fragile and could deform or tear from rough handling.
Also - my adhesive seems to be resistant to denatured alcohol, yours may not be. You don't want to remove the adhesive accidentally, so take a minute and check on a spare piece. Or just use water - it's not quite as effective but should be compatible with all adhesives.
I didn't make an outer cutout on the stencil shown in these pictures, but nowadays I do. For this one, I just used an exacto knife to cut it out and peel up a corner. At any rate, be careful when starting to peel - you don't want to tear the stencil!
Step 5: Slap It on and Paste It
Sticking the stencil on is really quite easy. It's got a bit of stretch, so some misalignment can easily be corrected. Tack down one corner, making sure the PCB pads are clearly visible through the holes in the Kapton. Continue down that row until both corners on that side are aligned, then work from that side to the other. You're done!
Here, I'm using 1.5mil Kapton film. That is very thin, but still generates nice "divots" for the BGA to fall into for perfect alignment. You can use the film as a stencil for a little bit of solder paste, and the thickness of the Kapton will determine how much paste is available at each pad. You don't need much extra since the BGA already has solder balls, but a little is nice to get the wetting started.
Squirt a rice-sized blob of solder paste down and spread it around into all the holes, then squeegee or scrape off. Again - be gentle, you don't want to tear your stencil.
I am using leaded solder paste with a lead-free BGA. If you do this, you must reflow to the higher temperature needed to melt the lead-free solder balls. You should also dwell at this higher temperature for about 30 seconds to ensure good mixing between the leaded paste and lead free solders.
Once you've got it aligned and all seems good, just toss in the reflow oven.
Step 6: Reflow and Go
The other neat thing about the Kapton sheet is it seems to repel solder bridges. So along with alignment, you get the added benefit of the Kapton helping to coax any errant solder paste into an existing solder ball.
In case you're wondering - yes, the Kapton film is sealed in there after soldering - it's now a permanent part of your PCB. No matter, it won't bother anything. At the very least, it will help crud like thermal grease getting worked in to create shorts between your pins. The one case in which it would be a drawback is epoxy underfill, as the epoxy would bond predominantly to the Kapton rather than the PCB.
But for the hobbyist, this method provides an easy and reliable way to align those big BGA packages without a fancy split-vision alignment system.
Go, my friends. Go and produce complex systems!
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