Thursday, April 14, 2016

Prj 145 - BBB LX9 FPGA Board Construction (Part 3)

This post captures a few notes on the fabrication of a BeagleBone Black minimal FPGA board. Previous posts covered the block diagram and schematic.  The board is a 2 layer OSH Park order at roughly 3" x 2".  One of the differences with this board is that I used 0603 resistors and capacitors for density reasons rather than 0805's I normally use.  I have used these in the past in a limited capacity.  The mechanics of mounting these are no different, however, I did find that the smaller parts slowed me down.  In the end, I think it was worth it as there were a couple of places where the 0805's would have made the board layout more difficult. Beyond the this, and the TQG package, there is nothing too challenging about this build.
Spartan 6 LX9 Board.

LX9 Board Mounted to Beagel Bone Black
This was the first time I had used a TQG package, so I was a little nervous about how it would turn out.  I have gotten reasonably good at working with 0.5mm pitch QFNs but only in the 40 pin range. Airgunning the QFNs works really well and they self align nicely if you get the solder paste application right.  I only have a jewelers loop not a microscope so manual alignment was a concern. My attempt on the first version of this board used an airgun.  This was not a good idea.  The problem with this is the shear area - its 22mm X 22mm.  It took forever to get the paste to melt and I had a hard time evenly distributing the hot air around the perimeter of the part.  There are hoods for air guns (which I do not have).  The board above used manual placement with a soldering iron.  I tacked down a pin on one corner, inspected, and then tacked down a pin on the opposing corner.  This was followed up with running a solder bead down each side and then wicking off the excess solder (you can see the flux residue from this around the part).  This worked out extremely well and was simple to do.  The picture below captures a closeup of the end result.
Closeup of hand soldering and alignment of TQG-144.
The only issue with the technique is that if too much solder is applied it tends to walk up the knee of the pins where it creates shorts with adjacent pins.  This high in the knee makes it difficult to wick off. I found that inspecting all of the pins from three different angles (front on, top angled left, and top angled right) allowed me to catch all instances of this.

Related:
Prj 145 - Beagle Bone Black Simple LX9 FPGA board (Part 1)
Prj 145 - BBB LX9 FPGA Board Design (Part 2)
Prj 145 - BBB LX9 FPGA Board Construction (Part 3)
Prj 145 - BBB LX9 JTAG Boundary Scan Utilities (Part 4)
Prj 145 - BBB LX9 C++ and VHDL (Part5)

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