Saturday, January 14, 2012

First BGA board

(NOTE: This was originally scheduled to happen a lot earlier but the fab screwed up shipping on the boards. I only got them today.)

A while ago I decided to get serious about doing a design based on the Spartan-6 FPGA. Unfortunately all but the smallest part in the family are BGA only. They're somewhat pricey ($40ish for the XC6SLX25) so I figured it'd be a good idea to practice on something less expensive!

Quick calculations given the DorkbotPDX batch order's design rules showed that anything under 1mm pitch was not possible in a full array since a via could not fit between adjacent balls. This rules out the CPG196, CSG225, CSG324, and CSG484 packages. The remaining options are FTG256, FGG484, FGG676, and FGG900. FTG256 looked like the easiest as it had the least pins.

I did a bit of catalog browsing and determined that the cheapest FTG256 part available from Xilinx was the XC3S50A, which at $10 a pop was still a bit expensive for testing BGA soldering processes.

Eventually I settled on CPG56 packaged CoolRunner-II CPLDs as my first victim (Digikey page). CPG56 is 0.5mm pitch but is only two concentric rings, not full array.
CPG56 packaged CPLD

The next step was to design a board. I went with a simple 2-layer design that did not break out all of the balls, but seemed to offer enough IOs to be useful for casual testing.

Once the boards arrived I quickly inspected them under my microscope. They were the typical DorkbotPDX batch boards - purple LPI soldermask and ENIG finish on the pads. Some of the unbonded pads (A7-A9) appeared to have lifted off the board or underetched during manufacture. Since the pads weren't being used in the board layout this was not a problem, though it did mean the chip would be attached a bit less securely. I forgot to take a photo of this but will try to upload one soon.

I have a Proctor-Silex toaster oven I had used for reflow several times in the past but never on a BGA. There is no thermocouple or automatic temperature profile control on the oven yet (a Type-K thermocouple is inbound from Sparkfun as I write this) so I used my standard manual profile for lead-free solder, adjusting the thermostat by hand for each step:
  • Heat quickly from room temperature to just below 100C
  • Soak at 100C for 30-60 seconds
  • Ramp up to 180C, hold for 15-30 seconds
  • Ramp up to 220C, hold for 15 seconds
  • Open door, wait 30 seconds, remove board
The first two attempts at soldering failed as the chip was not even close to correctly aligned. The first (pic coming soon) was so far off (about 250 μm) that none of the balls even touched the pads. Visible holes in the flux residue showed just how far off center it had been. The second was slightly better but still not usable.

One of the first two reflow attempts
On the third attempt I paid extra attention to centering the chip and it seemed to turn out well. I then hand-soldered the clock oscillator, voltage regulator, JTAG header, and the 0402 sized decoupling capacitors on the back.

When I attempted to solder the breadboard headers I realized I had used the wrong drill size and the pins didn't fit. Since the only ones I really cared about were power and ground I just soldered two wires into the holes.

Finished board, top view
Finished board, bottom view
Electrical test showed no shorts anywhere between adjacent balls. Optical inspection looked good too.

Side view of CSBGA
After hooking it up to a 3.3V power rail and a JTAG programmer, I was able to successfully program it with a simple design (divide-by-2 counter). Oscilloscope confirmed a nice 10 MHz squarewave on the output when driven by 20.