I have a design which uses a Xilinx FPGA, in 0.5mm lead pitch TQFP. The fastest signals are a 96MHz SDRAM interface. The rest are slower (maybe twelve signals at 24MHz and the rest below 5MHz).
I’m trying to design the PCB so it’s suitable for home etching (i.e, two-layer board, vias are tiny rivets in 0.6mm holes, no vias under SMDs, minimum 0.2mm track width/separation). On previous designs I have had no problems with 0.8mm lead-pitch TQFP packages; I thought the jump to 0.5mm would be easy.
Unfortunately this chip has twelve 3.3v supply pins, four 1.2v supply pins and twelve ground pins, all of which need vias down to the ground plane or to their decouplers. And since these vias are spread evenly around and necessarily quite close to the chip, routing the signals between them appears to be impossible within the design constraints I mentioned above.
Has anyone managed to make a similar device work on a home-made PCB? If I could only put vias under the chip I think it would be fairly straightforward, but I don’t know of any way of making home-made vias sit sufficiently flush with the board to allow me to solder a TQFP on top of them.
- Chris