A couple of weeks after my last post, my new boards are back! So now, I have my new boards from OSH Park, my paste mask from OSH Stencils and my replenished component stocks from Mouser. I have a new plan for assembly and a simpler board. I’m feeling hopeful!
Apart from the simpler layout and fewer components on this version, my plan is to correct the mistakes of the last build. The main changes are:
- Sparing use of solder paste – last time the paste mask moved and spread solder unevenly over the board, outside the pads. Ensure the paste mask is held in place and doesn’t lift to allow paste under the mask.
- Larger (oversize) holes in the paste mask to ensure the 0.8mm pitch devices, such as the SDRAM center bottom in the image, get enough paste to reflow properly.
- Assemble the board dry first and transfer the components quickly from one board to another once the paste is applied. This will reduce the actual build time. If I don’t have a spare board, a laser print of an actual size board would work as well (and might be better).
- The small components will be laid out first. This is at odds with typical build methods, but will allow me to determine if I can accurately place the small components before I commit the big expensive FPGA, ARM microcontroller, Flash and video chips.
There’s no going back on this build, I’m determined to get this working if only partially. If I have an issue in the build, I’ll try to analyse the the fault and if needed I’ll rework the SMT devices using hand tools.
The board is designed from left to right, so only populating the left hand side components will get an element of the board working, specifically the ARM microcontroller and flash memory. So a partial success could be just the ARM micro working, or the ARM and FPGA, or if we’re lucky, the ARM, FPGA and video chips.
I’ve made quite a few changes since the last build, so I’m feeling hopeful this one will work. Look out for details in a later post.