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Second Rev of Boards with new Paste Mask is Back.

A project log for Homebuilt LED Displays

This is an attempt to re-create a long obsolete Hex LED display.

BharbourBharbour 11/10/2018 at 23:270 Comments

I made some minor tweaks to the digit PCBs, mostly adding some pull up resistors to logic inputs. The main thing that I did was reduce the paste mask openings on the QFN packaged 74HC595 parts. The thermal pad mask was reduced to allow a paste spot 60% of the size of the pad and on the individual pads (0.012" x 0.031"), I set the paste mask expansion value to -0.002". When I mounted the parts, the amount of paste on the thermal pad seemed about right. The part set right down on the board, as nice as you could ask for. The individual pads needed more solder paste. After the -0.002" paste mask expansion, the pin pads got 0.008" x 0.027" openings. Comparing the area of the pad to the area of the shrunk paste mask, there was still about 58% of the pad exposed. The paste I used seems kind of stiff, it may be that I just did not apply it correctly, but I think that it will be worth getting another mask with -0.001 paste mask expansion. A project for another day.

The 4 digit carrier board that I build now has all 4 digits in place and driven by the test software. There is something very pleasing about the "rounded" corners that this display type affords.

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