Its based on the vowel digraphs. I have used words that are suitable from aged 4 up to aged 8 and have tried to include words that contain wh and ph more on the 2rd and 3rd boards.
Board 1 - ay,ou, ie, ea & oy sounds
Board 2 - ir, ue (oo and yoo), aw, wh, ph, ew (oo and yoo) & oe sounds
Board 3 - au, ey, a-e, i-e, e-e, o-e, u-e
All boards and word cards contained in one file