ds won't turn on? screen flashes

so I opened up my boyfriends ds to fix the L and R buttons, granted we did fix them but after closing it up there were lines on the bottom screen, hurt his eyes and it's just annoying. so I opened it up, checked everything and replaced the screen with my old dsi bottom screen, since the motherboard on that one was broken it's just spare parts anyway, and replaced the slightly faulty panel thing near the motherboard (I think it has something to do with the battery) with my old dsi one also, since it was in better shape. I proceeded to close up the ds back the way I opened it up, re hooked everything in as best I could (looked perfect) but then I put in the battery in, and it won't turn on, well won't stay on. the bottom screen will flash white and the power light will come on (it's fully charged and I've tried two different batteries) then it will shut off. I've reopened it and checked More times than I can count but I can't find a problem. any idea how to fix this? thank you for any help...

I have access to a replacement to any part but the motherboard

I hope this can get fixed because he's had this dsi since 2007 and I don't want to have to explain that it's finally done for