I developed a new chemical process to make double-sided PCBs with a laser cutter, and some common chemicals. This is intended to be used on SLA 3D-printed parts, but may work on many materials.
Refs:
http://infohouse.p2ric.org/ref/29/28004.pdf
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.527.5817&rep=rep1&type=pdf
http://shodhganga.inflibnet.ac.in/bitstream/10603/195088/3/chapter%202.pdf
https://sci-hub.tw/https://doi.org/10.1016/S0167-577X(03)00424-5
https://library.nrao.edu/public/memos/chem/CHEM_5.pdf
https://core.ac.uk/download/pdf/48645791.pdf
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