Developing process of flexible circuits is the unexposed active group of photographic film part response to dilute alkali solution to the produce soluble substances and dissolve, leaving the already sensitive graphical circuit pattern. The most common used liquid is made of Na2CO3, weight content ratio between 0.75% and 2%.

Dry or wet film load is small, so the correct solution configuration of flexible pcb is the most basic step, in order to accurately control the concentration, we use acid-base titration method calculate the concentration of Na2CO3.
Reference to below methods:
(1) Sampling 25ml developing liquid in 250ml beaker;
(2) by adding D.I. water 100ml and 3-5 drops of methyl orange;
(3) use 0.1N hydrochloric acid to titrate the color changed to red;
(4) Calculate: wt% Na2CO3 = 0.0212 × titre (ml).

Usually dry film in 1% Na2CO3 developing liquid, the loading is about 12-15 mil-ft2/gal. At a higher developing pressure and under the full water washing can be obtained higher loading.

DI water, soft water is recommended for developing of flexible circuits, the use of hard water would have caused the hard dirt to clog of the nozzles.