White Solder Mask - color not consistent

Hi All member

I have a problem on the stable down the color of the white solder mask PCB.

first, i tough the color differences was cause by the solder mask ink, but after a close look, the color change are cause by the copper under the solder mask layer.

The problem occur are after the final cure oven.

i have cross section the pcb to check the mask thickness, it was about 20 to 25um. i even try to print 2 layer of mask, the thickness was about 35 to 38um, still the copper color will cause the PCB look yellowish.

however, there still have some good looking pcb which is look white from the same batch.

please anyone, have any ideal how to control the white solder mask color to be more consistent.

material used.

Solder mask ink - OTC R-500 WH (special ink for LED PCB) Laminate - CEM3 1oz base copper 2 side. ( thermal conductive CEM3) legend - 2 side Finishing - HAL Lead Free or OSP

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