The principle of soldermask exposure
Exposure is the core of the entire soldermask process. The principle of solder mask exposure mainly uses the scattering of light to produce an optical reaction with the photosensitive polymer contained in the solder mask film, thereby achieving the desired effect.
Exposure is very critical in the solder mask process. When overexposed, due to the scattering of light, the solder resist film at the edge of the graphics or lines reacts with the light to form a residual film, which reduces the resolution and causes the developed graphics to become smaller and the lines Thinning; if underexposed, the result is opposite to the above situation, the developed graphics become larger and the lines become thicker.
If the exposure time is long, the measured line width is a negative tolerance; if the exposure time is short, the measured line width is a positive tolerance. In the actual process, a “light energy integrator” or “exposure ruler” can be used to determine the optimal exposure time.
Choice of exposure light source
The characteristics of the exposure light source directly affect the quality and efficiency of solder mask exposure. The spectrum emitted by the light source should match the absorption spectrum of the photosensitive material to obtain better exposure results.
At present, the wavelength of light absorbed by solder resist ink is 350~450nm. With shorter wavelength light, the edges of the imaged pattern will be neat and clear after exposure. Longer wavelength light energy has good penetrating power and can quickly solidify the deep layers of the ink. Since wavelengths below 300 nm are easily absorbed by glass and the polyester base of the film, the wavelength of the light source selected in the exposure machine is generally between 310 and 440 nm.