Application of PCB Surface Solder Mask

The solder mask of the printed circuit board is a permanent protective layer, which not only has the functions of anti-soldering, protection, and improvement of insulation resistance in function, but also has a great influence on the appearance quality of the circuit board. In the early stage of solder mask printing, solder mask film was used to make screen graphics, and then UV light-curable solder mask ink was printed. After each printing, excess solder mask remains on the pad due to screen deformation and inaccurate positioning. It takes a long time to scrape off, which consumes a lot of manpower and time. Liquid photosensitive solder resist ink does not need to make screen graphics, and uses empty screen printing and contact exposure. This process has high alignment accuracy, strong solder mask adhesion, good solder resistance and high production efficiency, and has gradually replaced light-curing inks.

1. Process Flow

Solder mask film making → film positioning hole punching → printing board cleaning → ink preparation → double-sided printing → pre-baking → exposure → development → heat curing

2. Key Process Analysis

(1) Pre-Baking

The purpose of pre-baking is to evaporate the solvent contained in the ink and make the solder mask non-stick. For different inks, the pre-baking temperature and time are different. If the pre-baking temperature is too high, or the drying time is too long, it will lead to poor development and reduce the resolution. If the pre-baking time is too short, or the temperature is too low, the film will stick during exposure, and the solder mask will be exposed to the corrosion of sodium carbonate solution during development, causing the surface to tarnish or the solder mask to expand and fall off.

(2) Exposure

Exposure is the key to the entire process. For positive pictures, when the exposure is too high, due to light scattering, the solder mask on the edge of the pattern or line reacts with the light. Mainly the photosensitive polymer contained in the solder mask reacts with the light. It results in a residual film, which reduces the resolution, causing the developed graphics to become smaller and the lines to become thinner.

If the exposure is insufficient, the result is the opposite of the above case, and the developed pattern becomes larger and the line becomes thicker. This situation can be reflected through the test: 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, the “light energy integrator” can be selected to determine the optimal exposure time.

(3)Ink Viscosity Adjustment

The viscosity of liquid photosensitive solder resist ink is mainly controlled by the ratio of hardener and main agent and the amount of diluent added. If the amount of hardener added is insufficient, an imbalance in ink properties may result. After the hardener is mixed, it reacts at room temperature, and its viscosity changes as follows.

Within 30min: The main agent of the ink and the hardener have not been fully integrated, the fluidity is not enough, and the screen will be blocked during printing.

30min～10h: The main agent of the ink and the hardener have been fully integrated, and the fluidity is appropriate.

After 10 hours: The reaction between the materials of the ink has been actively carried out, resulting in increased fluidity and poor printing. The longer the time after the hardener is mixed, the more fully the reaction between the resin and the hardener will be, and the gloss of the ink will also improve well. In order to make the ink glossy and print well, it is best to leave it for 30 minutes after the hardener is mixed to start printing.

If the diluent is added too much, it will affect the heat resistance and hardening of the ink. In short, the viscosity adjustment of liquid photosensitive solder resist ink is very important: the viscosity is too thick and the screen printing is difficult. The screen plate is easy to stick to the screen. The viscosity is too thin, and the amount of volatile solvent in the ink is large, which brings difficulties to the pre-curing.

The viscosity of the ink was measured using a rotational viscometer. In production, it is necessary to adjust the optimum value of viscosity according to different inks and solvents.