Factors Affecting Appearance of PCB: Pre-Processing for Silkscreen

With the rapid development of printed circuit board manufacturing technology, users now not only have higher and higher requirements for the internal quality of printed boards (internal quality refers to hole resistance, copper foil thickness, continuity test, solderability, etc.), but also the appearance of the printed board puts forward higher requirements, such as: the ink color should be uniform and free of impurities, and the surface of the copper layer should be free of foreign matter and oxidation points. The following discusses the influence of silk screen pretreatment on appearance quality.

The main influences of the pretreatment of silkscreen on the appearance of the printed board are the copper under the ink is oxidized, and there is hard damage (obvious scratches on the copper layer or substrate) after the pretreatment. The ink color on the copper foil is uneven in the area. The quality of the pre-treatment directly affects the appearance quality of the printed board, slightly causes rework, affects the production schedule, delays the delivery date, and reduces the company’s credibility. Severely, it may also cause the board to be scrapped. Finally, the company’s “orders” are reduced, which directly affects the company’s economic benefits.

In order to gradually reduce the losses caused by the pre-processing, and improve the company’s competitiveness. It is necessary to strictly control the pre-treatment process.

Purpose and method of silkscreen pretreatment：

The final goal of the pretreatment is to make the copper surface free of oxides, grease and impurities, and also have a certain degree of roughness. The impurity of the pre-treated board has a certain degree of roughness. The cleanliness of the pre-treated board is tested with the Zero-Ion 100A ion concentration tester, to see the concentration of impurity particles on the board after the treatment, the ion concentration is lower than 1.5μg/cm2, and the state of the copper layer surface has a relatively uniform concave-convex structure and no oxidation points. This can enhance the mechanical bond with the ink and prevent blistering and large-area shedding. Affect the electrical conductivity of the printed circuit board.

There are many pre-treatment methods, and the main ones commonly used in the industry are the following: nylon brush scrubbing, chemical cleaning treatment, alumina/pumice powder injection, alumina/pumice powder plus nylon brush, chemical cleaning plus nylon brush.