Screen printing is a common printing method for flexible circuit boards. In flexible circuit boards, single-sided printed circuit boards are more common, and dual panels are relatively few, but double-sided hard circuit boards are more common. The principle of the double-sided flexible circuit board is the same as the production principle of the commonly printed hard circuit board. The printed hard circuit board is the conduction between the two layers of the line by sinking copper; The double-sided flexible circuit board is made of conductive carbon ink or silver ink impregnated by small holes and filled with the upper and lower two layers of circuit. The conductive inks used in printing circuit plates are mainly silver ink and carbon ink. The following briefly introduces the screen printing process and the problems that need attention of double-sided flexible circuit boards with carbon ink as conductive ink.

1. Screen printing of upper-layer cables

The first selection of the data, baking pretreatment, in order to prevent the shortening of the data deformation to the post-process trouble. Select the side of the line with higher scale accuracy requirements as the upper line (numbered A to facilitate the difference between the base line B). Use carbon ink screen to print the A-side line, and pay special attention to controlling the thickness of the ink during the screen printing process to ensure that the resistance requirements of the line are satisfied.

2. Stamping carbon holes

The diameter of the carbon injection hole is generally 0.8 ~ 1.0 m m. Note that the carbon hole must be selected properly, the hole is too large, and the carbon ink soaked through the carbon hole is too much when the screen printing base line is printed, and the remaining ink will flow through the carbon hole and remain on the screen printing platform, which simply leads to short circuit of the line. And because of the limitation of the width of the line, the carbon hole cannot be made too large. However, if the carbon hole is too small, the stamping die manufacturing is more difficult, and the data of the manufacturing circuit board is not easy to fall after the stamping carbon hole, and it is necessary to rely on manual shedding. Otherwise, the carbon filling hole will be covered in the next process, which will hinder the ink activity and affect the conduction of the upper and lower layers of the line. And the carbon hole is too small, the amount of ink through is small, and the lack of the upper and lower layers of the line is conductive. In the process of stamping carbon holes, it is necessary to pay special attention to the A side (the upper line surface), the intention is to make the stamping tip (that is, the flash, the jagged protruded) towards the side without screen printing, and its effect will be fully reflected in a process. In order to add the conduction probability of two layers of lines, a carbon filling hole can be added at the same place, and the field of view between the two holes is determined by the width of the field of view, which cannot be too large. In view of the small carbon hole, the interval is close, the stamping die manufacturing is more difficult, generally manufacture two sets of molds, stamping one of the two small holes at a time, that is, half the carbon hole is completed in two times.

3. Screen printing of basic lines

Based on the positioning hole, print the base line on the unscreened surface. Because the tip is toward the B side, the tip is also covered with A layer of ink after screen printing, and under the gravity effect, the active carbon ink is more smoothly filled through the carbon hole, and is connected with the A-side line (the upper line), so that the A and B sides of the line are connected. If the tip is facing the A side, when the B side screen printing, the ink flows through the carbon filling hole to the A side line, which is obstructed by the tip, the tip has A certain height h, and the ink is not easy to flow through the A side line of the tip to connect, so the probability of A and B two layers of the line is small. The activity of screen printing conductive ink is also very important to the conduction of carbon filling holes, the ink is viscous, the activity is poor, and the conduction probability of the upper and base lines is also small. In the screen printing process, if the use of machine printing, the selection of vacuum suction equipment on the workbench is conducive to the activity of conductive ink, but it is usually simple to leave the ink on the workbench, and the residual ink must be removed in time, otherwise it will affect the quality and beauty of the line, and even form a short circuit of the line. If the use of technology printing, the work table does not have vacuum suction equipment, after screen printing should be placed on the circuit board for a period of time and then put into the drying oven dry, so that the conductive ink is fully active, so that the two layers of the line is conductive. Screen printing process to control the ink thickness to meet the requirements of the line resistance value.

4. Shape stamping and final product inspection according to the use of the circuit board, stamping forms a shape with certain scale requirements. Test whether the line is short circuit, whether it is open, and whether the resistance value meets the requirements.