Stencil Manufacturing Requirements For Pcb

1. The size of the frame and template

Before manufacturing circuit boards, in order to make the printing more perfect and suitable, custom-designed stencils are required. The most important template for steel mesh is the size and frame. The template adopts the stretching method of AB glue and nylon mesh, and the protective paint must be scraped off evenly at the joint between the aluminum frame and the glue (S224). In order to ensure that the template has sufficient tension (specified greater than 35N/cm, usually 30~50N/cm) and good flatness, the distance between the template and the inside of the frame is not large. It is less than 25mm, preferably in the range of 50-100mm. As shown in Figure 1.

Since the inner side of the template and the frame, the bonding area between the template and the nylon mesh, and the bonding area and the printing area require a certain distance, the internal size of the frame is not the maximum size available for the template, and the most useful maximum printing size is the internal size of the frame. The size is 220mm or larger. Of course, this also depends on the width of the printing press guide rail, the size of the wiping mechanism and the size of the frame. For example, the frame size of the DEK265 printer is 29"x29", and the maximum printing area is 510mmx489mm.

2. Tension

Template tension refers to the tension of the nylon mesh applied to the template, which can be measured by a tensiometer. The principle is to test the thrust required for the local sinking unit distance of the grid, in N/cm, as shown in Figure 2.

In order to ensure the flatness of the template, sufficient tension is required, and it is generally required to be greater than 35 N/CM, usually in the range of 30-50 N/cm.

3. Hole wall shape and roughness

The opening of the template should ideally have an inverted conical shape, that is, the size below the opening is about 0.01 mm wider than the upper size (depending on the thickness of the template). As shown in Figure 3:

In the above cases, the template opening should be polished:

QFP, SOJ, PLCC, SOP, socket and CSP devices (pitch less than or equal to 0.5mm) have all printed templates.

4. Dimensional tolerance

At present, the machining accuracy of electro-polishing templates of major template manufacturers in China is shown in Table 1.