1, the size of the frame and template
Before the board is manufactured, in order to make the printing more perfect and appropriate, a custom-designed steel mesh is required. The most important template for the steel mesh is the size and the frame. The template adopts the stretching method of AB glue and nylon mesh yarn, and a protective paint (S224) must be evenly scraped on the joint between the aluminum frame and the glue. In order to ensure that the template has sufficient tension (specified to be greater than 35N/cm, generally required to be 30~50N/cm) and good flatness, the distance between the template and the inside of the frame is required to be not less than 25mm, preferably in the range of 50-100mm. As shown in Figure 1.
Since the template and the inner side of the frame, the bonding area of the template and the nylon mesh, and the bonding area and the printing area require a distance, the inner dimension of the frame is not the maximum size available for the template, and the most useful maximum printing size is the inner size of the frame. Go to a size of 220mm or more. Of course, this also depends on the width of the printing machine rail, the size of the wiping mechanism and the size of the frame. For example, the DEK265 press has a frame size of 29"x29" and a maximum printing area of 510mmx489mm.
The templating tension refers to the tensile force of the nylon mesh applied to the stencil, which can be measured by a tensiometer. The principle is to test the thrust required for the partial sinking unit distance of the mesh, the unit is N/cm, as shown in Figure 2.
In order to ensure the flatness of the template, sufficient tension is required, and the general requirement is greater than 35 N/CM, usually in the range of 30-50 N/cm.
3. Hole wall shape and roughness
The template opening section should ideally have an inverted cone shape, that is, the size under the opening is about 0.01 mm wider than the upper dimension (depending on the thickness of the template). As shown in Figure 3:
In the above case, the template opening shall be polished:
QFP, SOJ, PLCC, SOP, socket and CSP devices with 0.5mm pitch and below, all printed template.
4. Dimensional tolerance
At present, the processing precision of the electro-polished template of the main template manufacturers in China is shown in Table 1.