The component size defines the thickness of the template.
The article explains why small and large components require templates of very different thicknesses.
Printing solder paste for very small components, such as CSP with a pin spacing of 0.3 mm or components in chip 01005 housings, is a key challenge in the printing process. This becomes especially difficult when there are also much larger components on the same PCB, such as RF screens, SMT connectors, or large components in a chip housing.
To keep the so-called An area ratio (illustrated in the figure below) greater than 0.55, which is commonly believed to guarantee good paste transfer to the substrate, smaller elements usually require a 3 mils template. Larger components, on the other hand, require a greater height or volume of the solder paste deposit, and therefore a suitable template thickness for them is in the range of 4 to 5 mils.
Figure 1: Concept of the area factor. Source: © 'Stencil Options for Printing Solder Paste for .3 Mm CSP's and 01005 Chip Components' William E. Coleman Ph.D., Photo Stencil and Chris Anglin, Indium.
Going deeper into the problem of the simultaneous presence of large and miniature components, it is worth looking at the 'work' of a thin and thick template during printing. On the one hand, large components require more solder paste to provide sufficient solder for the reflow process. If the same template is used to print paste for small parts, the holes are so small that it is difficult or even impossible to release paste from the aperture. The poor release of the paste is due to a combination of several factors, but the whole can be illustrated by the area factor concept. The area ratio is defined as the area of the hole divided by the area of the hole wall (Figure 1).
Figure 2: Advantages and disadvantages of using a thick template. Source: © 'Stencil Options for Printing Solder Paste for .3 Mm CSP's and 01005 Chip Components' William E. Coleman Ph.D., Photo Stencil and Chris Anglin, Indium.
The problems encountered when using a thick template for both small and large components are shown in Figure 2. A thicker template provides enough paste to form acceptable shapes for reflow solder joints, however, with small components, poor pad paste release results in insufficient pad paste release. solder paste deposit and the formation of defective solder joints.
There are several standard solutions to this 'dilemma'. The first - rare - option is to print the plate twice with two templates. First, small apertures are printed with a thin stencil, and larger deposits are made with a thicker stencil with special convex pockets to protect the first print. Another option is, of course, the template steppe tapes of variable thickness, which we will devote to a separate article. Another option is the use of modern technologies for the production of the template itself, ensuring good paste transfer efficiency for a surface ratio below 0.5. In this case, a thicker template can be used to print pads for all components. Modern technologies of cutting patterns with a surface coefficient ranging from 0.4 to 0.69 include laser cutting of patterns from fine-grained stainless steel, steel with PTFE coating and electroformed templates with or without a Teflon coating.