SMT: Solder Printing Technologies : Dispensing, Screen Printing
1. Printing Technologies
Solder printing can broadly be divided into 3 categories.
1.1 Dispensing
1.2 Screen printing
1.3 Stencil printing
1.1 Dispensing
Solder paste is dispensed by squeezing through the needle of a syringe. Here paste is dispensed sequentially on one land/ pad at a time. generally, dispensing is a manual or semi-automatic operation. Dispensing is appropriate when screening can not be used as in repair/ rework, prototype development, and prototype development or low volume production.
Dispensing is not considered for its speed of application, but for its versatility. For example, dispensing method can be used to print different materials, like adhesive, flux, and permitting the same tool to be used. Also, there is no vendor turn around time as in case of stencils or screens.
Dispensing may also have special application for fine pitch packages that require lower paste thickness than other components (i.e. lower than 8 mil) such as fine pitch SOICs, PLCCs). Though screen / stencil printing are dominant methods, dispensing is widely used for specialized applications for which screen printing is not viable.
1.2 Screen Printing
A screen made of a woven wire mesh is stretched over a frame with a glued-on photosensitive emulsion. The wire mesh supports the emulsion, which is etched where solder paste must flow to reach the substrate surface. The screens/ stencils have etched openings to match the land patterns on the substrate(PCB). The screen is attached to a aluminum frame, which imparts rigidity and tension, hence a flat but flexible screen surface. The wire mesh is usually made of polyester or stainless steel. Polyester materials are more resilient than stainless steel and last longer.
The diameter of the mesh wire and the thickness of the emulsion primarily determine the solder paste thickness that can be deposited. Screens are used mainly because of its lower cost, faster delivery and reusability. Also, hand printing is almost impossible with screens because it is very difficult to align the screen on the work area which is due to the poor visibility due to the presence of the screen mesh in the openings.
