Stencil design and printing is one of the most important steps when it comes to PCB assembly. Knowing how to properly design the stencil and implement paste dispensing for assembly takes considerable knowhow and in-depth experience. In this instance, there’s no substitute for experience since textbooks on the subject are few and far between.
Take for example a mixed signal PCB design with considerable analog circuitry. The analog segment requires a lot of heavy current and grounding. A thicker 6 to 8 mils stencil is recommended here. The thickest stencil would probably be 6 mils with a ratio of 1.1:1.
The ratio of 1.1:1 means 10 percent more paste is being deposited compared to the pad size. This provides a good fillet. As shown in Fig. 1, a fillet is the base of an IC or component, and a good fillet means enough solder is deposited, but not so much as to create a short.
The accuracy of stencil definition and design takes on even greater significance when a high-count layer board is involved. In these instances, thicker boards are used for a load board application, for example.
Unlike a regular 6 to 12 layer commercial board, this thicker board has 20 or more layers with more power and ground planes. What this means is that you’ll have a different thermal profile and in such a case, stencil design and paste dispensing takes on a whole new meaning.
When a stencil is properly designed, and solder paste is correctly deposited, the result is perfect printing. Put another way, this means few to no bridges or solder issues, less touch up and rework, and subsequent cost and time savings to hand you faster time-to-market.