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JULY 2025 I DESIGN007 MAGAZINE 45 D ES I G N E RS N OT E B O O K ▼ F i g u re 2 : I n - l i n e S M T a s s e m b l y p ro c e s s f l ow. Predetermine Power and Ground Distribution For the less complex single- and two-sided circuit structures, power and ground can be inte- grated within signal intercon- nects. The conductors assigned to distribute power and ground will typically be the first stage of the interconnect planning pro- cess, furnishing a significantly wider conductor than the signal- carrying conductors. When the power and ground are retained on dedicated circuit layers, they will contribute to improving sig- nal integrity by suppressing elec- tromagnetic (EMI) noise and interference. Power and ground can be dis- tinct areas of copper foil or the entire surface of internal lay- ers of the circuit board. How this is achieved will depend on the type of circuit board design sys- tem employed. Most CAD sys- tems furnish the ability to define a power and ground area that appears as a solid graphic repre- sentation. When an area of cop- per is retained on the outer sur- face of the circuit board, it will likely be positioned to service ground isolation of specific com- ponents or a group of related components. For mixed function applications, the designer may divide the power and ground planes on an inner circuit layer to isolate differing voltage and ground potentials. Preparing for Surface Mount Assembly Surface mount assembly is a highly controlled method for pro- ducing electronic circuits using a broad range of miniature passive and active components mounted or placed directly onto the sur- face of printed circuit boards. The component outlines and ter- minal configurations can be very different, relying on perfectly matched land pattern geome- tries to ensure a reliable electri- cal interface between compo- nent and circuit board. The flow diagram illustrated in Figure 2 furnishes the basic "print-place- solder reflow" process sequence for a single-side surface mount assembly. Surface mount assembly pro- cessing relies on specialized systems and strict process con- trols. The surface mount com- ponent type selected (passive and active, coarse-pitch, fine- pitch, BGA, CSP) will determine the assembly methodologies required. Solder materials developed for surface mount applications are primarily a tin alloy-based pow- der and flux composite. The sol- der paste material is commonly deposited onto the component land patterns using a contact stencil printing process that dis- tributes a precise volume of sol- der onto the components' land patterns. Although not practi- cal for many high-volume SMT assembly applications, dispens- ing solder paste has proved ideal for prototype or low-vol- ume applications, components with very exacting solder volume requirements, or where solder is to be deposited into narrow recesses not accessible using stencil printing. Note: The assembly special- ist should be consulted before specifying PCB surface plating or coating material. The following plating and coat- ing alternatives provide a flat, uniform, low-profile, solder pro- cess-compatible surface finish: