A well-designed board that follows the basic surface-mount technology considerations greatly improves the cost, cycle time, and quality of the end product. Board designers should comprehend the SMT-automated equipment used for assembly, including minimum and maximum dimensional limits and placement accuracy. Many board shapes can be accommodated, but the front of the board should have a straight and square edge to help machine sensors detect it. While odd-shaped or small boards can be assembled, they require panelization or special tooling to process in-line. The more irregular the board — non-rectangular with no cutouts — the more expensive the assembly cost.

Fiducials (the optical alignment targets that align the module to the automated equipment) should allow vision-assisted equipment to accommodate the shrink and stretch of the raw board during processing. They also define the coordinate system for all automated equipment, such as printing and pick-and-place.

The following guidelines may be helpful:

• Automated equipment requires a minimum of two and preferably three fiducials.
• A wide range of fiducial shapes and sizes can be used. Among the most useful is a circle 1.6 mm in diameter with an annulus of 3.175/3.71 mm. The outer ring is optional, but no other feature may be within 0.76 mm of the fiducial.
• The most useful placement for the fiducials is an L configuration, which is orthogonal to optimize the stretch/shrink algorithms. When possible, the lower left fiducial should be the design origin (coordinate 0,0).
• All components should be within 101.6 mm of a fiducial to assure placement accuracy. For large boards or panels, a fourth fiducial should be added.

If the edges of the boards are to be used for conveyer transfer, a cleared zone of at least 3.17 mm should be allowed. Normally, the longest edges of the board are used for this purpose, and the actual width is dependent on equipment capability. While no component lands or fiducials can be in this area, breakaway tabs may be.

Interpackage spacing is a key aspect of DFM, and the question of how close you can safely put components to each other is a critical one. The following component layout considerations are recommendations based on TI experience:

• There should be a minimum of 0.508 mm between land areas of adjacent components to reduce the risk of shorting.
• The recommended minimum spacing between SMD discrete component bodies is equal to the height of the tallest component. This allows for a 45° soldering angle in case manual work is needed.
• Polarization symbols need to be provided for discrete SMDs (diodes, capacitors, and so forth.) next to the positive pin.
• Pin-1 indicators or features are necessary to determine the keying of SMD components.
• Space between lands (under components) on the backside discrete components should be a minimum of 0.33 mm. No open vias may be in this space.
• The direction of backside discretes for wave solder should be perpendicular to the direction through the wave.
• Do not put SMT components on the bottom side that exceed 200 grams per square inch of contact area with the board.
• If space permits, symbolize all reference designators within the land pattern of the respective components.
• It is preferable to have all components oriented in well-ordered columns and rows.
• Group similar components together whenever possible.
• Room for testing must be allowed.