Factors to Consider for a Successful DFA
Design for assembly (DFA) is an ongoing struggle to balance assembly, fabrication and layout. The layout designer strives to create a product with ease of assembly in mind. The easier the product is to assemble, the cheaper the final product. In order to design the best assembly, the layout designer needs to understand the fabrication limitations of the components and the fabrication shop. The layout designer is a mediator between the requirements of the engineer, the fabrication shop abilities and the needs of assembly.
General
DFA is more than placing components at a safe distance from one another. The small pitch components of today are pushing the limits of fabrication tolerances. Understanding the fabrication shop’s minimum soldermask webbing is crucial for footprint development. Copper land patterns should take soldermask webbing into account. Oversizing the width of the pin footprint should not eliminate the soldermask webbing. Combining the pins into one soldermask opening is called soldermask ganging. Soldermask ganging increases the likelihood of shorts between the pins during assembly. Avoid ganging soldermask when possible.
Vias are a major factor in testability and DFA. Vias that are too close to a pin do not allow a soldermask webbing. The absence of the webbing will starve the solder from the pin. The paste will travel through the via and short components on the opposite board. Vias should be exposed for testability. Adequate clearance from pins allows test probes to reach the via and allows a soldermask web. Vias are concern for cold solder joints as well. On a multi-lamination board with four or more ground fills, a direct connect via can absorb the heat into the planes and cause cold solder joints.
Figure 1 - Solder bridge caused by an inadequate soldermask between a pad and via on the opposite side of the board.Read the full article in: http://www.pddnet.com