Pads are too wide or too close for the chosen density level. IPC-7351A prevents this by adjusting side extensions per lead pitch.
IPC-7351A forces you to use the component dimensions for the heel and the minimum component dimensions for the toe (or vice versa, depending on the feature). This "worst-case" math ensures that every part from every batch will sit correctly on the pads. ipc7351a
Its primary goal is to standardize the dimensions of the copper pads on a PCB to ensure the reliability of the solder joint. Before standards like IPC-7351A became prevalent, land patterns were often "guesstimated" or copied directly from component datasheets. This led to inconsistencies; one manufacturer’s SOIC-8 footprint might differ slightly from another’s, causing assembly yield issues. Pads are too wide or too close for the chosen density level
One of the most critical features is the definition of three distinct land pattern geometry variations (Density Levels) for every component family, allowing designers to optimize for different manufacturing goals: This "worst-case" math ensures that every part from
Pad Width = Lead Width + 2T + 0.10 (optional side fillet) = 0.40 + 0.20 + 0.10 = 0.70 mm