Impedance Coupon Reports Read against the Working Board
Peripheral coupon TDR reports require empirical offset adjustments derived from physical microsections to accurately reflect functional internal board impedance.
Solder Mask Encroachment
Mask Displacement
Solder mask encroachment describes a liquid photoimageable coating invading the exposed metal surfaces of a pad or via land during the final application stage of fabrication. Such solder mask encroachment occurs when the physical clearance provided for registration fails to account for the lateral spread of the fluid material across the base substrate. Manufacturers identify this condition by examining the degree of copper exposure loss on land patterns during incoming inspection or final visual audit.
The definition covers only the unwanted presence of the polymer layer upon regions designated for component attachment. It governs the physical bond integrity between the lead and the metallic surface, whereas boundaries exist where the coating serves a functional purpose as a dam between fine pitch pads. The condition creates an immediate reduction in the effective solderable surface area available for an electrical connection.
Registration Tolerance
Precise mechanical alignment between the artwork and the panel prevents the lateral migration of liquid resin onto exposed copper features. High precision optical alignment systems calculate the required gaps to account for slight panel expansion during thermal cycles. When equipment limits prevent perfect centering, the resulting layer variance forces the material into zones meant for component mounting.
These zones require absolute cleanliness to ensure the metallurgy accepts the molten alloy during reflow. If the layer shifts during the coating process, the resulting bridge blocks the wetting path for a lead or surface mount terminal. Technicians monitor the gap width using automated optical inspection equipment to confirm the gap meets the minimum requirements established by the board design file.
Electrical Consequence
Restricted surface area on a mounting pad reduces the fillet volume and compromises the mechanical strength of the soldered joint. A partial coverage of the metal inhibits the uniform flow of the alloy during the transition from solid to liquid states. Solder mask encroachment dictates the rate of defective joints because the restricted pad prevents standard wetting angles.
Boards with these errors exhibit lower shear resistance when subjected to physical shock or thermal cycling. The integrity of the connection relies on the full exposure of the copper surface.