Guard Rings and Leakage Control in Sensitive PCB Design When a circuit measures picoamps, the resistance of the board itself becomes part of the circuit. Surface contamination, humidity and flux residues create leakage paths that can be larger than the signal being measured, and the errors they produce look like…

Return Path Design Across Split Planes Every signal that travels on a trace returns on a reference. At low frequency the current takes the path of least resistance, which may be a long way around, but at high frequency the return current follows directly beneath the trace, because that is…

Impedance Controlled Stackup Design Guide An impedance controlled stackup is a build in which the dielectric thickness of each layer is chosen so that a defined trace geometry gives a defined impedance. It is a joint decision between the layout and the shop, because the impedance depends on dimensions that…

Intermetallic Growth And Solder Joint Reliability A solder joint is not a single material. Between the solder and the copper, the nickel or the gold that it wets, a layer of intermetallic compound forms. It is the reason the joint holds: a metallurgical bond exists where a layer of a…

Bake Before Assembly And Moisture In The Laminate A printed circuit board absorbs water from the air. The laminate takes it up through the resin and the glass, the amount depends on the humidity and on how long the board has been exposed, and the water is released as steam…

Board Finish Selection Guide A board finish is the thin layer on the exposed copper that keeps it solderable until the assembly is done. It is chosen late, it costs very little, and it decides whether the first assembly run goes smoothly or produces a batch of joints that will…

Flex Rigid Transition: Key Checks Before Release A flex rigid transition is the region where the flexible part of a board meets the rigid part. The laminate ends, the coverlay ends, the stiffener usually ends as well, and three mechanical discontinuities sit within a few millimetres of each other. The…

Motor Driver PCB Layout and Current Path Planning A motor driver switches large currents in a few nanoseconds, and the layout decides whether that energy stays in the intended loop or couples into everything nearby. The difference between a quiet driver and one that trips its own protection or upsets…

Nitrogen Reflow And Oxygen Concentration Control A reflow oven is normally filled with air, and the oxygen in that air reacts with the solder, the pads and the flux while the joint is being formed. Replacing the atmosphere with nitrogen reduces that reaction, and the result is better wetting, fewer…

Solder Bridging Causes And Process Fixes A bridge is a connection of solder between two conductors that should be separate. On a fine pitch package it is a single deposit that has spread across the gap; between a lead and a neighbouring pad it is a thread formed as the…