Copper Thickness Selection for Power and Ground Planes A power plane is usually specified by its finished copper weight rather than by a dimension, and the two are related but not identical. The figure on the drawing sets the current the plane can carry, and it also sets what the…

Via in Pad: Filling, Planarization and Assembly Rules Via in pad places the via inside the solder pad so the ball or the printed paste lands on the same feature that carries the signal to the next layer. The arrangement saves routing space on dense boards, but it moves a…

Thermal Interface Degradation Over Product Life A thermal interface is specified by its resistance when new and by its stability over the life of the product. The second number is the one that decides whether the design still works after a year. Pump Out and Dry Out A grease migrates…

Visual Inspection Criteria for Lead Free Solder Joints Lead free solder looks different from tin lead. It is more matte, it does not always form a smooth concave fillet and its surface is often grainy. Criteria written for the older alloy reject joints that are perfectly sound. Why the Appearance…

Reflow Assembly of Flexible and Rigid Flex Circuits A flexible circuit is thin, it moves and it is soldered in the same process as a rigid board. Each of those properties changes how the reflow has to be controlled. Why Flex Behaves Differently The material is thin, so it heats…

QFN and Bottom Termination Component Soldering A quad flat no lead package sits on its pads rather than on leads, so the joint is formed at the edge of the package and it is short. The joint that forms is often thinner than the design assumed. Why the Joint Is…

Barrel Crack: Plated Through Hole Reliability Under Thermal Cycling A plated through hole is a copper tube in a laminate that expands more than the copper does. Every thermal cycle loads the barrel at its two ends, and the crack that forms is the classic board level failure. The Load…

Impedance Coupon Design An impedance coupon is a test structure built into the panel that allows the impedance of a specific geometry to be measured. It converts a claim about the stack into a number, and it is the only practical way to verify that the geometry a design calculated…

Coupon Design for PCB Fabrication A test coupon is a piece of the panel that is not part of a product and exists to be measured. It is the evidence that the board was built to the specification, and it is what allows a customer to verify an impedance, a…

Selective Soldering: Key Checks Before Release Selective soldering is a process that solders individual through hole joints, or small groups of them, with a small nozzle rather than a full wave. It exists because a mixed assembly has a few through hole parts that cannot survive a wave, and because…