Burn-in and Screening Test Justification Screening is the application of a stress to a population so that the weak units fail before they are shipped. Burn-in is the same idea applied with power and temperature, and it is expensive enough that it should be justified. What Screening Can and Cannot…

Gold Embrittlement: Design Rules and Process Limits Gold dissolves into molten solder, and the joint that results is not the alloy the process was qualified with. The effect is well known in older tin lead processes and it is not confined to them. What Gold Embrittlement Is Gold dissolves quickly…

Tin Whisker: Design Rules and Process Limits Tin whiskers are thin filaments that grow out of a tin finish over time. They are conductive, they can bridge a gap that was designed to be several hundred volts, and they grow in the field rather than in the factory. What a…

Eight Layer Board: Impedance Control and Stackup Design An eight layer board is the point at which a design stops being a set of stacked signal layers and starts being a controlled transmission environment. There are enough layers to give every high speed net a reference plane, enough planes to…

Copper Deposition and Hole Wall Quality in PCB Fabrication Copper deposition is what turns a drilled hole in a laminate into an electrical connection. The hole wall is an insulator, and the only way to plate it is to make it conductive first. That first conductive layer is thin, fragile…

First Pass Yield, Defect Pareto and Rework Cost A yield figure on its own tells you that something is wrong and nothing else. The first pass yield, broken down by defect type and by location on the board, is what turns a number into an action. Which Yield to Measure…

Solder Mask Cure Verification and Resistance Testing Solder mask that has not fully cured looks almost identical to solder mask that has. It is the right colour, it covers the right areas and it passes a visual inspection without comment. The difference only appears later, when the board is exposed…

RTD Excitation Current and Measurement Circuit A platinum resistance thermometer is the default choice when temperature has to be measured accurately and repeatably over a wide range. Its resistance is nearly linear with temperature, it is stable over years, and the accuracy of the measurement then depends almost entirely on…

PIR Sensor Motion Detector Circuit Design A passive infrared detector responds to a change in the infrared radiation reaching it, and that single sentence explains most of its behaviour: it detects movement, it ignores a stationary person, and it can be triggered by anything that changes the temperature of what…

OSP Surface Finish In PCB Assembly An organic solderability preservative, usually shortened to OSP, is not a metal. It is a thin organic film that is applied to the copper surface and that protects it from oxidation until the board is soldered. Because there is no metal layer, the pad…