Thermal Via Array Design Under Power Devices A power device soldered to a pad has to get its heat out through the board. The heat leaves the die, crosses the package, crosses the solder layer, enters the copper pad and then has to travel somewhere. If it can only spread…

Solder Mask Plug: Design Rules and Process Limits A solder mask plug is mask material that is pushed into a via rather than draped over it. The distinction matters because a tent can crack and pull away, while a plug survives the assembly process and gives a surface that can…

Thermal Cycling Testing Of Solder Joints A solder joint that survives assembly has only proved that it can be made. Whether it will survive the life of the product is a different question, and the standard way to answer it is a thermal cycling test: the assembly is taken repeatedly…

Microvia Filling and Copper Capping Process Control for HDI A microvia is a small hole, usually formed by laser, that connects one layer to the next without passing through the whole board. As the diameter falls and the depth stays the same, plating the barrel becomes the easy part and…

Flux Residue Cleaning Design Guide Flux residue cleaning is usually treated as a process problem, and it is often a design problem instead. The residue collects where the board cannot be reached, the water cannot drain from a pocket, and a component that traps the chemistry is a component that…

Plating Uniformity and Copper Distribution Control on Panels Copper is deposited on a panel by an electrochemical process, and electrochemical processes never deposit perfectly evenly. The current takes the path of least resistance, so copper builds faster on edges and corners and more slowly in the middle of the panel…

Desiccant Storage And Moisture Control For PCBs A printed circuit board that is stored in a sealed bag with a desiccant and a humidity indicator card is a board whose moisture content is known. The bag keeps the atmosphere around the board dry, the desiccant absorbs what enters, and the…

Ionic Contamination: Design Rules and Process Limits Ionic contamination testing measures the material left on a board that can dissolve in moisture and carry current. The measurement is a conductivity, the result is expressed as an equivalent mass of salt per unit area, and the limit is written into the…

Reflow Conveyor Speed And Profile Balance A reflow oven is programmed with a set of zone temperatures and a conveyor speed, and the combination produces a profile at the board. The zone temperatures are what most people adjust, and the conveyor speed is what actually determines how long the board…

Edge Plating And Hard Gold Wear Resistance A gold contact surface is not simply a layer of gold. It is an electroplated deposit from a bath that contains cobalt or nickel as a hardening addition, and the resulting layer is harder and more wear resistant than pure gold. The distinction…