Coating Masking: Design Rules and Process Limits Conformal coating protects a board only where it is applied, and it causes problems wherever it is applied by accident. Connectors, test points, relays, optical surfaces and anything that must mate mechanically have to stay clear, which means the coating process has to…

Gold Finger: Design Rules and Process Limits Gold fingers are the part of a board a customer touches, plugs and unplugs, and they are expected to survive thousands of mating cycles without losing contact resistance. They are also one of the hardest features to plate well, because the geometry is…

Solder Wick: Design Rules and Process Limits Solder wick is a braided copper strip that is heated on a joint and draws the molten alloy into itself by capillary action. It is the standard way to remove solder from a joint, and the way it is used decides whether the…

Wave Soldering Thermal Relief Design A thermal relief is the pattern of copper spokes that connects a plated through hole to a plane without attaching it solidly. It exists to slow the rate at which heat is conducted away from the joint during soldering, so that the barrel and the…

Solder Joint Void Acceptance Criteria Voids are cavities inside a solder joint, and they are one of the few joint features that can be seen without cutting the board open. Because X-ray makes them visible, they attract attention out of proportion to their effect, and specifications are often written as…

Rolled Annealed and Electrodeposited Copper Foil Copper foil is made by two processes, and the foil that results is different in ways that matter to a flexible circuit and hardly at all to a rigid one. The difference is in the grain structure, and it shows up as ductility rather…

Component Shift After Reflow and How to Prevent It A component that has shifted during reflow is a placement problem that the placement machine cannot fix. The part was in the correct position when it was placed and moved while the alloy was molten, so the cause lies in the…

Panel Rail Design Rules The panel rail is the part of the panel that the machine holds, and it is often designed last, after the boards have been placed. That order of work is the reason so many panels handle badly: rails that are too narrow to support the panel,…

Solder Bridging Control on Fine Pitch Assemblies A solder bridge is a connection between two joints that should be separate. On a fine pitch assembly the joints are close together, the paste volume is small and the margin between a good joint and a bridge is narrow, so bridging is…

Cleaning After Hand Soldering Hand soldering leaves more residue than any machine process, because the operator applies flux locally, often more than the joint needs, and the heat source is a tip rather than a controlled profile. Some of that residue is harmless and some of it is not, and…