Double Sided Assembly: Design Rules and Process Limits A double sided assembly is reflowed twice, and the second pass is the one that causes trouble. The parts placed in the first pass are already on the board, and they have to survive a second trip through the oven without falling…

Press-Fit Connector Hole Design and Retention A compliant press-fit pin carries its own spring. It is inserted into a plated through hole that is slightly smaller than the pin, and the pin deforms elastically so that it presses against the barrel over the whole depth of the hole. The termination…

Panel Carrier and Fixture Design A carrier or fixture holds a board or a panel through a process step, and its design decides whether the assembly is supported, aligned and protected. On a product with an unusual shape or a thin board it is the difference between a process that…

Cold Solder Joint Formation: Heat, Cooling and Inspection A cold solder joint looks like a solder joint and behaves like a resistor. It forms when the alloy solidifies without having reached a proper wetting temperature, so the metal never bonds metallurgically to the pad and the lead. The joint passes…

Stencil Design for Mixed Technology Boards A mixed technology board carries both surface mount and through hole parts, and the stencil has to serve both in one print or in two. The design of the foil therefore has to accommodate apertures for fine pitch devices alongside large pads for connectors…

Solder Mask Cure Profile: Process and Verification The solder mask cure profile is the thermal cycle that turns a printed liquid film into a hard, chemically resistant coating. It is the step that decides whether the mask will survive the assembly soldering process, resist the flux chemistry, and hold its…

Step Stencil Design for Paste Volume Control A single stencil thickness is a compromise. The paste volume a joint needs depends on the pad area, the component pitch and the thermal mass behind it, and those requirements pull in opposite directions across a mixed board. A step stencil resolves the…

Wire and Cable Soldering Practice A wire or a cable is soldered because the joint has to survive movement, and that requirement changes everything about the technique. The strand, the insulation and the strain relief all matter more than the appearance of the joint. The failure modes are also different…

Board Level Reliability Test Plan and Sample Size A reliability test answers a specific question, and the question has to be written before the test is chosen. A plan that lists tests without stating what each one decides produces data that cannot be used to make a decision. Starting From…

No-Clean vs Water-Soluble Paste: Choosing a Flux System The choice between a no-clean paste and a water-soluble paste is usually presented as a cleaning question, but it is really a chemistry question with reliability consequences. Both systems can produce excellent joints, and both can fail if the residue they leave…