ENIG Finish And Black Pad Defects An electroless nickel immersion gold finish is popular because it presents a flat surface for fine pitch components, survives storage, and can be used for both soldering and wire bonding. It is also the finish with the most discussed defect in the industry, a…

Underfill Dispensing for Flip Chip and CSP Packages A flip chip joint is a very short column of solder between a silicon die and a substrate, and the two materials expand at very different rates. Every thermal cycle loads that column in shear, and after enough cycles a crack starts…

Card Edge Connector and Gold Finger Design A card edge connector is a board that plugs into a socket, and the contact area is the gold finger. It is a wearing surface, a current path and a mechanical datum at the same time, and it is usually defined by a…

Connector Insertion Force, Normal Force and Contact Wear A connector is a mechanical device that carries an electrical signal, and it fails for mechanical reasons far more often than for electrical ones. The force that holds the two halves together, the force needed to push them together, and the wear…

Mixed Alloy Soldering and Step Reflow Windows Two soldering alloys in the same assembly is a common requirement: a lead free main assembly with a temperature sensitive part, or a mixed alloy legacy board. The problem is that the second alloy must melt without disturbing the first. Why Mixed Alloys…

Parylene Coating for High Reliability PCB Assemblies Most protective coatings are applied as a liquid, and every liquid shares the same limitations: it runs, it pools, it leaves thin spots over sharp edges and it cannot reach into the shadow behind a tall component. Parylene is different because it is…

PCB Copper Foil Grain Structure and Etch Behaviour Copper foil is the starting point of every trace on a printed circuit board, and the way it is made decides how it etches, how it adheres and how it behaves at high frequency. Two foils of the same weight and the…

Gas Sensor Interface Circuit Design A gas sensor turns a chemical concentration into a small electrical change, and the circuit around it has to do two jobs at once: drive the element under stable conditions and read a signal that moves slowly. Most field failures in these designs come from…

Flow Meter Interface Circuit Design A flow meter converts a volume or mass rate into something an instrument can read: a train of pulses, a current, a frequency or a digital value on a bus. The interface circuit is chosen by that output type and by the environment, and the…

Stencil Cleaning and Paste Transfer Efficiency in SMT The stencil is the least expensive tool in the SMT line and the one that most directly determines whether the process works. Every paste deposit that ends up on a pad passed through an aperture, and every aperture that is partially blocked…