Coverlay Adhesive Selection For Flex Circuits A coverlay is the flexible equivalent of a solder mask. It is a layer of polyimide film coated with an adhesive that is laminated over the finished circuit to protect the conductors and to define the areas where components or connectors will be attached.…

Flexible Heater PCB Design And Power Density A flexible heater is a resistive circuit formed on a thin polymer substrate, usually polyimide, that can be bonded to a curved or irregular surface. It replaces a wound wire element with an etched foil pattern, which makes the heat distribution repeatable and…

UV Curing Solder Mask For Quick Prototypes A photoimageable mask is applied as a liquid or a dry film, exposed through a artwork and developed to open the pads. The exposure step uses ultraviolet light to cross link the coating where it should remain, and UV curing is therefore the…

In Circuit Test Fixture Design An in circuit test uses a bed of nails fixture to contact every accessible net on a board and measure its components and its connections. The fixture is the interface between the board designer and the test engineer, and it succeeds or fails on decisions…

Solder Bead Prevention in Assembly A solder bead is a small sphere of solder that sits on the board surface away from any joint. Beads are usually between the size of a grain of sand and a pinhead, they are not part of the circuit, and they are a problem…

ICT Fixture Probe Selection for Assembled Board Testing In-circuit test still catches the faults that optical inspection cannot see: a wrong value, a reversed diode, a short under a shield, a connector pin that was never soldered. The inspection methods that cover the rest of the faults are set out…

Electroless Nickel Thickness Control On Copper Pads An electroless nickel immersion gold finish is built on a layer of nickel that is deposited without current and therefore covers every exposed copper surface evenly. That nickel layer is the structural part of the finish: it stops copper diffusing into the gold,…

Solder Mask Thickness Selection For PCB Builds Solder mask thickness is rarely discussed until a board fails for a reason that traces back to it. The coating has to cover the copper, insulate adjacent features, protect the laminate during assembly and hold its shape around every pad and via. A…

Tombstoning Prevention in Reflow Tombstoning is the defect in which a two terminal chip component lifts at one end during reflow and stands upright, held by the solder at the other end. It is most common on the smallest passives, where the mass of the component is small enough that…

Wave Solder Flux Selection and Application in Practice In a wave soldering machine the flux is the only chemical agent in the process, and it has to do three jobs in a few seconds: clean the surface, protect it through the preheat, and promote wetting when the board meets the…