Thermal Reliefs in Pads and Planes What a Thermal Relief Does A thermal relief is a pad connection that is deliberately narrowed, usually to four spokes, so that a pad connected to a large copper plane does not conduct heat away too quickly. The plane is an excellent heat spreader,…

Common SMT Defects and Their Root Causes A Framework Rather Than a List Defects in surface mount assembly are usually grouped by appearance, but the useful grouping is by process step. Every joint passes through printing, placement, reflow and cooling, and a defect that appears at the end was created…

Plated Hole Size Tolerance and Drill Selection Drill Size Versus Finished Hole The drill that makes a hole and the hole that a component sees are not the same dimension. Plating adds copper to the wall of every plated hole, so the finished diameter is smaller than the drilled diameter…

The Assembly Drawing: What It Must Contain What the Drawing Is For The assembly drawing is the document that tells a contract manufacturer what to build and how to check it. It is not the schematic, and it is not the layout file. Its job is to convey everything that…

Countersinks, Counterbores and PCB Fasteners Why Holes Are Shaped A plain hole accepts a screw, but the screw head sits proud of the surface. Where the board is mounted against a chassis, where a head must not touch a neighbouring assembly, or where the surface has to be flat, the…

DFM Checks Before You Release a PCB What DFM Actually Catches A design for manufacture check is not a review of whether the circuit works. It is a check of whether the drawing can be built on the process that will make it, at the yield the price assumes. Most…

Conformal Coating: Coverage and Process Why Coating Is Applied A conformal coating is a thin polymer film applied over an assembled board to protect it from moisture, condensation, salt spray, chemical exposure and, to a lesser extent, mechanical abrasion and tin whiskers. Its value is greatest where the product will…

Rigid-Flex Design: Bend Radius and Layers What Makes Rigid-Flex Different A rigid-flex board is a single construction that contains both rigid sections with multiple copper layers and a flexible section that bends. The layers are continuous across the boundary, so the flexible area is made of the same stack as…

BGA Fanout and Escape Routing The Escape Problem A ball grid array presents its connections on a grid of pads, and only the outermost row is directly accessible. Every other ball has to reach the rest of the board through a via, and those vias have to be placed without…

Solder Voids in Thermal and BGA Joints What a Void Is A void is a cavity inside a solder joint, filled with gas or with a vacuum, where solder should be. It appears in an X-ray image as a lighter region inside a dark joint because there is less material…