Traces Vias And Pads

Chapter 1 built the board from the outside in; this chapter zooms into the three features you actually repair. It examines the trace that carries a signal or power from one point to another, the via that passes a connection through the board between layers, and the pad where a component solders down — their anatomy, the different types and geometries each comes in, how much current and how fast a signal they can carry, and, crucially for repair, how to read and follow them on real, dense, multi-layer boards. By the end you can look at any board and read its traces, vias, and pads for what they are and where they go.

• 2.1Trace Anatomy and FunctionYou have met the trace as the copper wiring of a board and learned how its size sets the current it carries; this section examines the trace itself — what it physically is and what it does. A trace is a narrow strip of etched copper, of a set width and thickness, running on one layer from one endpoint to another and kept clear of its neighbours by a spacing. Its job is to realize a connection: every set of points that must be joined together is a net, and the complete list of the nets a board should have is its netlist — the connectivity blueprint the copper implements. You will learn to tell a thin signal trace, whose width is set by signal quality, from a wide power trace that carries current; to understand routing, the way traces are laid out to connect every net across the board's layers; and, most useful of all, to read and follow a trace from pad to pad by eye and with a meter, which is exactly what you must do to diagnose a break or plan a repair.IntermediateLow Risk19 min read
• 2.2Via Types — Through-Hole, Blind, Buried, MicroYou met the via in the last section as the plated hole a trace hops through to change layers; this section examines the via itself and the several types it comes in. A via is a small drilled hole whose wall is plated with copper — that plated wall is the via barrel, and it is what carries a connection between the layers the via touches, ringed on each by a small annular ring of pad copper. The types differ in how far through the board the hole goes: a through-hole via runs all the way through and is visible on both sides; a blind via joins an outer layer to an inner one and shows on one face only; a buried via connects two inner layers and is hidden inside the board; and a microvia is a tiny, laser-drilled via used to pack dense, high-layer-count boards. Which type you face decides what you can see, reach, and repair — so learning to read vias is learning to reason about a board's hidden third dimension.IntermediateLow Risk20 min read
• 2.3Pad Types and GeometryYou have met the trace that carries a connection and the via that passes it between layers; the third feature you repair is the pad — the exposed patch of copper where a component solders to the board. A pad is where the board meets a part: the solder mask opens over bare copper, a lead or terminal sits on it, and solder joins the two, so every pad is a joint you may one day inspect, reflow, or rebuild. Pads come in two broad types by how the part mounts — a through-hole pad with a plated hole for a leaded component, and a flat surface-mount pad for an SMD terminal — and their geometry matters: the annular ring around a hole, the spoked thermal relief that ties a pad to a plane, and whether a pad's solderable edge is set by the copper or by the mask opening, which makes it a solder-mask-defined pad. Reading a pad's type and geometry tells you how it should be soldered and how to save it when it lifts or burns.IntermediateLow Risk20 min read
• 2.4Trace Width, Current, and ResistanceChapter 1 gave you a first read of a trace's size and the current it carries; this section returns to the trace to put numbers on it — its resistance, the voltage it drops, and how current heats it. Every trace has resistance, set by copper's resistivity times its length over its cross-section, so a trace grows more resistant as it gets longer, narrower, or thinner. A shortcut for a given copper weight is sheet resistance — the resistance of one square of copper — so counting how many squares long a trace is gives its resistance quickly. That resistance drops a voltage along a current-carrying trace and turns power into heat, warming the trace to a temperature rise above its surroundings; a trace's ampacity is really the current that produces an acceptable temperature rise, which is what published current tables capture from width and copper weight. These numbers let a repairer size a jumper or a rebuilt trace to carry the load without dropping too much voltage or overheating.IntermediateLow Risk21 min read
• 2.5Identifying Traces on Multi-Layer BoardsYou can follow a trace across a surface, you know the via types that carry a connection between layers, and you can read a board's stackup; this final section of the chapter brings them together to solve the hardest reading problem — following a trace across the hidden inner layers of a multi-layer board. The trouble is that an inner-layer trace is invisible: a trace dives into a via and reappears somewhere else, and you cannot see the copper in between. Two things let you follow it anyway. When you have the board's design package — its Gerber files, its fabrication drawing, and its assembly drawing — every inner trace and via span is laid out for you. When you do not, you fall back on reasoning from what you can see: locating a net's endpoints by reference designator and schematic, reading which layers a via could reach, using X-ray to glimpse inner copper, and confirming every step with an unpowered continuity check. Either way, following a connection you cannot see is what turns a dense multi-layer board from a black box into something you can trace and repair.IntermediateLow Risk21 min read
• Chapter Quiz35questions · 80% required to continue