EC-24.4 Stack-Up, Placement and Routing
You can choose a layer stack-up for a given board and justify each layer, place components so that the routing is possible before it is attempted, and route with the manufacturing constraints your fabricator actually quotes.
Before:EC-2. Electromagnetics and Transmission LinesEC-17. Industry and Research Experience
Layout looks like a puzzle and is really a sequence of decisions made in the right order. The stack-up comes first because it decides what impedances and what routing density are available. Placement comes second and determines perhaps eighty per cent of the outcome. Routing is last, and if it is difficult the answer is almost always to go back and move something.
Work through these
Choosing a stack-up: layer count, plane placement and dielectric thickness
Every signal layer wants an adjacent reference plane, and the spacing to it sets the impedance and the loop area. Adding two layers is cheaper than a redesign caused by not having them.
NPTEL: An Introduction to Electronics Systems Packaging (IISc Bangalore) · CoursePlacement: connectors, heat, sensitive parts and the mechanical envelope
Component positions are constrained by the enclosure, by where heat may be dissipated and by what must be kept away from what. A layout that is hard to route is usually a placement that was decided too quickly.
NPTEL: Electronic Systems Design — Circuits and PCB Design with CAD Software (IIT Delhi) · CourseTrace width and current, and the temperature rise that decides it
A trace carrying current heats up, and the accepted rise decides the width for a given copper thickness. The standard charts are conservative and are also the figure a reviewer will check against.
Vias: types, current capacity, and what each costs to manufacture
Through, blind and buried vias have very different manufacturing costs, and a design that needs the expensive kind should need it for a reason. Vias also add inductance in a supply path.
NPTEL: An Introduction to Electronics Systems Packaging (IISc Bangalore) · CourseManufacturing constraints: minimum widths, spacings, annular ring and drill sizes
Every fabricator publishes a capability table, and prices rise sharply outside its standard column. Designing to the standard column is the cheapest single decision available on most boards.
NPTEL: An Introduction to Electronics Systems Packaging (IISc Bangalore) · CourseThe output files, and checking them before they are sent
The manufacturer builds from the exported layers and drill files, not from your design file, and an export mistake is invisible in the tool. Opening the outputs in a separate viewer is a five-minute check that prevents a scrapped batch.
NPTEL: Electronic Systems Design — Circuits and PCB Design with CAD Software (IIT Delhi) · Course
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