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Design E

Official Guidebolt Electrical Design Guide

Last Updated: 2021-03-22

Introduction

Great electrical designers have a deep understanding of resistance, capacitance, inductance in the complex source-sink-impedance dynamics of signal-power transmission. Forward and return paths (ground shift), parasitics (leakage, distortion) and electromagnetics (EMI, EMC), form-tradeoffs (size/weight) and thermals (real-time distribution, ambient expansion, operating range, cycling, MTBF effects). Whiskering and corrosion (ECM, CAF), part-protection (SMT/THT shock-resist, IP rating, TVS, moisture-sensitivity, conformal-coat/encapsulation), user-safety (fuses, flammability, total-impact of material-lifecycle). Stack-up definition (layers, surface finish, impedance control), production stress (reflow conduction-paths, flux activation, soldermask-defined-pads), quality-assurance (optical inspection, QVL/PPAP), performance-limits and failure-modes (FIT, cascading-events). User experience (silkscreen, connector-mechanisms) and ease-of-maintenance (top vs bottom assembly, universal footprints/pinouts), modularity (universal interfaces, multiple firmware options).

Great Resources

Linear Circuit Design Handbook, 2008, by Analog Devices

Art of Electronics, 3rd Edition, by Paul Horowitz and Winfield Hill

Comparing Bus Solutions, rev C, by TI

Basics

Unit Standard: Metric. Dimensions are mostly mm and um.

Company-wide Lists: qualified vendor list (QVL), qualified parts list (QPL), shared symbol/footprint library.

Lifetime: 5 years MIN, 15 years TARGET MTBF

Safety: RoHS-compliant (RoHS3, EU Directive 2015/863, without exemptions), halogen-free (IEC 61249-2-21), red-phosphorous-free, antimony-free, REACH-compliant (U Regulation 1907/2006, without candidate substances).

Reliability: AEC-Q (target, grade 1), UL94-V0 flame resistance, glow-wire connectors, tin-finishes are matte-tin (reduced whiskering), operating temperature -40C to 85C MIN (-40C to 125C TARGET), MSL1 TARGET, MLCCs have soft-termination (bend-crack resistance), bulk capacitors are solid-material (ex. aluminum-polymer over aluminum-electrolytic for higher lifetime).

Usability: 1608 metric size TARGET (ease-of-assembly/repair), 4 mounting holes (square or rectangle pattern TARGET).

Ergonomics: rounded corners (major radius 5mm MIN).

Options: conformal-coating, encapsulation.

PCB

Fabrication Class: must be IPC-2 or IPC-3 for traditional fab.

Surface Finish: must be ENIG.

Soldermask Color: Green

Silkscreen Color: White

Electrical Testing: Yes

Impedance Control: Yes for high-frequency or analog PCBs.

Material: high thermal stability (ex. low-CTE, cycling tolerance, high electromechanical stability, high thermal conductivity (ex. 0.7W/mK is a good non-ceramic benchmark), good impedance-properties (ex. low relative-permittivity, low dissipation-factor).

Assembly: Must use lead-free solder. Preferred alloy is SAC305. Must use low-activity no-halogen (category L0) flux. Preferred flux is no-clean IPA-washable (good performance without cleaning but can be cleaned).

FOOTPRINTS

All pads must have rounded corners with radius 0.05mm MIN.

BGA footprint pads are generally recommended to be soldermask-defined.

SMT pads and reflow THT pads can be solid-connected to conductor-planes.

Easy-maintenance SMT pads and selective THT pads should have thermal-reliefs.

VIAS

We prefer full-depth vias (normal plated-thru-hole, no blind or buried).

Vias-in-pad are good for space and thermals. We use them with exposed-pad-ICs when we have space.

Via sizes:

Drill Dia, Annular Ring Dia

Small: 0.2mm, 0.25mm (vias-in-pad standard)

Compact: 0.25mm, 0.3mm (advanced-fab standard)

Normal: 0.4mmm, 0.8mm (traditional-fab standard)

0.4/0.8 is the default via size in Kicad (popular). It requires a large-enough drill bit that breaks/wears less than smaller sizes (cost-effective).

Easy: 0.5mm, 1.2mm (IPC-2221 Level-A)

SPACING

board-edge to silkscreen, min 0.2mm

board-edge to external-layer-conductor, min 0.25mm

board-edge to internal-layer-conductor, min 0.5mm

board-edge to hole-edge, minimum 1mm

via-hole-edge to pad-edge spacing, minimum 0.2mm

via-hole-edge to via-hole-edge spacing, minimum 0.5mm

Footprints are spaced to support part-holding with tweezers (0.8mm tip-thickness).

Same-net pads can be closer together. Different-net pads should be further apart.

SILKSCREEN

Attempt to have a reference-designator for each part-footprint.

Attempt to have a complete perimeter outline per part-footprint. In some cases a broken/dotted line is important for flux-scatter (ex. high-reliability BGA assembly).

Must have a functional-label per external-pin.

MINIMUM:

  • Company Name (ex. Guidebolt)
  • Part ID (ex. SOMEPART-MK1R3)
  • Batch ID (ex. 2021-01-20_XYZFAB)
  • MFG Origin (ex. Made in Canada with imported parts)

SPECIAL:

  • flammability marker (ex. UL94V-0)
  • WEEE directive marker
  • RoHS
  • REACH
  • CE
  • UL Recognized
  • UL Listed