A design specification (sometimes called a technical specification or detailed design document) describes how a system will be designed to satisfy its requirements. Where the System Requirements Specification answers 'what must the system do?', the design specification answers 'how will it be done?' It includes architecture decisions, component selections, interface definitions, material choices, dimensional constraints, and design rationale. Design specifications are the primary deliverable of the detailed design phase.
The content of a design specification varies by product type. For an electronic system, it typically includes: block diagram, component selection and rationale for each major part, PCB layout constraints, interface specifications (pin-out, voltage levels, protocols), power budget, thermal analysis, and BOM. For a mechanical system: drawings, material selection and justification, geometric tolerances, surface finish requirements, and assembly instructions.
Design specifications serve multiple audiences: engineers who build from them, reviewers who assess them, manufacturers who produce from them, and future engineers who maintain or evolve the product. Writing for all these audiences requires clear structure, defined terminology, and explicit traceability back to requirements. A design specification that cannot be traced to requirements introduces gold-plating risk; a requirement that has no design specification entry is an unaddressed gap.
Practical Example
The design specification for a wireless sensor node includes: processor selection (STM32L4 — justified by ultra-low-power requirement and available FreeRTOS support), RF module (RFM95W — justified by LoRa protocol selection from trade study), power architecture (LiFePO4 battery + solar harvesting — justified by 2-year unattended operation requirement), and thermal analysis confirming operation from -20°C to +60°C.
How SpecZero handles this
In SpecZero, the combination of the concept planner (concept selection and BOM) and decision log (reasoning for key choices) together constitute a lightweight design specification that can be exported or referenced during formal design reviews.
Related terms
System Requirements Specification(SRS)
A formal document defining all requirements a system must satisfy, derived from stakeholder needs.
Design Decision Log
A structured record of key engineering decisions, capturing the objective, alternatives, analysis, and rationale.
Concept Trade Study
A structured comparison of design alternatives against a set of requirements and evaluation criteria.
Design Freeze
A formal milestone after which no further design changes are permitted without a formal change order process.