Design for Testability

Understanding Design for Testability: A Foundation for Product Success

In the competitive landscape of modern product development, the ability to efficiently verify and validate your designs can make the difference between market success and costly failure. Design for Testability (DFT) is a systematic approach that integrates testing considerations into every phase of the product development cycle, from initial concept through to manufacturing and field service. For manufacturers across Atlantic Canada and beyond, embracing DFT principles has become essential for maintaining quality standards while controlling production costs.

At its core, Design for Testability encompasses the methodologies, techniques, and design practices that make products easier to test, diagnose, and repair. Rather than treating testing as an afterthought, DFT requires engineers to consider how a product will be verified during the design phase itself. This proactive approach typically reduces testing costs by 30-50% while simultaneously improving fault coverage and reducing time-to-market.

For Nova Scotia's growing technology and manufacturing sectors, implementing robust DFT practices offers a competitive advantage. Whether developing electronic assemblies, mechanical systems, or complex electromechanical products, the principles of testability apply universally and scale effectively from prototype quantities to full production volumes.

The Economic Case for Design for Testability

The financial benefits of implementing DFT practices extend throughout the entire product lifecycle. Studies consistently demonstrate that defects discovered during the design phase cost approximately 10 times less to correct than those found during production, and 100 times less than defects discovered in the field. For Maritime manufacturers competing in global markets, these cost differentials can determine profitability.

Key economic benefits of DFT implementation include:

• Reduced test development time by 40-60% through structured test point access

• Lower manufacturing test equipment costs due to simplified test requirements

• Decreased warranty and field service expenses through improved fault isolation

• Shorter production cycle times resulting from streamlined test procedures

• Improved first-pass yield rates, often exceeding 95% with proper DFT implementation

• Enhanced customer satisfaction through higher product reliability

Consider a typical electronic control unit manufactured in Nova Scotia for the marine or aerospace industry. Without DFT considerations, testing such a unit might require expensive custom fixtures, lengthy test cycles, and skilled technicians to interpret results. With proper DFT implementation, the same unit can be tested using standardised equipment, automated procedures, and clear pass/fail criteria that any trained operator can execute.

Return on Investment Calculations

When evaluating DFT investments, engineering teams should analyse both direct and indirect cost savings. Direct savings include reduced test fixture complexity, shorter test times, and lower rework costs. Indirect savings encompass improved production throughput, reduced engineering support requirements, and enhanced brand reputation through consistent quality.

For a typical production run of 10,000 units annually, implementing comprehensive DFT practices during the design phase might add $15,000-25,000 to development costs. However, the resulting savings in test time alone—assuming even a modest 2-minute reduction per unit—translates to over 330 hours of production time saved annually. When combined with improved yields and reduced field returns, the return on investment typically exceeds 300% within the first year of production.

Core Principles of Design for Testability

Effective DFT implementation relies on several fundamental principles that guide design decisions throughout the development process. Understanding and applying these principles ensures that testability becomes an integral part of the design rather than a bolted-on consideration.

Controllability

Controllability refers to the ability to establish specific states or conditions within the product during testing. A highly controllable design allows test engineers to isolate individual functions, set known input conditions, and exercise specific operating modes. This requires thoughtful inclusion of test modes, accessible control inputs, and the ability to override normal operating sequences when necessary.

In practical terms, controllability might include provisions for injecting test signals, bypassing power-on sequences, or forcing specific states in sequential logic. For mechanical systems, controllability often involves accessible adjustment points, removable covers, and the ability to operate subsystems independently.

Observability

Observability is the complement to controllability—it represents the ability to monitor internal states, measure parameters, and verify correct operation. High observability designs provide adequate test points, include built-in measurement capabilities, and make internal signals accessible without disturbing normal operation.

For electronic assemblies, observability typically requires strategic placement of test points on critical nodes, consideration of signal integrity at measurement locations, and sufficient physical access for probes or test fixtures. The general guideline suggests providing test points for any node that cannot be adequately verified through boundary testing alone.

Partitioning and Modularity

Breaking complex systems into testable modules significantly simplifies the overall testing challenge. Each module should have well-defined interfaces, clear functional boundaries, and independent testability. This approach enables parallel test development, simplifies fault isolation, and allows for module-level qualification before system integration.

Effective partitioning considers both functional divisions and physical accessibility. A well-partitioned design might group related functions on separate circuit board sections, use modular mechanical assemblies with defined interfaces, or implement software architectures that support unit testing and integration testing as distinct activities.

DFT Techniques for Electronic Systems

Electronic products present unique testing challenges due to their complexity, miniaturisation trends, and the abstract nature of signal processing. Several established techniques address these challenges and have become industry standards for testable electronic design.

Boundary Scan (JTAG) Implementation

The IEEE 1149.1 standard, commonly known as JTAG or boundary scan, provides a standardised method for testing interconnections between integrated circuits without physical test probes. For modern electronic assemblies using ball grid array (BGA) packages and fine-pitch components, boundary scan often represents the only practical method for verifying solder connections.

Implementing boundary scan requires selecting components with JTAG capability, routing the test access port (TAP) signals correctly, and providing appropriate connector access. The incremental cost of boundary scan implementation typically adds less than 2% to board cost while providing access to verify over 90% of digital interconnections.

Built-In Self-Test (BIST)

BIST techniques embed test functionality directly within the product, enabling autonomous verification without external test equipment. This approach proves particularly valuable for products deployed in remote locations or those requiring field verification—common scenarios for equipment used in Atlantic Canada's resource extraction, marine, and telecommunications industries.

Effective BIST implementations include memory testing algorithms, analogue-to-digital converter verification sequences, communication interface loopback tests, and power supply monitoring functions. The key is balancing test coverage against the additional circuitry and software required to implement BIST capabilities.

Test Point Strategy

Despite advances in JTAG and BIST, physical test points remain essential for comprehensive testing. A well-designed test point strategy considers:

• Power supply rails requiring voltage verification (typically all rails ±5% of nominal)

• Clock signals and timing-critical nodes

• Analogue signals that cannot be verified through digital methods

• Reset and control lines affecting system behaviour

• Ground reference points distributed across the assembly

• RF signals requiring impedance-matched access

Test point placement must also consider manufacturing constraints. For in-circuit test (ICT) fixtures, maintaining minimum 2.54mm (100 mil) spacing between test points and keeping them at least 3mm from board edges ensures reliable probe contact and fixture manufacturability.

Mechanical and Electromechanical Testing Considerations

While electronic DFT techniques are well-documented, mechanical and electromechanical systems require equally thoughtful testability provisions. For Nova Scotia manufacturers producing equipment for harsh maritime environments, mechanical testability directly impacts field serviceability and long-term reliability.

Dimensional Verification Access

Critical dimensions must be measurable without disassembling the product. This requires datum surfaces that remain accessible after assembly, inspection ports or windows for internal features, and reference features that coordinate with measurement equipment. For products requiring periodic inspection, such as pressure vessels or safety-critical mechanical systems, designed-in measurement access reduces inspection time and improves accuracy.

Functional Test Provisions

Mechanical systems should include provisions for functional verification that simulate operating conditions. This might include:

• Pressure test ports with appropriate fittings and isolation capability

• Load application points designed to accept test fixtures

• Motion measurement surfaces with appropriate finish and geometry

• Fluid sample ports for systems containing lubricants or process fluids

• Vibration measurement locations on rotating equipment

Environmental Testing Considerations

Products destined for Atlantic Canada's demanding climate conditions require environmental qualification testing. Designing for these tests means including appropriate mounting provisions for environmental chambers, ensuring sensor access for temperature and humidity monitoring, and providing power and signal connections compatible with test chamber feedthroughs.

Software and Firmware Testability

Modern products increasingly rely on embedded software, making firmware testability an essential component of overall DFT strategy. Testable software architectures enable efficient verification, simplify debugging, and support field diagnostics.

Debug and Diagnostic Interfaces

Every embedded system should include debug interfaces appropriate to the application. Common provisions include UART ports for console access, JTAG or SWD connections for processor debugging, and bootloader modes supporting firmware updates. For production testing, these interfaces enable rapid verification of processor function, memory integrity, and communication peripherals.

Diagnostic Modes and Logging

Production-ready firmware should include diagnostic modes that exercise hardware functions and report results. Effective diagnostic implementations provide structured test sequences, clear pass/fail reporting, and logging capabilities that capture relevant parameters for quality records. For products requiring traceability—common in aerospace, medical, and defence applications—firmware-based logging can automatically generate test documentation.

Hardware Abstraction Benefits

Well-architected software that separates hardware-dependent functions from application logic improves testability at multiple levels. Hardware abstraction layers enable simulation testing of application software, independent verification of hardware interfaces, and easier adaptation when hardware revisions occur. This architectural approach typically adds 10-15% to initial development effort but reduces total lifecycle testing costs by 25-40%.

Implementing DFT in Your Development Process

Successful DFT implementation requires organizational commitment and process integration. Design reviews must explicitly address testability, test engineers should participate in design decisions from project inception, and testability metrics should be tracked alongside traditional design parameters.

Recommended DFT implementation steps include:

• Establishing testability requirements during product specification

• Including test engineers in design reviews from concept phase onward

• Creating testability checklists specific to your product types and manufacturing capabilities

• Documenting test strategies before detailed design begins

• Reviewing test point coverage and accessibility at each design milestone

• Validating testability through prototype testing before production release

For companies in the Maritime provinces competing for contracts with major OEMs or seeking certification to standards such as AS9100 or ISO 13485, documented DFT processes demonstrate engineering maturity and quality commitment that evaluators recognise and value.

Partner with Experts for Testable Product Development

Implementing effective Design for Testability requires experience across multiple engineering disciplines and understanding of both design constraints and manufacturing realities. Getting testability right during initial design eliminates costly revisions and ensures your products can be efficiently manufactured, tested, and serviced throughout their lifecycle.

Sangster Engineering Ltd. brings decades of product development experience to clients across Nova Scotia, Atlantic Canada, and beyond. Our engineering team understands the unique challenges of designing products for harsh environments, demanding quality standards, and cost-competitive manufacturing. Whether you're developing electronic controls, mechanical systems, or complex electromechanical products, we can help you implement DFT practices that reduce costs, improve quality, and accelerate your time to market.

Contact Sangster Engineering Ltd. today to discuss how our product development expertise can help you design testability into your next project from the ground up. Let us help you build products that are not only functional and reliable but also practical to manufacture, test, and support throughout their service life.

Partner with Sangster Engineering

At Sangster Engineering Ltd. in Amherst, Nova Scotia, we bring decades of engineering experience to every project. Serving clients across Atlantic Canada and beyond.

Contact us today to discuss your engineering needs.