Design for Testability in Electronics

Understanding Design for Testability: A Foundation for Reliable Electronics

In the competitive landscape of electronics manufacturing, the ability to efficiently test and validate products can mean the difference between market success and costly recalls. Design for Testability (DFT) represents a systematic approach to electronic design that incorporates testing considerations from the earliest stages of product development. For engineering firms across Atlantic Canada, where precision manufacturing and quality assurance are paramount, implementing robust DFT practices has become essential for delivering reliable electronic systems.

Design for Testability encompasses a broad range of techniques, methodologies, and design principles that make electronic circuits and systems easier to test, debug, and validate. Rather than treating testing as an afterthought, DFT integrates test access points, built-in self-test capabilities, and diagnostic features directly into the design architecture. This proactive approach typically reduces overall development costs by 20-40% while significantly improving product quality and time-to-market.

The Economic Case for DFT Implementation

Many engineering teams, particularly those working with tight budgets common in Maritime technology sectors, question whether the upfront investment in DFT is justified. The data consistently demonstrates that proper DFT implementation delivers substantial returns on investment across multiple dimensions.

Cost of Fault Detection at Different Stages

The electronics industry follows the well-documented "Rule of Ten," which states that the cost to detect and repair a fault increases by approximately ten times at each subsequent stage of production:

• Design Stage: $0.10 to $1.00 per fault detected

• Board Assembly: $1.00 to $10.00 per fault detected

• System Integration: $10.00 to $100.00 per fault detected

• Field Service: $100.00 to $1,000.00+ per fault detected

For Nova Scotia manufacturers serving industries such as ocean technology, aerospace, and defence—sectors where product failures can have serious safety implications—these costs can escalate even further when considering liability, reputation damage, and regulatory compliance issues.

Quantifiable Benefits

Organisations implementing comprehensive DFT strategies typically experience:

• Reduction in test development time by 30-50%

• Improvement in fault coverage from 70% to 95% or higher

• Decrease in field failure rates by 25-60%

• Shortened production test times by 40-70%

• Lower warranty and service costs by 20-35%

Core DFT Techniques for Modern Electronics

Effective Design for Testability requires familiarity with a range of techniques, each suited to different applications and complexity levels. Understanding when and how to apply these methods is crucial for engineering teams developing products for demanding applications.

Boundary Scan Testing (IEEE 1149.1)

Boundary scan, standardised as IEEE 1149.1 (commonly known as JTAG), represents one of the most powerful and widely adopted DFT techniques. This method uses a serial scan chain to access pins on integrated circuits without requiring physical test probes.

The boundary scan architecture includes:

• Test Access Port (TAP): A four-wire interface (TDI, TDO, TCK, TMS) plus optional TRST

• Boundary Scan Register: Shift register cells at each IC pin

• TAP Controller: State machine governing test operations

• Instruction Register: Defines the current test operation

For complex printed circuit boards with ball grid array (BGA) packages where traditional probe access is impossible, boundary scan provides the only practical means of testing interconnections. Modern implementations achieve fault coverage exceeding 98% for stuck-at faults on supported nodes.

Built-In Self-Test (BIST)

BIST techniques embed test pattern generators and response analysers directly within the integrated circuit or system. This approach is particularly valuable for memory testing, where exhaustive pattern testing would otherwise require prohibitively long test times.

Common BIST implementations include:

• Memory BIST: Automated testing of RAM blocks using march algorithms

• Logic BIST: Pseudorandom pattern generation for combinational and sequential logic

• Analogue BIST: On-chip stimulus generation and response measurement for mixed-signal circuits

A typical Memory BIST controller can test a 256KB SRAM block in under 100 milliseconds, whereas external testing might require several seconds—a critical consideration for high-volume production environments.

Scan Chain Design

Scan design converts standard flip-flops into scan flip-flops that can be connected in serial chains for test access. During normal operation, the circuit functions conventionally. During test mode, the scan chains allow direct observation and control of internal state elements.

Key scan design considerations include:

• Scan Chain Length: Typically 500-2,000 flip-flops per chain for optimal test time

• Scan Insertion Rate: Target 95%+ of all sequential elements

• Compression Ratio: Modern techniques achieve 50:1 to 200:1 compression

• At-Speed Testing: Launch-on-capture or launch-on-shift for timing validation

PCB-Level DFT Considerations

While much DFT literature focuses on integrated circuit design, printed circuit board-level testability is equally critical for system manufacturers. Engineering teams in Atlantic Canada, often working on low-to-medium volume production runs, must balance comprehensive test coverage with practical manufacturing constraints.

Test Point Placement Strategy

Strategic test point placement enables in-circuit testing (ICT) and flying probe testing access. Effective test point strategies consider:

• Minimum Test Pad Size: 0.9mm diameter recommended for bed-of-nails fixtures; 0.5mm minimum for flying probe

• Test Point Spacing: Minimum 2.54mm (100 mil) pitch for fixture-based testing

• Coverage Goals: Access to all nets, or prioritised access to critical signals and power rails

• Placement Location: Secondary (bottom) side preferred to minimise interference with primary side components

For a typical mixed-signal board with 1,000 nets, achieving 90% test coverage might require 400-600 dedicated test points, depending on component accessibility and board density.

Design for In-Circuit Testing

In-circuit testing remains a cornerstone of PCB manufacturing test, capable of detecting:

• Component presence and orientation

• Solder shorts and opens

• Wrong component values (resistors, capacitors within ±5% typical)

• Basic semiconductor functionality

To maximise ICT effectiveness, designers should:

• Provide test access to both ends of critical components

• Include isolation jumpers for back-driving protection

• Avoid sharing test points between multiple nets

• Document programming and calibration requirements

Functional Test Integration

Beyond structural testing, functional test considerations must be integrated into the design process. Functional testing validates that the assembled product performs its intended functions under realistic operating conditions.

Design for Functional Test Access

Effective functional test design includes:

• Test Connectors: Dedicated interfaces for production test equipment, clearly labelled and documented

• Debug Headers: UART, SPI, or I2C interfaces for software-level diagnostics

• Measurement Points: Accessible nodes for analogue measurements (voltage references, oscillator frequencies, power supply rails)

• Loopback Capabilities: Internal paths to verify communication interfaces without external equipment

Test Mode Implementation

Many products benefit from dedicated test modes that enable accelerated or simplified testing:

• Manufacturing Test Mode: Activated via hardware strap or software command to enable full diagnostic access

• Burn-In Mode: Elevated stress conditions for infant mortality screening

• Calibration Mode: Interfaces for production calibration of analogue parameters

• Field Service Mode: Limited diagnostics accessible to service technicians

Security considerations must balance test accessibility with protection against unauthorised access in deployed products—particularly critical for Nova Scotia firms serving defence and security-sensitive applications.

Emerging DFT Technologies and Trends

The electronics industry continues to evolve, bringing new challenges and opportunities for testability. Engineering teams must stay current with emerging technologies to maintain competitive advantage.

IEEE 1687 (IJTAG)

IEEE 1687, also known as Internal JTAG, extends the boundary scan concept to provide standardised access to embedded instruments within integrated circuits. This standard enables:

• Hierarchical access to on-chip debug and test resources

• Standardised instrument description through ICL and PDL languages

• Flexible configuration of test access networks

• Compatibility with existing JTAG infrastructure

3D IC and Chiplet Testing

As advanced packaging technologies like 3D stacking and chiplet-based designs gain adoption, new DFT challenges emerge:

• Testing of through-silicon vias (TSVs) and micro-bumps

• Known-good-die (KGD) verification before stacking

• Post-assembly testing of the complete stack

• Thermal considerations during at-speed testing

Machine Learning in Test

Artificial intelligence and machine learning are increasingly applied to test optimisation:

• Adaptive test pattern selection based on historical data

• Predictive analytics for field failure correlation

• Automated test coverage analysis and gap identification

• Real-time test limit optimisation

Best Practices for DFT Implementation

Successful DFT implementation requires organisational commitment beyond individual engineering decisions. The following best practices help ensure consistent results across projects.

Early Engagement

DFT considerations should begin during the requirements phase and continue through detailed design:

• Include DFT requirements in product specifications

• Conduct testability reviews at each design milestone

• Engage test engineers during schematic review

• Allocate PCB real estate for test features during floorplanning

Documentation Standards

Comprehensive documentation ensures that test features are properly utilised:

• Test point assignments and locations

• Test mode activation procedures

• Expected measurements and tolerances

• Calibration procedures and requirements

Design Review Checklists

Formalised DFT checklists should address:

• Boundary scan chain connectivity and compliance

• Test point coverage and accessibility

• Power supply sequencing and test isolation

• Programming and debug interface availability

• Environmental stress screening compatibility

Partner with Experienced Engineering Professionals

Implementing effective Design for Testability requires expertise that spans electronic design, manufacturing processes, and test engineering disciplines. The choices made during early design phases have lasting impacts on product quality, manufacturing cost, and field reliability.

At Sangster Engineering Ltd., our team brings decades of combined experience in electronics design and development, serving clients throughout Nova Scotia, Atlantic Canada, and beyond. We understand the unique challenges facing regional manufacturers—from harsh maritime environmental requirements to the quality demands of ocean technology, defence, and aerospace applications.

Whether you're developing a new product and want to ensure testability from the start, or you're facing test coverage challenges with existing designs, we can help. Our engineering team provides comprehensive design services that incorporate DFT best practices appropriate for your production volumes, cost targets, and quality requirements.

Contact Sangster Engineering Ltd. today to discuss how we can support your next electronics development project with professional engineering services tailored to your needs. Let us help you design products that are not only functional and reliable but also efficiently testable throughout their production lifecycle.

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.