Electronics Guide

Fixture Construction

A typical bed-of-nails fixture consists of several key components:

• Probe plate - A precisely drilled plate holding spring-loaded probes in positions corresponding to test points on the PCB
• Wiring harness - Connections from probes to the test system interface, often using wire-wrap or ribbon cable assemblies
• Vacuum or mechanical hold-down - Systems to press the board firmly against the probes for reliable contact
• Receiver plate - A guide structure ensuring proper alignment of the board with the probe array
• Support structure - A rigid frame maintaining probe positions and fixture geometry

Probe Technologies

Spring-loaded probes come in various styles suited to different applications:

• Standard crowned tips - General-purpose probes for plated through-holes and test pads
• Spear or chisel tips - Designed to penetrate oxidation or flux residue on component leads
• Cup or tulip tips - Concave tips that center on spherical contacts such as solder balls
• Blade or knife tips - For contacting fine-pitch component leads
• Serrated tips - Multiple contact points for improved reliability on contaminated surfaces

Probe selection depends on the contact surface, required current capacity, contact resistance specifications, and cycle life expectations. High-performance probes may achieve over one million cycles before requiring replacement.

Design Considerations

Successful fixture design requires careful attention to several factors:

• Test point accessibility - Ensuring adequate clearance around test points for probe placement
• Probe spacing - Maintaining minimum pitch between probes to prevent short circuits and mechanical interference
• Board support - Preventing flexing during probe contact, especially for thin or large boards
• Thermal management - Accommodating board warpage and probe expansion during powered testing
• Maintenance access - Enabling easy replacement of worn or damaged probes

Architecture and Operation

Boundary scan architecture includes several key elements:

• Test Access Port (TAP) - A four or five wire interface (TCK, TMS, TDI, TDO, and optional TRST) providing access to on-chip test logic
• TAP Controller - A state machine managing the test interface and controlling boundary scan operations
• Boundary scan register - A shift register with cells at each device pin, allowing capture and control of pin states
• Instruction register - Controls the operation mode of the boundary scan logic
• Data registers - Store test patterns and captured results

Test Capabilities

Boundary scan enables several types of testing:

• Interconnect testing - Detecting opens, shorts, and stuck-at faults between devices on the board
• Cluster testing - Testing non-boundary scan devices through boundary scan capable neighbors
• In-system programming - Programming flash memory, CPLDs, and FPGAs through the JTAG interface
• Functional testing - Applying test vectors and capturing responses at device boundaries
• Debug access - Providing on-chip debug capabilities through the JTAG interface

Implementation Benefits

Boundary scan offers significant advantages in modern electronics manufacturing:

• Reduced fixture complexity - Fewer physical probes needed when using on-chip test access
• Access to hidden nodes - Testing connections under ball grid array packages and other inaccessible points
• Consistent test coverage - Standardized methodology across different board designs
• Defect isolation - Precise identification of fault locations for efficient repair
• Design reuse - Test programs portable across product variants sharing boundary scan devices

Test Strategy Development

Creating an effective functional test strategy involves several steps:

• Requirement analysis - Identifying critical functions and performance parameters that must be verified
• Test case definition - Developing specific test conditions and expected results for each requirement
• Stimulus and measurement planning - Determining the signals and measurements needed to exercise and verify each function
• Pass/fail criteria - Establishing quantitative limits for each measurement based on specifications and manufacturing capability
• Test sequence optimization - Ordering tests to minimize total test time while maintaining effective screening

Test System Architecture

Functional test systems typically include:

• Signal generators - Providing stimulus signals such as DC power, RF signals, digital patterns, or analog waveforms
• Measurement instruments - Capturing device responses including voltages, currents, timing, frequencies, and power levels
• Switching matrices - Routing signals between instruments and multiple test points on the device
• Load simulation - Representing actual operating conditions including resistive, capacitive, and active loads
• Test executive software - Coordinating test sequences, collecting data, and making pass/fail decisions

Test Program Development

Effective test program development follows established practices:

• Modular test architecture - Creating reusable test modules that can be combined for different product configurations
• Parameterized testing - Using configuration files to adapt test programs to product variants
• Error handling - Implementing robust exception handling to maintain system stability
• Data logging - Recording comprehensive test data for traceability and analysis
• Operator interface - Providing clear guidance for test operators and technicians

Stress Conditions

Common stress parameters used in burn-in include:

• Elevated temperature - Operating devices above normal ambient, typically 85 to 125 degrees Celsius
• Temperature cycling - Repeatedly transitioning between temperature extremes to stress solder joints and material interfaces
• Elevated voltage - Operating at voltages above nominal to accelerate voltage-dependent failure mechanisms
• Power cycling - Repeatedly powering devices on and off to stress power-on transients
• Vibration - Applying mechanical stress to identify marginal connections

Burn-in System Components

A complete burn-in system includes:

• Environmental chambers - Controlled temperature and humidity enclosures accommodating multiple units under test
• Burn-in boards - Carrier boards providing power and signal connections to devices within the chamber
• Power distribution - Systems delivering stable power to many devices simultaneously
• Monitoring systems - Continuous or periodic testing to detect failures during burn-in
• Data collection - Recording device responses and failure data for reliability analysis

Dynamic versus Static Burn-in

Burn-in approaches differ in how devices are exercised during stress:

• Static burn-in - Devices are powered but not actively exercised, primarily screening for temperature-related failures
• Dynamic burn-in - Devices execute test patterns during stress, enabling detection of functional failures and improving coverage of complex ICs
• Monitored burn-in - Combines dynamic operation with continuous or periodic testing to detect exactly when failures occur

Key Parameters

Common parametric measurements include:

• DC parameters - Supply current, input/output voltage levels, leakage currents, and threshold voltages
• AC parameters - Propagation delays, setup and hold times, rise and fall times, and maximum operating frequency
• Analog parameters - Gain, bandwidth, offset, linearity, and noise characteristics
• Power parameters - Static and dynamic power consumption, power supply rejection, and thermal characteristics
• RF parameters - Frequency accuracy, output power, sensitivity, and spurious emissions

Measurement Techniques

Accurate parametric measurements require attention to:

• Force and measure methodology - Applying precise stimulus while measuring device response
• Kelvin sensing - Using separate force and sense connections to eliminate lead resistance errors
• Guard techniques - Preventing leakage currents from affecting high-impedance measurements
• Settling time - Allowing sufficient time for signals to stabilize before measurement
• Averaging and filtering - Reducing noise effects on measurements through statistical techniques

Limit Setting

Establishing appropriate test limits balances several considerations:

• Specification compliance - Ensuring tested units meet published device specifications
• Guard bands - Providing margin between test limits and specification limits to account for measurement uncertainty
• Process capability - Setting limits achievable by the manufacturing process with acceptable yield
• Customer requirements - Meeting any application-specific requirements beyond standard specifications
• Statistical process control - Using parametric data to monitor and improve manufacturing processes

Data Collection and Management

Effective yield analysis begins with comprehensive data collection:

• Test data aggregation - Consolidating results from multiple test stations and operations
• Traceability linking - Associating test results with lot, date, equipment, and material information
• Parametric data storage - Recording actual measured values, not just pass/fail results
• Failure mode classification - Categorizing failures by symptom, location, or root cause
• Data quality assurance - Validating data integrity and identifying anomalies in collection

Analysis Techniques

Common yield analysis methods include:

• Pareto analysis - Identifying the most significant contributors to yield loss
• Trend analysis - Tracking yield and failure rates over time to detect shifts or patterns
• Correlation analysis - Finding relationships between yield and process variables
• Distribution analysis - Characterizing the statistical distribution of parametric measurements
• Spatial analysis - Identifying patterns related to position on wafers or panels

Continuous Improvement

Yield analysis supports ongoing process improvement through:

• Root cause identification - Using data patterns to pinpoint sources of defects
• Process control feedback - Providing early warning of process drift before yield impact becomes severe
• Design feedback - Identifying design features that create manufacturing challenges
• Supplier quality management - Tracking component or material related failures back to sources
• Cost reduction - Quantifying the impact of yield improvements on manufacturing cost

Coverage Metrics

Several metrics quantify test coverage:

• Fault coverage - The percentage of possible faults detectable by the test program
• Stuck-at fault coverage - Coverage of faults where nodes are stuck at logic high or low
• Structural coverage - Percentage of physical connections and components verified
• Functional coverage - Extent to which product functions have been exercised
• Defect coverage - Estimated percentage of actual defects detected, based on defect models

Coverage Analysis Methods

Tools and techniques for analyzing test coverage include:

• Fault simulation - Modeling faults in circuit descriptions and simulating test detection
• Design for Test analysis - Evaluating testability features during design phase
• Test point analysis - Determining accessibility of circuit nodes for probing
• Automatic test pattern generation - Creating test vectors optimized for fault detection
• Coverage gap identification - Finding untested areas requiring additional test development

Optimization Strategies

Improving test coverage while managing costs requires strategic approaches:

• Multi-strategy testing - Combining in-circuit, boundary scan, and functional testing for complementary coverage
• Defect-based prioritization - Focusing test development on the most likely and impactful defect types
• Test time reduction - Eliminating redundant tests and optimizing measurement sequences
• Design for testability - Incorporating test access points and built-in test features during product design
• Adaptive testing - Adjusting test depth based on product history or incoming quality indicators