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What Is The Reliability Test in PCB ?

Printed circuit boards (PCBs) must withstand years of service in demanding operating environments, from consumer devices to automotive, aerospace, medical and industrial applications. Validating PCB reliability requires testing samples under accelerated stress conditions across temperature, vibration, humidity, mechanical shock, and power cycling.

This article explores common PCB reliability tests, failure modes revealed, and how test results validate design margin and process quality. We’ll examine test methodologies, challenges interpreting results, qualifications standards, and how reliability testing reduces field failure risks. Read on for an in-depth look at this vital aspect of proving out PCB technology before volume deployment.

Reasons for PCB Reliability Testing

Thorough reliability testing provides confidence that PCBs will survive expected conditions over target lifespan requirements. Key drivers for reliability testing include:

  • Risk reduction – Identify and correct flaws before volume production
  • Process validation – Quantify manufacturing consistency and quality
  • Material selection – Prove out substrate, bonding, and coating materials
  • Design verification – Validate robustness of board stackup, layout, components
  • Qualification – Certify boards meet applicable standards like IPC or military specs
  • Improvement – Feedback to enhance design rules, processes, and QC

By stressing PCB samples beyond normal use environments, latent weaknesses are revealed enabling corrective actions. This prevents field failures and costly recalls.

PCB Failure Mechanisms and Fault Models

To develop effective reliability tests, the potential failure mechanisms in PCBs must be considered:

Electrical Shorts

  • Conductive debris creating short circuits
  • Electrical clearance inadequate under bias/humidity
  • Dendrite formation on closely spaced conductors

Open Conductors

  • Copper traces cracked by thermal cycling or shock
  • Solder joint fractures from vibration or fatigue
  • Contact pad lift or delamination

Intermittent Contacts

  • Vibration causes temporary loss of contact
  • Fretting corrosion and oxidation increases contact resistance

Dielectric Breakdown

  • Electrical arcing under high voltage bias and moisture

Cracking/Fracture

  • Thermal expansion mismatch causes component fracture
  • Vibration or shock cracks PCB substrate or joints

Corrosion/Dendrites

  • Environmental chemicals induce electrical leakage
  • Electrochemical migration of metal grows between conductors

Tests target replicating these fault conditions in an accelerated manner.

PCB Reliability Test Methods

Common PCB reliability tests include:

Temperature Cycling

  • Cycles board between hot and cold extremes
  • Assesses thermal expansion stresses and cracking
  • Standard cycles: -55°C to 125°C

Power Cycling

  • Power boards on and off repeatedly
  • Evaluates stresses from thermal transients
  • Checks for intermittent contacts or cracks

Vibration

  • Subjects PCB to vibration profiles
  • Screens for cracked solder joints and metal fatigue
  • Common test frequency range: 10-2000Hz

Mechanical Shock

  • Applies high G impulses to board
  • Shakes loose marginal solder joints
  • Tests for fractured components or traces

Humidity/Temperature/Bias (HTB)

  • Exposes PCBs to high heat, humidity, and voltage
  • Accelerates conditions promoting arcing, corrosion, electrochemical migration

Mixed Flowing Gas (MFG)

  • Flows mixed gas over powered boards
  • Accelerates surface corrosion and metal migration

Test standards define profiles tailored to replicate use conditions and accelerate anticipated failure mechanisms.

Interpreting PCB Reliability Testing Results

RFT Testing

The main results obtained from reliability testing are:

Pass/Fail Data

  • Were samples functional after testing?
  • Used to validate passing qualification requirements

Parametric Drift Data

  • Change in electrical parameters like resistance and capacitance
  • Gives insight into level of degradation or intermittent issues

Failure Analysis

  • Inspect failed samples to determine root cause
  • SEM, thermal imaging, bench probing used

Bathtub Curve

  • Plotting cumulative failures over test duration or cycles
  • Indicates failure rates and susceptibilities

The goal is not necessarily zero failures but rather validating design margin exists for minimal field failure rates. Extensive results enable feedback to improve manufacturing processes, design rules, components, materials and modeling.

Industry Reliability Qualification Standards

Many reliability standards exist covering various aspects of electronics:

IPC Standards

  • IPC-SM-785 – Guidelines for accelerated reliability testing of surface mount solder attachments
  • IPC-9701 – Performance test methods and qualification requirements for high frequency boards

JEDEC Standards

  • JESD22 – Extensive reliability test methods for electronics components and boards

Telcordia GR Standards

  • Telcordia GR-63-CORE – Network equipment reliability assurance
  • Telcordia GR-1221-CORE – Electrostatic discharge tolerance

MIL-STD Standards

  • MIL-STD-202 – Environmental stress testing methods for electronics
  • MIL-PRF-31032 – Certification requirements for high reliability PCBs

IEC Standards

  • IEC 60068 – Environmental testing of electronics across vibration, shock, temperature, humidity and more

Qualifying for these established standards validates achieving baseline reliability benchmarks needed for commercial and industrial applications.

Building in Design Margin

fct test

Aside from process controls, the PCB design can incorporate margin to enhance reliability:

  • Derating – Reduce voltage, current, and power limits to levels below maximums
  • Redundancy – Include backup circuits or pathways
  • Guard Banding – Provide spacing margins in layout – increase creepage, clearance
  • Simulation – Use accelerated models that consider aging effects
  • Sizing – Right-size traces, pads, and components for stresses
  • Materials – Select robust substrate materials and protective coatings
  • Diversification – Use component families with alternate internal construction
  • Monitoring – Built-in sensors to monitor temperature, vibration, etc.
  • Quality Components – Select reputable suppliers and proven parts

Building in margin provides insurance should unanticipated overload or use conditions emerge. This avoids “just good enough” designs prone to field failures.

Incorporating Reliability Testing in PCB Development

A strategic approach to reliability testing balances risk reduction with development costs:

Material and Component Testing – Start reliability testing at the material level before full PCB design to validate robustness.

HALT/HASS – Highly Accelerated Life Testing reveals failure points through extreme stressing.

FMEA – Failure Mode and Effects Analysis predicts risk areas to guide qualification.

Early Prototypes – Test initial prototypes under accelerated conditions to guide design revisions.

Process Validation – Apply reliability tests for new technologies before volume ramp.

Qualification Testing – Certify boards meet requirements as part of architecture freeze.

Ongoing Monitoring – Continue testing during volume production for process control and outlier detection.

Regression Testing – Validate reliability when revising designs or upgrading manufacturing lines.

Reliability cannot be an afterthought but must factor in throughout the PCB development, qualification, and production lifecycle.

Conclusion

Rigorously testing PCBs under accelerated stress conditions reveals design flaws and manufacturing defects before volume deployment. While passing established reliability qualifications provides a baseline level of confidence, continuous failure analysis and process improvements are needed to meet modern demands for service life spanning decades in challenging operating environments.

By incorporating reliability engineering principles spanning design, materials selection, modeling, testing, inspection, and continuous feedback, electronics producers can deliver robust PCB products with minimized risks of field failures. The result is dependable electronics underpinning safety-critical infrastructure across end-markets from automotive to medical to aerospace applications.

Frequently Asked Questions

What is the typical sample size used for qualification testing?

Sample sizes vary based on program criticality. High volume consumer devices may use 3-5 samples. High reliability military boards may test 25+ samples per lot. Setting sample criteria depends on risk analysis and desired statistical confidence.

How should I interpret intermittent faults during testing?

Intermittent failures often point to marginal design margin, manufacturing variances, or component inconsistencies. Any tendency for intermittent operation under stress should be investigated through root cause analysis, as it may indicate susceptibility to field failure.

What is an appropriate length of time for power or temperature cycling testing?

There is no set rule. Choosing numbers of cycles and test duration involves tradeoffs between risk reduction, development time, and cost. Hundreds to thousands of cycles spanning days to weeks of continuous testing is typical.

Should every PCB design go through rigorous qualification testing?

It depends on production volume and end use environment. High volume consumer goods and safety-critical boards should undergo extensive qualification. Low volume lab equipment may rely more on design margin.

How are results from accelerated testing correlated to real world conditions?

Careful analysis is required using acceleration factors and reliability models. But uncertainties remain, so it is recommended to test with greater than actual severity and duration to provide engineering margin.

 

 

 

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