Resistor selection for aerospace and defence designs no longer stops at checking a part number against a qualification list. Engineers must interpret qualification results in the context of real operating conditions and verify that the manufacturing system can reproduce those results consistently over decades of service life.
This article uses the TT Electronics Welwyn CR Series thick‑film chip resistors as a case study to show how to combine formal qualification with process controls and application‑specific analysis when designing for long‑term programmes.
Key features and benefits
The Welwyn CR Series from TT Electronics is a family of high‑reliability thick‑film chip resistors designed and manufactured in Bedlington, United Kingdom, and approved to EN 140401 and IECQ‑CECC 40401 for qualified resistance ranges. The series covers multiple chip sizes, wide resistance values and several tolerance options, making it suitable for signal, control and moderate power functions in avionics, defence and other long‑life systems.
- Formal qualification basis
- Qualified to EN 140401 and IECQ‑CECC 40401 requirements for specified resistance ranges
- Tested using recognised methods for environmental endurance, mechanical integrity and electrical stability (for example, load‑life, temperature cycling, moisture resistance, vibration, mechanical shock, short‑time overload)
- Key electrical parameters such as resistance value and temperature coefficient of resistance are verified before and after stress testing
- High‑reliability thick‑film technology
- Uses established thick‑film construction on ceramic substrates with screen‑printed resistive films and laser trimming for tight tolerance values
- Passivation systems are optimised to ensure long‑term stability under humidity, handling and cleaning conditions typical of aerospace and defence production
- Thick‑film technology gives good surge and overload robustness compared to many thin‑film parts, which is helpful in power‑related and mixed‑signal functions
- Manufacturing and process evidence
- TT Electronics draws on decades of production experience and millions of component‑hours of load‑life testing as evidence for long‑term performance of the technology and processes
- Statistical process control, lot traceability and disciplined engineering change management underpin repeatability of the qualified design in ongoing production
- The same process discipline supports FIT estimate generation for system‑level reliability modelling in safety‑critical platforms
- System‑level benefits
- Formal qualification plus strong process controls reduce both technical risk (unexpected drift or failure) and programme risk (supply continuity, unannounced changes)
- For programmes expected to remain in service for twenty years or more, this combination of evidence is as important as initial qualification itself
- Component‑level robustness supports lower maintenance needs and fewer redesigns due to obsolescence or uncontrolled changes
Typical applications
While the CR Series is a general‑purpose high‑reliability SMD chip resistor family, the combination of qualification pedigree and process controls makes it particularly suitable for long‑life aerospace, defence and high‑reliability industrial equipment.
Typical use cases include:
- Avionics and flight control
- Flight control computers and flight management systems where resistance stability over temperature and time is critical for sensor interfaces and control loops
- Actuator drive electronics and position feedback conditioning circuits
- Radar, communications and electronic warfare
- RF and IF signal‑conditioning stages where thick‑film parts are used for biasing, gain‑setting and termination functions in radar and communications systems
- Power management and housekeeping supplies in radar and electronic warfare equipment exposed to vibration, temperature cycling and occasional overloads
- Power management and control
- DC‑DC converter control circuits, feedback networks and protection sense resistors in power supplies for airborne and defence platforms
- General analogue and digital control circuits in mission computers, remote I/O, and control distribution units
- Space and long‑life industrial systems (where qualification and approvals align)
- Satellite sub‑systems and ground support equipment when the applicable EN and CECC approvals meet project requirements
- Long‑life industrial controls or transportation systems where life cycles can exceed twenty years and controlled change processes are mandatory
In all of these cases, the key is not only that the resistor family is qualified, but that the manufacturing site, change control and available traceability can support the expected service life of the platform.
Technical highlights
The table below summarises key technical aspects of the CR Series based on the TT Electronics datasheet and qualification overview. Exact electrical ratings for a given size and value should always be taken from the latest manufacturer datasheet and product‑level documentation.
| Parameter | Typical CR Series characteristics (per TT data) |
|---|---|
| Technology | Thick‑film SMD chip resistor |
| Package sizes | Multiple standard chip sizes (for example, 0805, 1206 and others as per datasheet) |
| Resistance range | Wide range within approximately 1 Ω to 100 MΩ according to manufacturer datasheet |
| Tolerances | Options including tight tolerances (for example, down to around ±0.1%) on selected values |
| Qualification and approvals | EN 140401 and IECQ‑CECC 40401 for defined ranges |
| Rated ambient temperature | Typically around 70 °C, with derating up to a maximum temperature as per datasheet |
| Operating temperature range | Extended range suitable for aerospace and defence applications (see datasheet) |
| Load‑life performance | Characterised by long‑term load‑life testing data used for FIT estimation |
From a design‑in perspective, a few of these points deserve closer interpretation:
- Temperature range and derating
The rated power is usually specified at a reference ambient temperature, often 70 °C for thick‑film chip resistors, with linear derating to a maximum allowed temperature. Designing with margin below the derating curve is critical in high‑reliability applications to reduce long‑term resistance drift and failure risk. - Temperature coefficient of resistance
The series offers specified temperature coefficient of resistance values over the operating range, enabling engineers to estimate resistance variation versus temperature and its impact on gain, bias and sensing accuracy. - Humidity and environmental robustness
Qualification testing includes moisture resistance, thermal cycling, vibration and mechanical shock. These tests help demonstrate that soldered resistors will maintain stability after board‑level assembly stresses and exposure to avionics or defence environmental conditions. - Surge and overload behaviour
Where surge or short‑time overload is relevant, designers should compare the application stress profile against the family’s short‑time overload and surge test results and consider additional protective circuitry if the application stresses are more severe.
How qualification and manufacturing controls work together
For aerospace and defence component engineers, checking that a resistor family carries the right qualification mark is only a starting point. Qualification reports confirm that representative devices passed a defined sequence of electrical, mechanical and environmental tests, but the long‑term behaviour in the field depends on how closely production follows the same design and processes.
TT Electronics highlights several aspects beyond the qualification itself:
- Ongoing process controls
- Statistical process control on critical steps such as thick‑film deposition, firing and laser trimming helps keep key parameters within narrow distributions over time.
- Substrate quality, passivation and termination processes are monitored to avoid unplanned shifts in solderability or moisture behaviour.
- Traceability and change management
- Lot traceability allows field or test data to be linked back to specific production lots, which supports root‑cause analysis and corrective action.
- Disciplined engineering change management, including configuration control and change notification, helps ensure that any process or material change is assessed for impact on qualified performance before implementation.
- Long‑term reliability evidence
- Accumulated load‑life testing, measured in millions of component‑hours, gives engineers additional confidence that the failure modes and drifts are well understood.
- This data supports more credible FIT estimations for system‑level reliability models used in safety‑critical and mission‑critical projects.
In practice, this means that when you specify a CR Series resistor today and again ten years into a long‑running programme, you can expect the part to behave consistently within the limits set by qualification and datasheet conditions, assuming no significant changes are introduced without proper notification and control.
Design‑in notes for engineers
When designing aerospace, defence or other high‑reliability systems around thick‑film chip resistors, engineers can strengthen their component choices by combining formal qualification data with application‑specific checks.
Checklist for interpreting qualification data
- Map qualification tests to your environment
- Compare the qualification load‑life conditions (power, temperature, duration) to your worst‑case stress profiles.
- Ensure that thermal cycling, vibration and mechanical shock test levels reflect or exceed your expected platform conditions.
- Assess resistance stability mechanisms
- Review load‑life and thermal cycling resistance drift data at the value range you intend to use.
- Check humidity‑related drift behaviour if the equipment may see condensation, high humidity storage or aggressive cleaning processes.
- Understand temperature and TCR impact
- Calculate expected resistance change across the equipment temperature range using specified temperature coefficient values.
- For precision analogue functions, consider whether resistor matching or tighter TCR classes are needed to keep gain and offset errors within budget.
Using CR Series in real circuits
- Analogue and signal conditioning
- For sensor interface and signal conditioning networks, choose resistance values and tolerances that keep both initial error and drift within allowable limits across life and temperature.
- Where noise is a concern, balance the trade‑off between high resistance values (lower current, higher noise) and lower values (higher current, lower noise, more power dissipation).
- Control and power‑management functions
- In power supply control circuits and feedback dividers, ensure worst‑case power dissipation stays well below the rated power at maximum ambient temperature and under fault conditions.
- Use derating and layout to keep actual resistor body temperatures comfortably within the thick‑film system’s long‑term stability envelope.
- Assembly and mechanical considerations
- Verify that PCB layout and reflow profiles follow the manufacturer’s assembly guidelines to avoid micro‑cracking or solder joint damage that can affect long‑term stability.
- In high‑vibration environments, consider board‑level measures such as underfill or mechanical support where necessary, and verify against the qualification vibration conditions.
Programme‑level considerations for buyers and component engineers
- Clarify documentation and control needs
- Define up‑front whether your programme requires formal lot traceability, specific documentation packs, or unique test reports for each lot.
- Agree on configuration control and change notification expectations, including how deviations and controlled changes will be communicated.
- Align with obsolescence and lifecycle strategies
- Confirm roadmap and obsolescence policies with TT Electronics for the CR Series line items you intend to standardise on.
- Where possible, qualify alternative values or package sizes within the same series to provide flexibility if certain values go EOL or face long lead times.
By following these principles, design teams can move beyond a checklist approach to qualification and use the CR Series as a well‑understood, well‑controlled building block in complex, long‑life aerospace and defence systems.
Source
This article is based on information published by TT Electronics on resistor qualification principles for aerospace and defence applications and on official documentation for the Welwyn CR Series thick‑film chip resistors, combined with independent engineering interpretation.
