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DD IEC PAS 62326 — Printed Boards Device Embedded Substrate and Flexible Wiring Boards Standard Series
Printed Boards — Device Embedded Substrate and Flexible Printed Wiring Boards (Draft for Development series)
📐 DD IEC/PAS 62326 is a British Standards Institution (BSI) “Draft for Development” (DD) series of publications that adopt International Electrotechnical Commission (IEC) Publicly Available Specifications (PAS) for use in the UK. Prepared under BSI Technical Committee EPL/501, Electronic assembly technology, the series covers two key areas of printed board technology: the terminology, reliability and design guide for device embedded substrates (Part 14), and the performance guide for single- and double-sided flexible printed wiring boards (Part 7-1). The two parts complement each other and serve the technical needs of high-density electronic assembly and flexible interconnection.
💡 The DD series is a characteristic BSI “Draft for Development” publication: it is not a formal British Standard but an interim document that adopts an IEC/PAS for trial use, allowing industry experience to be accumulated. It is normally reviewed after a few years by the international body, which decides whether to upgrade it to a formal standard or extend its validity.
1. Series Overview and Positioning 📌
DD IEC/PAS 62326 builds on the IEC 62326 (printed boards) family and converts the IEC publicly available specifications into engineering documents usable in the UK. Unlike formal BS EN standards, the DD documents emphasise trial use and feedback: BSI solicits comments at publication so that UK experience can be reported back to the IEC and help drive conversion into an international standard. The series therefore has guidance value in engineering practice but should not be misread as a mandatory product-acceptance standard.
The batch contains the following two parts:
| Document | Subject | Year | International source | Core content |
|---|---|---|---|---|
| DD IEC/PAS 62326-14:2010 | Printed boards — Device embedded substrate: terminology, reliability and design guide | 2010 | IEC/PAS 62326-14:2010 | Terminology system, test methods (TEG test vehicle) and design guidance for device embedded substrates |
| DD IEC/PAS 62326-7-1:2007 | Performance guide for single- and double-sided flexible printed wiring boards | 2007 | IEC/PAS 62326-7-1:2007 | Base materials, test methods, performance levels and visual inspection requirements |
⚠️ These two parts correspond to different sub-parts of IEC/PAS 62326, not to a new and old version of the same part. When specifying, select the appropriate part by product type (rigid-flex, flexible wiring, embedded devices) and do not mix them.
2. Device Embedded Substrate: Terminology, Reliability and Design Guide 🔧
Part 14 addresses device embedded substrates, a high-density packaging technology in which active or passive devices are embedded inside (rather than on the surface of) the substrate, enabling shorter interconnection paths, smaller packages and better electrical performance. The part first establishes a complete terminology system, distinguishing between “device embedded substrate technology”, “substrate”, and “Jisso mounting and interconnection”, and defines the structural levels and terminology.
2.1 Structural Levels and Process Terminology
The document structures the device embedded substrate into several levels and describes the manufacturing flow with standardised terms:
- Device embedding by conventional process: devices are buried within the substrate through conventional lamination or drilling steps.
- Electrical connection by vias after device embedding: after embedding, laser- or mechanically-drilled vias connect the device to the substrate wiring layers.
- Embedding of various devices over multiple layers: at different stages, different devices are embedded into different layers, forming a multi-layer embedded structure.
This layered description lets engineers understand the design intent of an embedded substrate precisely and provides a common language for different process routes.
2.2 Reliability Evaluation: TEG Test Vehicle
Reliability evaluation is a major part of this section. The document proposes a TEG (Test Equipment Group) test vehicle — a purpose-built “test vehicle” that reproduces the actual operating environment on a test circuit and test specimen, assessing electrical performance, thermal cycling and mechanical stress. The choice of test items and test equipment must match the real application environment, for example addressing thermal-expansion mismatch, via fatigue, and delamination risk at the device-substrate interface.
💡 Key to reliability design: after embedding, the difference in coefficient of thermal expansion between the substrate and the device creates interface stress. The design should control delamination and fatigue risk through proper via layout, material matching and structural redundancy.
3. Flexible Printed Wiring Boards: Performance Guide 🧵
Part 7-1 provides a performance guide for single- and double-sided flexible printed wiring boards. Unlike rigid boards, flexible boards experience bending, folding and dynamic flexing in service, so their performance requirements must balance electrical performance with mechanical flexibility. The document sets requirements at several levels:
- Test methods: specify the electrical and mechanical tests, including bend and fold endurance and other environmental/durability tests.
- Performance levels: classify performance by severity of application, providing a grading basis for different uses.
- Base materials: specify the suitability of flexible insulating base materials (e.g. polyimide).
- Visual inspection: define the inspection environment and items to ensure finished appearance and structural integrity.
Because flexible boards are widely used in foldable electronics, wearables and high-density interconnection, the performance grading and test methods directly affect product service life and reliability.
4. Engineering Practice Points 🛠️
- 🔍 Selection path: first determine whether the need is “device embedding” or “flexible interconnection”, then choose Part 14 or Part 7-1; both often appear together in one module.
- 🔧 Design synergy: embedded-substrate design must be co-optimised with packaging, routing and via processes to avoid short circuits or thermal failure between embedded devices and adjacent wiring.
- 📊 Reliability verification: use the TEG test vehicle for accelerated ageing, focusing on failure modes under thermal cycling, damp heat and mechanical vibration.
- 📋 Documentation: DD documents are provisional; procurement and design files should state the specific part and edition used, and monitor BSI review and upgrade activity.
5. Frequently Asked Questions ❓
❓ Q: Is DD IEC/PAS 62326 a formal British Standard?
A: No. The DD (Draft for Development) is a BSI provisional document adopting an IEC/PAS for trial use. It cannot replace a formal BS EN standard, but serves as an engineering reference where no formal standard exists.
A: No. The DD (Draft for Development) is a BSI provisional document adopting an IEC/PAS for trial use. It cannot replace a formal BS EN standard, but serves as an engineering reference where no formal standard exists.
❓ Q: How does a device embedded substrate differ from surface-mount (SMT)?
A: SMT solders devices onto the board surface; a device embedded substrate buries devices inside the substrate and interconnects them through vias. The embedded structure gives shorter interconnection, smaller packaging and better electrical performance, but requires more complex process and reliability control.
A: SMT solders devices onto the board surface; a device embedded substrate buries devices inside the substrate and interconnects them through vias. The embedded structure gives shorter interconnection, smaller packaging and better electrical performance, but requires more complex process and reliability control.
❓ Q: How should performance levels of flexible printed wiring boards be understood?
A: The document grades performance by application severity; higher levels correspond to stricter test conditions and durability requirements. Select the level by the actual bending, flexing and temperature environment.
A: The document grades performance by application severity; higher levels correspond to stricter test conditions and durability requirements. Select the level by the actual bending, flexing and temperature environment.
🔍 Q: Why use a TEG test vehicle for reliability evaluation?
A: The TEG (Test Equipment Group) test vehicle is a purpose-built test substrate that reproduces the real embedded-device structure and applies electrical, thermal and mechanical loads, exposing latent failures under accelerated conditions to guide design improvement.
A: The TEG (Test Equipment Group) test vehicle is a purpose-built test substrate that reproduces the real embedded-device structure and applies electrical, thermal and mechanical loads, exposing latent failures under accelerated conditions to guide design improvement.