Printed Circuit Boards or PCBs fall into various categories depending on their form. They can be rigid, flexible, rigid-flex, and high-frequency. The vast majority of PCB are of the rigid form. The industry uses the flex type when the board must occupy a non-planar position, typically serving as a replacement for a cable. Rigid-flex boards present a rigid board at one or both ends of a flex board, especially for installing electrical parts like connectors. Multichip modules use high-frequency boards typically as substrates.
Classification of PCBs
The industry classifies PCBs based on certain criteria such as:
- Reinforcing Materials — Paper, Glass fiber, PTFE fabric, Kevlar fabric, Polyester or Polyethylene terephthalate, Silicon Carbide
- Dielectric Materials — Polyimide, Epoxy, Cyanate Ester, Busmaleimide Triazine, PTFE, Polyester, Phenolics
- Circuit Function — Analog, Digital, Mixed, Microwave, RF
- Electronic Components Interface — THT or Through-Hole Technology, SMT or Surface Mount Technology, Mixed Technology, HASL or Hot Air Surface Leveling, ENEPIG, ENIG, Immersion silver, Immersion tin
- Board Construction — Single layer, Double layer, Multi-layer, Flex, Rigid-Flex
- Design Complexity — Circuit density, Interconnecting structures (PTH or plated through holes, blind vias, buried vias, via-in-pads, and low, moderate, or high manufacturability.
A significant portion of laminate materials in industrial applications are polyimide with glass reinforcements. The glass transition temperature of polyimides is above 200 °C. Their coefficient of thermal expansion is nearly 55 ppm/ °C in out-of-plane direction. Their CTE is nearly 15 ppm/ °C for in-plane direction.
The above thermal properties of polyimides offer a good match to ceramic-based thermocircuits soldered to the boards. Matching the thermal properties helps to reduce thermal stress that transfers to the solder joints of the packages. This thermal stress accumulates with each thermal cycle, especially with wide delta T’s in association with ground-based environmental testing (-45 °C to +85 °C).
Industries use epoxy-based laminate materials when the thermal condition of the applications allow, such as during experiments inside aerospace vehicles. Epoxy-based laminates typically have thermal property values like Tg of 150-170 °C, in-plane CTE of 10-15 ppm/ °C, and out-of-plane CTE of 50-70 ppm/ °C.
PCB Laminate Materials
Industries select PCBs for specific applications based on differences in materials, processing steps, or both.
- Reinforcement — The material offers mechanical strength together with electrical properties.
- Coupling Agent — Helps to bond organic resin with inorganic glass, while transferring stresses through the structure.
- Resin — Acts as a load transferring agent and a binder.
- Curing Agent — Enhances cross/linear polymerization in resin.
- Flame Retardant — Reduces flammability of laminates.
- Filler — Reduces cost of laminate and thermal expansion.
- Accelerators — Reduces curing temperature, increases reaction rate, controls cross-link density.
PCB Requirements for High-Reliability
The industry selects PCB materials for high reliability applications with an aim to optimize performance of the final assembly. This includes minimizing thermal vacuum outgassing over relatively large swings of temperature, reducing the accumulation of stress over several thermal cycles, and over-exposures to temperature variations linked to ground testing and over the life of the project.
The industry selects inks, via fills, and solder mask for minimizing outgassing. They appropriately specify, test, and qualify the PCB to ensure the boards meet the industry’s outgassing requirements. Depending on the application, the needs can be diverse.
Standards for High-Reliability PCBs
The industry couples material properties and selection of standard materials with design criteria for ensuring manufacturability. Design for high-reliability PCBs commonly follow industry standards like:
- IPC-2221 — Generic Standard on Printed Board Design
- IPC-2222 — Sectional Design Standard for Organic Rigid Printed Boards
- IPC-2223 — Sectional Design Standard for Flex Printed Boards
- IPC-2225 — Sectional Design Standard for MCM-L or Organic Multichip Modules and their Assemblies.
- MIL-STD-55110 — General Specification for Performance of Rigid Printed Wiring Board
- MIL-PRF-31032 — General Specification for Printed Circuit Board/Printed Wiring Board
- ECSS-Q-ST-70-10C — Qualification and Assurance of printed circuits boards
- IPC A-600 — Acceptability of Printed Boards (Class 3 requirements)
- IPC-6011 — Generic Performance Specification for Class 3 Printed Circuit Boards
- IPC-6012 — Qualification and Performance Specification for Rigid Printed Boards, Class 3
- IPC-6013 — Qualification and Performance Specification for Flexible Printed Boards, Class 3
- IPC-6015 — Qualification and Performance Specification for Mounting and Interconnecting Structures for Organic MCM-L or Multichip Modules
- IPC-6018 — Microwave End Product Board Inspection and Test, Class 3
- IPC-6012DS — Qualification and Performance Specification for Rigid Printed Boards for Space and Military Avionics Applications, Addendum to IPC-6012D.
The industry uses a test-plan whose goal is to design-in the variations in printed circuit boards, thereby correlating the effects of these variations to the risk of PCB failure. They also conduct reliability tests such as mechanical flexure and temperature cycling on test samples. The aim is to discover whether design parameters need controlling with a minimum dimension so that there is no loss of reliability.
Akber Roy is the CEO of RUSH PCB Inc. He has 20 years of experience in electronic contract manufacturing, pcb manufacturing. RUSH PCB website provides detailed printed-circuit-board development resources.