Chapter 7: Printed Circuit Boards

What a printed circuit board does

A printed circuit board (PCB), also called a printed wiring board, does two jobs. It gives components mechanical support, and it provides their electrical interconnections through copper tracks bonded to an insulating board instead of individual wires. Compared with point-to-point wiring it is compact, light, repeatable and reliable, and it suits automated assembly and testing. With its components fitted it becomes a printed circuit assembly — the "card" or "board" inside every avionics LRU, FADEC, display and control panel. In many units the cards are shop-replaceable units (SRUs) that plug into a backplane (motherboard).

Materials

• FR-4 — woven glass-fibre cloth impregnated with epoxy resin. "FR" means flame retardant. It is strong, stable and the standard material, typically 1.6 mm thick.
• Phenolic paper (FR-1/FR-2) — cheap consumer boards; rarely used in aircraft equipment.
• Polyimide — withstands higher temperatures, and is the base of flexible and rigid-flex circuits.
• PTFE and ceramic-filled laminates — low-loss materials for radio-frequency and microwave circuits such as radar and radio altimeters.
• Metal-core (aluminium-backed) boards — spread heat from LEDs and power devices.
• Copper foil is specified by weight per square foot: 1 oz/ft² ≈ 35 µm thick and 2 oz ≈ 70 µm. Heavier copper and wider tracks carry more current; wider spacing withstands higher voltage.

Single-sided, double-sided and multilayer boards

• Single-sided — copper on one side only. Simple and cheap, used for simple circuits.
• Double-sided — copper on both sides, joined by plated-through holes (PTH): drilled holes whose walls are plated with copper.
• Multilayer — several thin copper-clad cores and bonding sheets (prepreg) laminated under heat and pressure. Avionics boards commonly have 6–20 or more layers. Inner layers are often complete power and ground planes, which give low-impedance supply and return paths and shield against interference. Layers are joined by vias: through vias pass through every layer, blind vias join an outer layer to an inner one, and buried vias join inner layers only.
• Flexible and rigid-flex circuits — polyimide films that bend or fold into tight spaces and replace connectors and wiring harnesses, for example behind display panels.

How a board is made (in outline)

The track pattern is printed photographically onto copper-clad laminate, and the unwanted copper is etched away. Holes are drilled and their walls plated. A solder mask (usually green) is applied over everything except the pads, to stop solder bridging between tracks and to protect the copper. The pads receive a solderable surface finish (tin, or nickel-gold). Finally a silkscreen legend is printed: component outlines, reference designators and polarity and pin-1 marks. Bare boards are then electrically tested.

Mounting components

• Through-hole technology (THT): leads pass through plated holes and are soldered on the far side, usually by wave soldering. Strong joints suit connectors and large or heavy parts, but they use more board area.
• Surface-mount technology (SMT): surface-mount devices (SMDs) are placed on pads coated with solder paste and reflow-soldered in an oven. They are much smaller and lighter, can go on both sides of the board, allow high density, and perform better at high frequencies. Their joints are small and need magnification to inspect. A ball grid array (BGA) hides its joints under the package, so they are checked by X-ray.
• Most boards mix both technologies.

Board features

• Tracks, pads (lands), vias and planes — the copper.
• Edge connectors — gold-plated fingers that plug into a backplane connector. A keying (polarising) slot stops a card being inserted the wrong way round or into the wrong slot. Card guides and ejector levers help remove cards without flexing them.
• Test points, links, heat sinks and stiffeners are added as the design needs them.

Conformal coating

Most aircraft circuit assemblies are protected by a conformal coating: a thin polymer film — acrylic, polyurethane, silicone, epoxy or parylene — sprayed, brushed or dipped over the finished board so it follows every contour. It protects against moisture and condensation, contamination, salt, fungus and dust, and helps against vibration. Most coatings glow under ultraviolet light, which makes coverage easy to inspect. For a repair, the coating is removed locally by the method that suits its type (solvent, heat or careful abrasion — parylene must be abraded), the work is done and cleaned, and the same type of coating is re-applied and cured.

Electrostatic discharge (ESD)

Many components on a board — above all MOS and CMOS devices and modern fine-geometry chips — can be destroyed by a discharge of static electricity. A person does not feel a discharge below about 3 000 V, yet some devices are damaged by less than 100 V. Worse, a discharge can cause latent damage: the part keeps working but has been weakened, and fails later in service.

• Recognise the ESD-sensitive device symbol — a yellow triangle with a hand crossed through — and the caution labels on LRUs and packaging.
• Wear a tested wrist strap connected through about 1 MΩ to the common ground point, work on a grounded ESD mat, and use grounded soldering irons and ionisers where provided.
• Handle cards by their edges; never touch pins, tracks or connector contacts.
• Keep parts and cards in shielding (metallised) bags or conductive containers until they are needed, and fit connector caps to removed LRUs.
• Avoid synthetic clothing and plastic items at the work area.

Inspection, defects and repair