Multilayer pcb Wikipedia

Multilayer pcb Wikipedia

Multi-layer printed circuit boards have trace layers inside the board. This is achieved by laminating a stack of materials in a press by applying pressure and heat for a period of time. This results in an inseparable one piece product. For example, a four-layer PCB can be fabricated by starting from a two-sided copper-clad laminate, etch the circuitry on both sides, then laminate to the top and bottom pre-preg and copper foil. It is then drilled, plated, and etched again to get traces on top and bottom layers.

Inner layer automated optical inspection (AOI)

The inner layers are given a complete machine inspection before lamination because afterwards mistakes cannot be corrected. The automatic optical inspection system scans the board and compares it with the digital image generated from the original design data.

Laminates

Laminates are manufactured by curing under pressure and temperature layers of cloth or paper with thermoset resin to form an integral final piece of uniform thickness. The size can be up to 4 by 8 feet (1.2 by 2.4 m) in width and length. Varying cloth weaves (threads per inch or cm), cloth thickness, and resin percentage are used to achieve the desired final thickness and dielectric characteristics. Available standard laminate thickness are listed in.

Often encountered materials:

FR-2 (Flame Retardant 2), phenolic paper or phenolic cotton paper, paper impregnated with a phenol formaldehyde resin. Cheap, common in low-end consumer electronics with single-sided boards. Electrical properties inferior to FR-4. Poor arc resistance. Generally rated to 105 °C. Resin composition varies by supplier.

FR-4 (Flame Retardant 4), a woven fiberglass cloth impregnated with an epoxy resin. Low water absorption (up to about 0.15%), good insulation properties, good arc resistance. Well-proven, properties well understood by manufacturers. Very common, workhorse of the industry. Several grades with somewhat different properties are available. Typically rated to 130 °C. Thin FR-4, about 0.1 mm, can be used for bendable circuitboards. Many different grades exist, with varying parameters; versions are with higher Tg, higher tracking resistance, etc.

Aluminium, or metal core board or insulated metal substrate (IMS), clad with thermally conductive thin dielectric - used for parts requiring significant cooling - power switches, LEDs. Consists of usually single, sometimes double layer thin circuitboard based on e.g. FR-4, laminated on an aluminium sheetmetal, commonly 0.8, 1, 1.5, 2 or 3 mm thick. The thicker laminates sometimes come also with thicker copper metalization.

Flexible substrates - can be a standalone copper-clad foil or can be laminated to a thin stiffener, e.g. 50-130 µm

Kapton, a polyimide foil. Used for flexible printed circuits, in this form common in small form-factor consumer electronics or for flexible interconnects. Resistant to high temperatures.

Pyralux, a polyimide-fluoropolymer composite foil.Copper layer can delaminate during soldering.