Understanding how PCBs are made

There are two basic technologies associated with PCB fabrication: design and manufacture. First one is covered under the EDA topic. In this section we'll focus on the processes needed to make the actual board once the masks have been produced from the design database.

In the past PCB was build on photosensitive resin coated ceramic or FR4 substrates, exposing the board to UV light with a mask or stencil between the light source and the board with the purpose of setting the features included in the mask (the design) into the resin. NExt the material was developed using a chemical agent to attack the non-reacted resin. Then copper or aluminum plating was done to create the circuitry on the channels created. An optional step to cover again with a second step of photosensitive resin could be done to protect the metal traces and leave exposed only the areas for component attachment. Only one layer of circuitry was allowed with this process; in many ways this is still sufficient if the application is not too demanding: low number of components, relatively moderate requirements for power and heat dissipation.

More complex designs require that this basic process be repeated several times to create a multilayered board, where alternate coatings with resin, exposure, development, etching, plating and recoating are conducted to grow metal and isolator layers. This is required to enable sufficient escape lines for all signals and enough input lines for power and groud. A basic diagram of this type of construction is shown below:

Notice the brown line in the center of the costruction, this is the core, normally made of FR4 or an similar epoxylated material reinforced with fiber glass.
After that there is an alternate build up between bright yellow lines that simbolize Cu and orange planes that simbolize the isolating layers, normally constructed using a glass fiber/epoxy laminate called prepreg or ABF.
In addition to connect different metal layers there are through hole vias (through the core) or blind-buried vias (connecting one plane to the next neighbor metal plane).

In this sequence of steps the green represents the photosensitive material, the gray is the mask with the design features to be printed. The non-blocked areas allow the light to pass and set the resin, shown as the purple areas. Once the material is developed, those areas remain while all non-exposed areas get etched away. Copper plating ensues on the open areas; the hardened resin is stripped away and more prepreg material is laminated to create another layer.

With so many individual steps, it should be obvious that careful preparation ond quality control are absolutely essential. The final quality of the board will depend on the individual care on each of the individual steps. Many control steps are implemented to make sure the right processing paramenters are used. UV underexposure will normally result in shorts, while overexposure might be beneficial to insure sharp features it is a costly and time consuming process. Similarly plating baths chemistry has to monitored constantly to avoid contamination or weak adhesion.

Some interesting developments

Samsung Electro-Mechanics developed substrate at 0.08 millimeters is thinner than a regular sheet of paper and can be used to stack up to 20 layers of flash memory and static random access memory chips.

The company said samples are being sent to semiconductor
manufacturers around the world for test. If proven effective, it plans
to begin commercial production later this year.

Useful links
The Future of PCB Design
http://www.mentor.com/products/pcb/expedition/techpubs/mentorpaper_17838...

Matsushita's Optical Waveguide Realizes 10Gbps w/ 1m Circuit
http://techon.nikkeibp.co.jp/english/NEWS_EN/20080118/145670/