PCB Fabrication method - Types of PCB fabrication?

 PCB Fabrication method

PCB Fabrication is the process of creating physical PCB from the layout files supplied in the form of Gerber files and drill details files. Gerber is a universally accepted standard for transferring relative coordinates and treat information and component details of a layout for PCB fabrication. Gerber files are a collection of art work.

There are common techniques are adopted for PCB fabrication. 

1. Photo Engraving (PCB Etching)

This is the oldest technique used for PCB fabrication. Even today it is still popular. Photo engraving is based on photography technique. In photography we prepare negative first and develop photography from it. Photo engraving make use of photo masks and then chemical etching. The layout first is converted into a film very similar to the negatives of the photograph. The process of creation of PCB film from the file is termed as photo plotting. The photo plots of each layer is obtained. The tracks and the copper using using areas in the layer are opaque and other portions are transparent in these photo plots. These films are used as "photo masks" for photo engraving.

2. Copper clad Broad 

It is also called as copper clad laminate (CCL). Copper clad laminate to CCL, is a type of base material of PCBs. They can have copper clade on either one side or both sides of reinforcing material after being soaked in resin. Copper clade board are manufactured as adhesive based or adhesive less-based laminate. The adhesive less are manufactured by cast, plate or laminate methods.

Copper clade laminates are manufactured by curing under pressure and temperature layer of cloth or paper with thermo-set resin to form an integral final piece of uniform thickness. Different sizes are selected to manufacture copper clade boards, similarly varying cloth weaves (thread percm), cloth thickness and resin percentage are used to achieve the desired final thickness and dielectric characteristics of reinforcing material. The cloth or fibre material used, and the cloth to resin ratio determine the laminate's type designation. Therefore the characteristics are the level of which the laminate is fire retardant, the dielectric constant, the loss factor, the tensile strength, the shear strength the glass transition temperature (Tg), and the z-axis expansion coefficient (how much the thickness changes with temp). Glass fibre offers the best-dimensional stability. FR-4 is by far the most common material used today. The board stock with unetched copper on it is called "copper clad laminate".

CCLs can be classified into various categories

• Based on mechanical rigidity.

• Based on insulation material and structures.

• Based on thickness.

• Based on reinforcing material types.

• Based on applied insulation resin.

3. PCB Etching

PCB etching is used for the removal of copper to achieve the desired circuit pattern. The most common etching systems are based on alkaline ammonia and cupric chloride. Ferric chloride or ammonia per sulphate chemical bath is used for removing unwanted copper from the substrate. Cupric chloride is the second main stream etchant of choice as it is more compatible with photopolymer resist and is the etchant of choice for precision etching of small features. Ferric chloride solutions are used as etchants for copper, copper alloys, Ni/Fe alloys and steel in electronics photo engraving arts but because of costly disposal of the copper containing etchant due to environmental restrictions, it is used in limited way. The temperature of etchant bath is normally maintained at 45 degree to 50 degree to achieve higher etching rate. Other methods like bubbling is also used.

Technical, economic and environmental needs have brought about major improvement in etching techniques. The batch type operations used to have variable etching rate and more downtime. Hence they are now replaced by continuous, constant etchrate process and entensive automation is brought in it.

4. Photolithography 

Photo plotting method is used to create 'PCB film' from the art files. Here the tracks and copper using areas in the layer are opaque (Black) and other portions are transparent in the film. These PCB film are called "photo mask". A copper clade board is used for photo engraving. A photo sensitive materials called "photoresist" which is resistant to acids and other etching materials is applied to the copper sheet. The photo mask (PCB film for the corresponding circuit in that layer) is kept on the top of the photoresist materials coated copper sheet and it is aligned properly. The photoresist is light sensitive. This arranged sheet is then exposed to strong ultraviolet light (UV). The photosensitive emulsion has the property of becoming insoluble in certain devlopers wherever it is polymerized (hardened) by exposure to UV radiation. Hence the place on the target copper clad board where the PCB film is transparent gets hardened and the opaque regions remain as such. The photo resist is then devloped by washing in a solvent that removes the unhardened part.

Photolithography technique is also employed in the manufacture of integrated circuits.

5. PCB Milling 

It uses a milling machine to remove the unwanted copper from the copper cladded substrate. This gives better PCBs. The engraving and milling processes used for PCB creating are "substrate" processes since copper is removed from the substrate board to create electrical isolation.

6. PCB Printing

This is the latest technology adopted for fabrication of PCB. It is similar to the ordinary printing. The only difference is that instead of ink and tonner, conductive ink is used and papers are replaced by fernible substrate. But it needs special type of printer.