Inside PCB factory: how a printed circuit is made

The PCB manufacturing process has a number of variations but, despite many small variations, the main stages of PCB manufacturing are nearly identical. In this article we will find out what are the steps of a standard manufacturing process in the electronic industries and how a printed circuit board is made.

Control and review of gerber files

When a circuit design is approved for production, designers export it in the format supported by their manufacturers. The most used software is called Extended Gerber . The Gerber file contains information on electrical connections such as tracks, vias, and pads , directions for drilling and milling the printed circuit board. Almost all PCB factories run a process called Design for Manufacture (DFM), in which an experienced team of engineers performs checks to verify the feasibility of the design.

Substrate preparation

Typically, PCB substrates can be made from a variety of materials. The commonly used substrate is a fiberglass sheet known as FR4. In order to create a circuit with the tracks for the components, the plate is coated with copper on both sides. Through an automated process the sheets are cut and perforated according to the customer's project.

The drilling process (perforation) is carried out using numerically controlled machines and the holes can have a diameter smaller than the diameter of a hair. In fact, there are machines capable of making holes with a diameter of 100 microns and the diameter of a hair measures on average 150 microns. After perforation, the cards are immersed in tanks containing a liquid with copper salts to carry out the Plated Trough Holes process . in this process a thin layer of copper is plated on the walls of the holes to allow conductivity between two or more layers of copper.

How a printed circuit is made: photoengraving process

To obtain the desired circuit, a photoengraving process is used. A chemical called a photoresist is applied to the copper panels inside a special room with yellow light. This substance has the property of being easily removed from caustic soda in case it is exposed to UV rays; instead it is very resistant to corrosion if not exposed. The copper panels containing the photoresist thin film are then exposed to UV light through a mask that defines the required traces. In this way, the image of the tracks is transferred to the thin film. Then, through the treatment with caustic soda, the development is carried out which allows the removal of the photoresist in the desired areas.

Copper not protected by the photoresist will be removed during the etching step. This phase consists of a treatment with ferric chloride, in order to obtain the engraving of the areas where no track is needed . Finally, a chemical action called stripping removes the photoresist film left over the tracks.

Optical inspection and manufacture of the welding mask

In this phase the panels are cleaned and subjected to AOI ( Automated optical inspection ). A machine performs an optical inspection of the panels to confirm the total absence of defects. Specifically, a camera scans the circuits and makes the comparison between the digital image and the Gerber file. The panel that passes the inspection passes to the phase of making the solder mask .

A photosensitive paint is deposited over the entire surface of the circuit, creating a uniform coverage. Also for the realization of the solder mask layer a photographic process is used in which all the parts that must be protected are polymerized. A chemical attack, on the other hand, removes uncured parts, such as solder pads. The solder mask layer is a particular resin that acts as a mask and is applied with the aim of delimiting the areas to be welded , protecting the tracks, improving the electrical insulation and reducing the possibility of short circuits.

How a printed circuit is made: silk-screen printing and profiling

Almost complete panels are subjected to an inkjet printing process for the realization of screen printing. Screen printing is usually applied to the side of the components and is used to indicate all essential information relating to the printed circuit board. The panels finally move on to the last phase of coating and curing.

Generally, a panel consists of a certain number of cards, which must be able to be extracted from the original panel. The method used is V-Scoring and consists of leaving small tabs along the edges of each PCB, or V-shaped grooves on both sides of the board so that it can be easily extracted from the panel.

Electrical testing and shipping

All the cards produced are subjected to track connectivity tests. An automated procedure confirms the functionality of the PCB and its conformity with the original design. The methods used are the Test Fixture and the Flying Probe Testing . The first uses an equipment ( fixture ) on which a series of needles that make contact with the test points of the circuit are suitably positioned. The second method, on the other hand, consists in moving one or more pairs of probes to check the electrical connections of each single track of the printed circuits . Finally, a visual inspection of the cards ensures that everything is in order before moving on to the packing and shipping stages.

How a printed circuit is made: assembly process

For all customers who want it, the printed circuit industries also offer a component assembly service. The main assembly stages are:

• the drafting of the solder paste
• the placement of components
• welding

Spreading the solder paste: to position the components on the board, it is necessary to spread solder paste over all those areas that represent the component pads. The drafting takes place through the use of a stencil , that is a stainless steel mask made with the laser cutting technique.

Positioning of the components : during this phase, the board is inserted inside a Pick & Place, that is a machine that takes the components from the reels (tape loaders for SMD devices) and places them on the board. The components positioned on the board are held in their position until the next step thanks to the surface tension of the solder paste.

Soldering Process: When all components are placed on the board, it is transferred into a reflow oven suitable for SMD component soldering processes. Furnaces for PCB assembly represent a significant renewal compared to manual soldering as, thanks to them, a high yield of circuits and compact soldering is obtained.

After completing the soldering process, the boards are subjected to a second automatic optical inspection (AOI). In this phase the automatic machines equipped with cameras are able to inspect the boards to detect poor soldering , the absence of components and in some cases the wrong component.

According to the technology to be developed, the production process could contain other steps aimed at optimizing the yield, limiting the presence of defects and producing circuits of the highest quality.

Curated by Antonino Pagano

The article Inside PCB factory: how a printed circuit is made comes from Tech CuE .