How PCBs are made – the basics

The PCB manufacturing is important step towards successful build of electronic equipment. The quality of device, smooth operation may be influenced by printed circuit board – if it is well built, device works as planned, and if it has flaws then it may bring some deviations from normal operation. Anyway there are billions of electronic devices around the globe and all of them use PCBs as basis for circuits. Earlier devices were larger, used lots of through-hole components, but today in modern world electronics is very miniature and dense, thus PCBs become much more complex requiring more and thinner tracks, more layers and should work on high frequencies. So not only PCB designer has tough task, but also a PCB manufacturer should be able to follow the trends.

But despite the requirement change, the whole process of PCB manufacturing remain practically the same.

PCB materials

General purpose PCBs are manufactured using widely known FR4 board material also known as glass-reinforced epoxy laminate material. FR4 is a composite material composed of woven fiberglass cloth with an epoxy resin binder that is flame resistant.  FR4 material is commonly used and of its low cost. IT is manageable to work up to 1GHz frequencies. If you step to higher frequencies, then you may need PTFE material which can be used for up to 10GHz electronics. The main downside is that it is expensive as it is harder to process the boards.

The manufacturing of PCBs

The manufacturing of PCBs normally is based on chemical etching of unwanted copper leaving only tracks on board. Common etchant is ferric chloride. There are also other etchants used depending on manufacturer and technology.

The track pattern on PCB is applied by using photographic process. During this process the copper on bare PCB is covered with thin layer of photo resist. Then it is exposed to light through photographic film containing track image – this way tracks are passed on to photoresist. In following step tracks are developed in special solution where exposed to light photo resist is etched.

The next stage involves etching of copper. The copper covered with photo resist isn’t affected by etchant. This way all unwanted copper is removed leaving only required tracks. Etching process require knowing the concentration of ferric chloride, the temperature and etching time.

Other methods involve different techniques such as milling. Accurate computer guided milling machines remove unwanted copper. Milling takes long time and is suitable only for prototyping or manufacturing of small quantities of PCBs.

Multilayer PCBs

Today even less complex electronic devices use more than two layered PCBs. The reason is same – increasing amount of surface mounted parts and EMI compliance. It is more convenient to trace signal, power lines in different layers leaving more room for components and designers imagination.

Multilayer PCBs are made similarly as single layer, just material is much thinner. Thinner individual boards are then glued together that are interconnected with vias. Manufacturing of multilayer PCBs require accuracy when matching individual boards together. There can be many problems when heat gluing boards together. They can warp. So designing of these boards require some knowledge.

Drilling holes and placing vias

Almost all PCBs have some number of holes and vias. Holes normally are used to mount larger components, fix the board to enclosure or be used as vents. Vias are so called plated holes the carry connection from one side to another or between layers. Drilling is done automatically with drilling machine which reads CAD file with hole size and position information. The price of board may depend on amount of holes and vias, so if possible use less of them.

Plating and solder resist

After holes are drilled, the next operation is plating and applying solder resist layer. Plating ensures better component soldering and connectivity. This process also is known as tinning. The cheaper methods use solder to cover exposed pads, for better results there may be gold used. Solder resist in other hand isolates other areas from solder adhesion. It prevents from solder bridging, and also serves as protective layer. You can choose different colors of solder resist for instance the most common is green.

Silk screen

The last but not less important layer of PCB is silk screen. It carries all information to the board. Here can be marked positions of different components and their values, company logo, board identification, warnings and version. There are several silk-screening methods available:

  • Silk screen-printing can be utilized when the registration tolerance is 5 mil.
  • LPI (Liquid Photo Imaging) provides more accuracy when line widths are greater than 4mil.
  • DLP (Direct Legend Printing) is the most accurate and has the highest cost for consumables.

Optional steps

After PCBs are manufactured they are cut or routed in to individual boards. Also the may be tested for connectivity. If you are planning large production of devices, is advised to build a prototype to test the layout before running all batch of PCBs. This step takes time, but can save tons of money. You never know if you need small adjustment to make, replace some parts or do other minor adjustments.

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