PCB manufacturing is an important step towards the successful build of electronic equipment. The printed circuit board may influence the device’s quality and smooth operation – if it is well built, the device works as planned, and if it has flaws, it may bring some deviations from normal operation. Anyway, billions of electronic devices worldwide, and all of them use PCBs as the basis for circuits. Earlier devices were larger, used lots of through-hole components, but today in the 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, PCB designers have a tough task, but a PCB manufacturer should 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 the chemical etching of unwanted copper, leaving only tracks onboard. The common etchant is ferric chloride. There are also other etchants used depending on manufacturer and technology. You can find more details about making PCBs at gerberlabs.com.
The track pattern on PCB is applied by using a photographic process. During this process, the copper on bare PCB is covered with a thin layer of photoresist. Then it is exposed to light through photographic film containing track images – this way, tracks are passed on to photoresists. In the following step, tracks are developed in a special solution exposed to light photoresist is etched.
The next stage involves the etching of copper. The copper covered with photoresist isn’t affected by the etchant. This way, all unwanted copper is removed, leaving only required tracks. The etching process requires knowing the concentration of ferric chloride, the temperature, and the 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 the same – increasing amount of surface-mounted parts and EMI compliance. It is more convenient to trace signals, power lines in different layers, leaving more room for components and designers imagination.
Multilayer PCBs are made similarly to a single layer; just the material is much thinner. Thinner individual boards are then glued together that are interconnected with vias. The manufacturing of multilayer PCBs requires accuracy when matching individual boards together. There can be many problems when heat gluing boards together. They can warp. So designing these boards requires some knowledge.
Drilling holes and placing vias
Almost all PCBs have some number of holes and vias. Holes normally are used to mounting larger components, fix the board to the 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 a drilling machine which reads CAD file with hole size and position information. The board’s price may depend on the number 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 the 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; there may be gold used for better results. Solder resist, on the other hand, isolates other areas from solder adhesion. It prevents solder bridging and also serves as a protective layer. You can choose different colors of the solder resist. For instance, the most common is green.
Silkscreen
The last but not less important layer of PCB is silkscreen. 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:
- Silkscreen 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 into individual boards. Also, they may be tested for connectivity. If you are planning a large production of devices, it is advised to build a prototype to test the layout before running all PCBs batch. This step takes time but can save tons of money. You never know if you need a small adjustment to make, replace some parts, or make other minor adjustments.
