PCB Design Tutorial - Section 1 - How a PCB is made
At Black Stick we think it's important to know exactly what a PCB is and how it is constructed before undertaking design. So let's begin:
The PCB or Printed Circuit Board.
We're not going to delve into the history of PCBs and how they came to be. There is plenty of material online that you can dig up if this is of interest to you. Instead we will try to make this as straight forward as possible.
The first type of PCB we will look at will be a double sided PCB. So called because there are 2 layers of thin copper sheets separated by some “glass and glue” which is called an epoxy resin fibre glass substrate or FR4 (see Image 1 below).
Image 1 – copper clad FR4
This material can come as a “Core Construction” (as in image 1 above), which means the copper is already stuck on to the FR4. Or it can be built up from a sheet of “epoxy resin” which then has copper bonded to it. This is called Build Up technology (Image 2).
Image 2 – Copper separate from FR4
The “Copper weight” is how you describe the thickness you want to use. It is defined as the weight of copper evenly distributed over 1 sq. ft. (1 square foot).
0.5 oz (ounce) copper will give a thickness of 17.5 μm (microns) or 0.7 thou (thousandths of an inch).
1.0 oz (ounce) copper will give a thickness of 35 μm (microns) or 1.4 thou (thousandths of an inch).
2.0 oz (ounce) copper will give a thickness of 70 μm (microns) or 2.8 thou (thousandths of an inch).
There are of course many types of PCB construction, but for this tutorial we will concentrate on the FR4 construction.
PCBs can be built as single layer boards with copper on one side, up to 32 layers and above, and the materials range from paper, glass, ceramic and metal, to name just a few. There is a vast amount of information online on this subject.
So what happens to the PCB now we have a blank one.
We use a drill file (which is a text file that is generated once we complete our PCB design), to tell us where there needs to be holes drilled on the PCB.
One of the things you should take note of at this stage of is that there are no connections between the top layer of copper and the bottom. Although we drilled holes, there is no copper in these holes to connect the two sides. These are what we would call un-plated or unsupported holes (Image 3)
Image 3 – Drilled holes
Next we have to apply molten copper to the PCB so that it floods down the holes and there is a connection on all of the holes between the two layers. This is called “Plating” (Image 4).
Image 4 – Copper plating down the barrels of the holes
Next we spray a photo sensitive coating to the whole PCB. This coating will be used to protect the copper that we want to keep on the board.
We then place a photographic transparency (it can also be called an Acetate, or artwork pattern and is normally a negative) of our PCB layers over this coating and shine a UV light onto this (image 5).
This will harden the photo sensitive coating at the exposed areas.
Image 5 - UV Protective Covering Areas to be hardened are exposed
The photographic transparency is removed and the PCB is then washed (with an alkeline developer). The unexposed photo sensitive coating is then cleaned away, leaving the copper that we wish to remove exposed. The copper we wish to keep still has a cured photo sensitive covering protecting it.
Now the PCB is etched with a solution (normally Ferric Chloride) that reacts with the copper and removes it from the PCB leaving the areas that were coated with the hardened photo sensitive resist untouched.
We now wash off the remaining hardened photo sensitive coating and we are left our etched PCB.
Image 6 - All unwanted copper is etched away
We need to protect the copper now, as copper reacts with air and oxidises easily. First we add what is called a “solder mask coating”. This will cover all of the PCB in Solder Mask, but we need to remove certain areas that we want exposed e.g. components pads (these are areas where components will attach to the PCB).
Similar to the copper etching process, we place a photographic transparency (normally a negative) over the solder mask and with UV light we harden the areas we want to keep.
The PCB is then washed again and we are left with the exposed areas that we need to solder components to (Image 7).
Image 7 - Solder Mask coating
As these exposed areas are still copper, we need to protect these areas from oxidisation and still be able to solder to them. We do this by adding a surface finish. This is normally a metallic coating like silver, gold, nickel gold, or HASL (Hot Air Solder Level, which is a means of adding solder to the exposed copper). There are also non metallic coatings available called OSP's (Organic Solderability Preservatives).
Don't get too caught up in the differences just now as we will cover the strengths and weaknesses at another point.
The PCB is now placed in a bath of the chosen finish and the remaining exposed copper is coated in this material (Image 8).
Image 8 Solder Finish
We can now apply our silk screen, which is a non conductive visible ink showing the outlines and reference designators (numbering) of the components on the PCB (Image 9). This can be done by screen printing (similar to T-shirt printing), or by direct printing (similar to ink jet printing) which is becoming more common.
Image 9 Silk Screen
The silk screen helps during the assembly, test, inspection and repair processes, by showing components identification numbers (reference designators or REFDES for short), pin numbering, and orientation of components.
We now have a finished PCB, ready for soldering components to.
In the next section of the Black Stick PCB design tutorial we will be teaching you the basics of libraries, their structure and component footprint and schematic symbol creation.
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