Circuit :  Andy Collinson
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Introduction to PCB Prototyping

After designing any electronic or radio circuit and testing on breadboard, the next step is to create a PCB Prototype. The prototype stage will confirm that your final circuit will work as intended once the components have been soldered in place. In essence, the prototype PCB is a pre-production method of testing.

The prototype PCB layout may be created on any schematic capture and PCB drafting program, such as the Kicad, shown left. The PCB layout on most programs, (including Kicad) can produce an output file in gerber format. This special format includes dimensions and drilling templates and many companies can produce the PCB direct from the gerber file.

At this stage, the PCB layout needs to be accurate, with no drafting mistakes, as PCB's are not correctable. The layout needs to model the circuit, and be fully functional, because when the prototype PCB is created, it will serve as a more basic version of the finished production board.

Developing a PCB Prototype

The steps involved in creating the PCB prototype (for the home constructor) are complex and time consuming. The layout needs to be created on copper clad board, the tracks must mimic the connections on the schematic and the size of each component needs to be known in advance, as do the size of the pads for each component and lead thickness. After the design is laid out, then chemicals are used to remove the excess copper leaving only tracks and pads. Any mistake will result in a non-functioning board. For all these reasons, it is easier to email your design in gerber format to a company to produce your PCB prototype.

After developing a prototype PCB and soldering components in place, you will have a working model of your design. The prototype version is more reliable than a layout made on solderless breadboard and your circuit can be tested to make sure it performs as expected.

Prototyping is the last step before moving to a full production PCB. If you plan to mass produce your PCB, then the prototyping stage should be overseen by an technician or company that has sufficient expertise in the area of PCB manufacturing.

One such company with expertise in this field is PCBGogo. The factory has just been relocated to the Baoan District in Shenzhen, China and has upgraded its production facilities. As shown below left,

Looking as clean as an Operating Theatre, the production facilities include new drilling and milling machines, electroplating machines, reflow ovens and a wave soldering machines. There is also a completely new computerized SMT workshop. The machine, above right is a flying probe test machine capable of checking track connectivity of boards up to 20 layers.

PCB Materials

When your prototype has been drafted, its time to consider the types of PCB materials available:
Normal FR-4 Board This board is the most common type of material used for PCB. It is made from fibreglass and the letters FR4 stand are for fire retardant. This type of board is excellent for all types of layouts.
Aluminium Board An aluminium based PCB is a special board consisting of multiple composite layers, the aluminium layer provides excellent heat dissipation and ideally suited to power supplies or circuits that generate a lot of heat, (more on the section below).
Rogers Rogers PCB boards are a speciality board made from a core material such as (PTFE) teflon. They are less lossy at high frequency and therefore an excellent choice of material to use in any high frequency or fast switching design. (See also section on High Frequency Boards for IoT).
HDI Buried/ Blind Vias The HDI (High Density Interconnect) board is designed for high density construction and the connections between board layers (known as vias) can be made very small or buried between layers. PCBGogo can also create track layers and widths down to 3mil. This type of board is ideal for SMD and micro-controllers where many connections and tracks need to be occupied in small areas.
Copper Base The Copper based PCB is the most expensive PCB, but offers the best thermal conductivity and the best current carrying capacity compared to other materials. The copper base board is ideal for high frequency circuit design, power supplies and any circuit subject to high and low temperature variations. The thermal conductivity is better at heat management than aluminium boards and can withstand thermal ageing. As for current carrying capacity, the copper layer can be created with foils of 35μm~280μm, and has the lowest ohmic resistance. Any multi layer PCB board that has an additional copper substrate layer will also benefit from better heat dissipation, and will have excellent shielding and grounding.
Flexible PCB For most projects the PCB will be housed in a solid enclosure and not be subject to movement. However, what if you're designing a project that is subject to movement like a hand-held tester ? Then a flexible PCB may be the solution. A flexible PCB Flexible PCBs (FPC) are PCBs that can be bent or twisted without damaging the circuit or tracks. This allows the board to bend freely and conform to the desired shape. The choice of material for the substrate is flexible, such as Polyamide, PEEK, or a conductive polyester film.

Normal FR-4 Board PCB

The FR-4 PCB is the most common type of material used in PCB manufacture. The letters FR-4 is a name and also rating of the material. The substrate (inner layer) material is a Fibreglass-Reinforced epoxy-laminated sheet. The letters "FR" also stand for flame retardant, while the suffix "4" is the class of the material. FR-4 PCB's can be made with multiple layers. It is common to see dual sided or two layer boards, but if many interconnections are required extra intermediate layers can be used to route connections from one area of the board to another.
The copper thickness (shown left) refers to the thickness of the copper foil used for tracks and pads. The thickness of the substrate (shown in green) is also a consideration. A dual sided PCB has tow copper layers board separated by the substrate. The whole PCB has then the characteristics of a capacitor. This is not normally something to worry about unless your circuit is working at microwave or ultra high frequencies, where other boards may be a better choice. The solder mask (see right) is the protective layer covering the tracks (but not the pads). The solder mask is available in a number of different colours, red, green, yellow, blue and purple for example. The final layer is a silkscreen layer. This layer is used to identify component designations and can be created in white, black or yellow or omitted, if not required.

Aluminium PCB

If you're a home constructor, then you are limited to the materials available usually, copper clad epoxy or fibre-glass boards. You will not have access to speciality boards containing aluminium or materials designed for high frequency. Aluminium PCB boards are a speciality board, the aluminium base being a prominent feature. The board composition, (shown left) can contain up to 4 layers: a substrate layer (aluminium layer), a dielectric layer (insulating layer), a circuit layer (copper foil layer), and an aluminium base membrane (protective layer).
The benefits of an aluminium base PCB, include better thermal management. As all components generate heat, especially in power supplies and audio amplifiers, or high speed switching for example, the ability to dissipate heat is crucial. Excessive heat can alter component parameters, the performance of the circuit may change and heat is often the cause of premature failure.The efficiency of aluminium based PCB are 10x bettter at dissipating heat than fibre-glass PCB's. This is something you should consider if your prototype is for a power supply or amplifier. Aluminium type board are also available at PCBGogo.

High Frequency Boards for IoT

PCB's intended for high-frequency, high speed switch or IoT (Internet of Things) designed to work at mobile 4G or 5G technology need special consideration. One area of difficulty is the ground plane design. The ground plane will ensure that signal quality is maintained and not degraded, and in addition will also reduce spurious emissions and reduce EMI radiations. One way to ensure this is by adding extra layers to your PCB that can supplement the ground and power rails, (see image).

For all high-frequency PCB designs, a multi-layer PCB is recommended. Multi-layered PCBs allow components to be soldered both sides of the board, resulting in a higher assembly density and reducing the volume of the PCB, making them very suitable for small and compact packages. In addition, the multi-layer boards shorten the connections between components and improve the speed of signal transmission. The image (shown right) is a cross section of a multi layer as used at PCBGogo.

The selection of materials used for multi-layer boards is very important and will affect overall performance. First, the material used for the substrate layers (labelled core in the cross section) will affect dielectric loss at high frequency. A substrate layer with low water absorption is more suitable because this can reduce the effect on dielectric constant (DK) and dielectric loss (Df) at high frequency. Second, the DK and the Df of the substrate must be small and stable, which helps to improve overall quality of signal transmission. In addition, the substrate layers and copper foils (top, ground, power and bottom) must all have the same thermal expansion coefficient; otherwise, excessive heat will result in copper foil separation. The performance of the high frequency PCB prototype, will depend on the material used for substrate. FR-4, a fibreglass material, Rogers and Teflon are some of the most popular materials for high-frequency PCB boards. Finally, high-frequency PCB materials need to be high quality, have heat and chemical resistance, and strong peel-off resistance. All these factors result in a prototype PCB that will be reliable, and meet the demands of the circuit.

Links

Quick-turn PCB Prototype
Turnkey PCB Assembly