Making Your Idea into Reality
Going from prototyping circuits on a breadboard, to designing our own printed circuit board (PCB) is like going off training wheels. There is a lot to know about this process, so let’s dive in.
PCB design happens during the Electrical engineering (EE) process. EE creates the “brains” of how your device will work. Without electronics, you basically have just a bunch of metal and plastic in your hands.
Things to know before starting the PCB design:
1. PCB size — this depends on your product size (or enclosure size). Product size is defined during electronic engineering design process. You can see our video about it here.
2. PCB layers — the more layers, the more complex the manufacturing of the PCB will be. (Note: even a single layer PCB can be a complex one, but here we are talking about complexity to manufacture the PCBs. The more layers the PCB has, the more costly it will be to manufacture it).
- 2 layers usually for simple toy products
- 4 layers usually for IoT related products
- 6 to 8 layers usually for Smart phone or smart watch.
3. Your PCB manufacturer requirements. Make sure you read the guidelines about pacing, traces size, power isolation, and file naming before you start designing.
Information you will need to provide to the PCB manufacturer:
- Number of Layers (ex. 2, 4, 6,…etc.)
- Material (FR-2 (phenolic cotton paper), FR-3 (cotton paper and epoxy), FR-4 (woven glass and epoxy…etc.)
- Thickness (0.5 mm, 1.0 mm…etc.)
- Color (Red, Black, Green…etc.)
- Surface Finish (ENIG (Electroless Nickel/Immersion Gold), DIG (Direct Immersion Gold), OSP (Organic Solderability Preservatives…etc.)
- Copper Weight (1 oz (35 um), 2 oz (70 um), 0.5 oz (18 um)…etc.)
- Gerber file
PCB design process:
For this step you need to create a schematic. It is a document, like a blueprint, that describes how components relate to each other and work together. To create a schematic file, you will need a software tool. We like Quadcept, as it is optimized for designing PCBs for manufacturing (for example, you can export your Bill of Materials (BoM) directly from the tool) and, being cloud-based, it can be conveniently used anywhere. (They also have a free Community Version of the tool for makers and students).
There are also many others you can choose from:
After you have the selected tool installed, you need to get component specifications for each of your selected components. They are usually available on your vendors’ websites. The model files will help you to draw the schematic. When you upload the model to the software tool, the component will be available in the database. Then all you need to do is to follow the data sheet to connect the lines to each pin out of the components. (Note: specifics of the design process will depend on the software tool of choice).
Each schematic symbol needs to have an associated PCB footprint that defines the physical dimensions of the components, and placement of the copper pads or through-holes on the PCB. You should already have your components selected (or select them now), and we covered this procedure in the EE design flow video (See video).
A good schematic is really important, it will serve as a reference file when you do debugging and it is a great communications tool with other engineers. Also, manufacturers can test the device by test points on this document.
PCB Layout + Gerber file
To design PCB layout and create a Gerber file you can use the same software tools that we mentioned for circuit design. Unlike the schematic, PCB layout is allocating the actual components to the exact location on the PCB and show the trace to connect each component together between the PCB layers. As mentioned in the beginning of the article, the higher number of layers you have, the more complex manufacturing it will need and it will be more costly.
Divide the PCB into logical sections according to the functionality (e.g. power supply, audio output, etc.). Then make sure to group the components of each section in the same area. This way you can keep conductive traces short and reduce noise and interference.
User interface (UI) is also something you need to keep in mind when designing your PCB. Locations of the components like audio jacks, connectors, LEDs, etc. need to be adjusted for the best user experience possible.
When you finish the layout design, you produce a Gerber file. This file will be used by your PCBA manufacturer. There are many companies out there that provide these services, and from and from HWTrek’s expert pool we recommend Kingbrother, NexPCB and HQPCB.
Component placement on the PCB is very important. Some components might interfere with each other and cause unexpected behaviors. For example, if you have both, Bluetooth and Wi-Fi modules, they have the same 2.4 Ghz bandwidth and can interfere with each other if not placed correctly.
When you send out your Gerber file to the PCB manufacturer they can print out the circuit board. This will be the basis to build up further, to add the components to the PCB and manufacture PCBA (Printed Circuit Board Assembly).
At this point in your EE design, you should already have components selected. You can either ask your PCBA manufacturer to order the required components for you or do it yourself if you have vendors selected. Things to keep in mind:
- Lead time: as these components come from different vendors, keep in mind the lead time. It can be up to 8–16 weeks for some components.
- Packaging: order components in reels for SMT machine automatic pick-up, not in separate packages.
- Minimum Order Quantity: check the MOQ of your component. If you’re buying less than the minimum, make sure the selected components are in-stock. For small quantities (up to 50) you can order online from DigiKey or Mouser. For greater quantities, ask your manufacturer for recommendations.
- Losses: Order 10% more to account for losses (this does not apply to the expensive components)
Mounting the Components on the PCB
There are two main methods for placing components on the PCB surface:
Through-hole (thru-hole) is manual method of fitting components with wire leads into holes on the PCB surface. It is also often called DIP or Dual In-line Package process. (See the SMT in process in this video.)
SMT (Surface Mount Technology) method is the most widely used in mass manufacturing. It is done by fast and precise SMT machines that save you time, money, and avoid human error.
Things to remember:
- Your component type number should not exceed the number of reels the SMT machines of your manufacturer can support.
- Optimize and consolidate your components, so to have just one SMT run.
- Check what footprint pad sizes your manufacturer supports. Otherwise the SMT machine will not be able to mount the components correctly.
- Some bigger components cannot be mounted by the machine and still need manual thru-hole work. Thus, both of these technologies can be used on the same board.
- Any components that you will need to add manually with thru-hole method, will add to the manufacturing cost.
Reflow soldering is a process that makes the components “stick” to the PCB. The PCBA goes through a reflow oven or an infrared lamp that heats up the board until the solder melts, permanently connecting the components to the board.
The tricky part here is not overheating or damaging the components, as each package has a different thermal profile. A reliable PCBA manufacturer will take care of this process and all you need is to provide the component specifications to them.
Other soldering methods:
- Wave soldering is mostly used for the components added manually using the thru-hole method. In these cases, your PCBA will first go through reflow oven and then after adding other components manually, it will go through a wave soldering machine.
- Iron soldering can be used in specific cases, but not usually in mass manufacturing.
Testing & QA
In this step, a sample of PCBAs will be tested to ensure quality. Common mistakes are: components that are not connected, misaligned components and shorts that connect portions of the circuit that should not be connected. Most common tests:
- ICT (In-circuit Test). When you design the PCB, you often will reserve some test points for debugging, programming, and other purposes. The ICT machine will use these test points to do the open/short test and will check if the values of the passive components (resistor, inductors, capacitors) are in within specifications.
- AOI (Automatic Optical Inspection). The manufacturers use “golden sample” — a reference PCBA to compare with others. For this test, the hardware creators will need to provide the specifications and tolerance to the manufacturer to set the parameters.
- X-ray. The PCBA manufacturers will use X-ray to check the soldering conditions for BGA (Ball Grid Array) components. See X-ray testing in action in this video.
- Watch: Hardware & Coffee Ep. 2: PCB Design & Manufacturing [VIDEO]
- PCB basics from SparkFun explains each part of the PCB and all the terminology
- EEVblog PCB Design For Manufacture Tutorial — Part 1, Part 2 and Part 3
- Circuit Basics How to Make a Custom PCB
Originally published at blog.hwtrek.com on July 5, 2016.