How to order: PCB fabrication and SMT assembly
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This repository contains all the files needed for fabrication and assembly of b-parasites. This is usually done by submitting an order to a company that can fabricate the printed circuit board (PCB) and also mount most of the components like resistors, capacitors and oscillator (a process called SMT - surface mount technology - assembly). This page describes what files are needed, what are their purposes and where to find them.
These two steps, PCB fabrication and SMT assembly, are described below.
In this step, we provide the fabrication company with files that describe the physical properties of our circuit board. This information is stored in different files (each one representing a "layer"). The information in these layers include:
- Outline or shape of the board
- Copper traces
- Silkscreen (for example the "Open Hardware" logo and components labels like R1 or C2)
- Mounting holes, if any Usually, Gerber files (and drill files) are for storing this information. Most fabrication companies accept a ZIP file containing a set of Gerber files, each describing one of these physical properties of the PCB.
In this repo, you will find the Gerber files in the kicad/gerber directory. You can ZIP those files and upload them to the fabrication company of your choice.
Each fabrication company has their own fabrication capabilities. These are a set of constraints for our designs, such as how narrow the copper traces can be, or how close to one another they can be placed. For example:
The Gerber files in the project are known to be compatible with JLCPCB, but are also likely compatible with other fabrication companies.
If you have ordered b-parasites from other companies, please let me know and I'll mention it here.
b-parasite uses tiny SMT (surface mount technology) components such as 0402-sized resistors and capacitors. While these could technically be mounted and soldered by hand or with a home toaster oven, the job requires a lot of skill and patience. Luckily for us, some fabrication companies let us relatively cheaply offload this job to robots known as pick-and-place machines.
For fabrication companies to be able to pick-and-place and solder our components for us, we need to be very specific about which components we want and where we want them, relative to our board physical layout.
Once again, we usually specify this information via two files, which we need to submit with our order:
This is a CSV file that maps components labels in our design (such as R1 or C2) to company-specific part numbers. For example, in this file we say that the resistor R1 is the part with reference 123ABC on a certain company's stock.
The kicad/parasite.csv file contains the BOM for JLCPCB's SMT assembly service with JLCPCB-specific part numbers, but it should be adaptable for other fabrication companies.
This file tells the fabrication company where to put each components while placing them on board. As with the BOM file above, this is unfortunately company-specific. The kicad/parasite-top-pos.csv file contains the position of each component as required by the JLCPCB's assembly service. Again, this should be easily adaptable for other fabrication companies.
Sometimes engineers from JLCPCB will get in touch to confirm or suggest changes to the orientation of components (see this comment and issue #42).
Here are some sanity checks:
Q4's collector (+) should be aligned with the footprint dot. For example, in version 1.2.0:
With the exception of the nRF52 Bluetooth module, all SMT parts in b-parasite are relatively common and fabrication companies should have a large number of them in stock. The nRF52 module (E73-2G4M08S1C), on the other hand, is currently a little elusive, likely due to the chip shortage we're currently going through.
The solution I have personally used is sourcing the nRF52 module on aliexpress and soldering it by hand. While not trivial, this is much easier than soldering the tiny resistors and capacitors. In this case, I used the SMT assembly service for all other components, and just mounted the nRF52 module myself.
Most fabrication companies usually offer SMT assembly only on a single side of the board. Since the CR2032 battery clip is mounted on the bottom side of the board, we need to solder it manually. Luckily, the clip has large pads, which makes soldering it by hand less challenging than the other, smaller components.
I have had success with the following CR2032 battery clips:
- MYOUNG MY-2032-05 on LCSC. This fits perfectly in the b-parasite. Prefer this one if possible.
- Q&J C70373 on LCSC. This one doesn't fit in as nicely but also works.