Designing and assembling my first PCB
The beginning
About a couple months ago I purchased an Arduino Nano
ESP32 dev board. I had this sudden itch and I wanted to
play around with hardware. I don't really have much
experience in this space, besides working on firmware
for an IoT company over a decade ago, but I've written
tons of software over the years. I was very surprised how
quickly I was able to get the built-in LEDs to blink
with Arduino IDE and some help from LLMs. After that I
moved on to figure out how to build and flash firmware
directly from the command line without having to deal
with all these custom abstractions. I like operating
from CLI. That was also somewhat easy and gave me
confidence that I can go back to my normal tools (nvim)
for working with code.
The devboard itself does not have much going on. Next
was getting some peripherals so I ordered a small LCD
and BME280 temperature/humidity sensor breakout boards.
Both of these I was able to wire up to the ESP32 chip and
get them to talk over the I2C protocol.
Naturally, we cannot continue assembling pre-existing
modules for a variety of reasons. I think they are great
for prototyping, but I like getting out of the prototyping
phase as soon as possible and getting into a more "release"
or "production" workflow. I was thinking maybe I should
recreate the Arduino board with all these components
hardwired so I can ditch the breadboard. That sounded
great, but it felt a little bit ambitious. There's lots
of components and it would make it hard for me to test
everything. Instead, I decided to create the BME280
sensor module. This would let me get a feel for what it
takes to design something starting with schematics and
ending up with a custom PCB. In the picture above you
can see the small brown board that I got from Amazon,
that's a BME280 sensor board which only has a handful of
components. If everything goes as planned I should be
able to swap-in my custom board and everything should
continue working as before. That was the plan.
Schematic and PCB design
There seem to be several tools available for
schematic/pcb design. I needed something that's free and
runs on Mac OS. Some people praise EasyEDA, others like
KiCad. I didn't do much research on this topic, it seemed
like either of them would fit the bill, but I picked KiCad
since it's free GPL licensed software.
All sensors, chips and components come with what's
called a datasheet. A datasheet is a technical document
made available by the manufacturer describing how the
component functions, at what temperatures it can
operate, reflow (soldering) temperature curves, size and
exact dimensions, example wiring and many other things
depending on the component.
For my sensor module, I needed to wire the BME280 for an I2C interface to make it plug-and-play. The module that I purchased from Amazon actually supports both I2C and SPI. So what I'm doing is not an exact copy, but actually a more narrow implementation. The
BME280 datasheet
has pin-out and connection diagrams
starting on page 38. It actually provides examples for
both SPI and I2C connections. I took the provided I2C
connection diagram and transferred it to KiCad.
I wouldn't call KiCad the most intuitive
application for first-time users. However, I was able to
draw up the exact schematic as was shown on the
datasheet.
To transfer a schematic to a PCB you need to select
what's called a footprint for each component. For example,
there's only one type of BME280 sensor and it really has
only one shape/size (aka footprint) available. That's not
the case for other types of general use components such as
resistors or capacitors. What footprints you pick will
dictate the size of your board, how easy it is to assemble
and most likely many other factors that I'm not aware of
(such as heat dispersion).
Through my research I discovered that most commonly
you're going to encounter SMD and THT components. THT or
through-hole components are more old-school looking tech
(though it's not old), generally larger in size and they
get installed by pushing component legs through the holes
in the PCB and soldering the legs to the board after. This
can be done in most cases with a regular soldering
iron.
SMD stands for surface mounted devices and are what
you'd find in almost all modern electronic
devices. They are much smaller in size, and as the size
gets smaller, they will require more specialized
equipment for installation. When I started looking at
the footprint library in KiCad I got very confused because
it wasn't immediately clear to me whether I needed to find a
footprint for the exact component brand that I was
planning to use or not. Lots of general SMD components
have standardized footprints, I discovered. They follow
standard codes like 1206, 0805, 0603, etc which
translate to dimensions 0.12" by 0.06" or (3.2mm x
1.55mm) for a 1206 component. I went with the 0805 size as this
seemed to be suitable for hand soldering, though it's
pushing the limits.
After you assign a footprint for each component you can then import them into the PCB editor and layout your PCB.
The layout process did not seem too complicated,
however, you need to be aware of how you are routing
the connections, or they could otherwise prevent
other ones from getting to their destination. I was
able to layout everything on the front layer of the
board. The only thing I did special (maybe that is not
so special) was ground filling empty space on the
front and back and then connecting front to back
using via. This seems to be a common pattern
used in the PCB design. Otherwise, it can get
really tricky to route the connections without blocking
other ones, even for a small simple board like the one
I'm making here.
Sourcing components and ordering PCB
The component search was somewhat interesting too. I
purchased my components from DigiKey. Although, it's
probably best to have accounts with several
electronics shops, just in case there's a limited
inventory. I was able to find all components on
DigiKey except for the BME280 sensor itself. The
BME280 sensor was out of stock on several sites and it
looked like it would take several months to get the
backorder processed. I skipped the BME280 and decided to
rip it off from the Amazon module I purchased earlier.
The resistors and capacitors were somewhat easy to
find, I just had to be careful picking the correct
footprint and configuration (resistance etc).
KiCad also lets you generate a bill of materials
(BOM). It's just a list of components and their
configuration and where they need to be placed. The PCB
manufacturers can sometimes perform the assembly for you if you
provide them with the BOM. I did not do that, I wanted to
hand assemble to get a feel for it.
To order a PCB you need to export gerber and drill
files. The gerber files define trace layout and the
drill file is for the CNC machining. I exported both of
these with default settings and packaged them into a zip
file which I then provided to JLCPCB. From there you
finish the order form. I did not make many changes and
used defaults. JLCPCB is a Chinese company, order to door
took about 2-3 weeks and cost me under $10 dollars.
There are faster options, but they would break the bank on
delivery costs.
My tools and assembly
As part of my current homelab, I only have two soldering tools plus a multimeter.
Hakko FX888DX-010BY
I purchased this iron because it lets you control the
temperature and it has pretty good reviews. It heats up
really fast. I usually run my iron at 650F. However,
temperature adjustment is critical so you know exactly
what temperature you're getting so you don't damage the
components.
Quick 861DW
This device is called a hot air station, sometimes
reflow or rework station. It is used heavily in electronics
repair shops to replace components. You also can use it to
solder SMD components. Once you go below 1206, using a
soldering iron gets tricky. The way SMDs get soldered is
by spreading solder paste over connections, placing the
components on top and then microwaving the board using a
hot plate or hot air gun to melt the solder. The hot air gun is the most versatile
tool and it's good for smaller assembly.
I picked up a Quick 861DW because it's considered a pro
entry-level device (according to LLMs at least). The most
important part for a device like this is airflow control.
I run this device on airflow setting 15 (which is very low
volume) and 250C. SMDs are tiny and very light devices,
anything that does not give you good air control will blow
away the components.
The assembly
It took me about 15 minutes to assemble the board. That
involved desoldering the BME280 sensor from the Amazon
board, applying solder paste and laying out the
components and soldering everything up. The only thing I
struggled with was the sensor. The sensor is tiny and it
has 8 connection pads underneath it so I wasn't sure if
I would short any of the connections by accident because
I couldn't see what was going on under the chip. I kept the
area for the sensor clean and made sure there was flux on it
and I left the rest to the surface tension gods.
The results
I was giving this whole thing a 50/50 chance of working
on the first try. I wasn't even sure if I routed
everything correctly on the PCB. The "via" and grounding were a
bit confusing. Then, soldering the sensor was slightly tricky
and I wasn't sure if I shorted connections anywhere. However,
to my surprise, the board I designed and assembled was
plug-and-play on the first try!
I did not have to modify the firmware, I did not have
to put anything in between the board. It was literally a
plug-and-play experience because I exposed the same I2C
interface that I used originally.
This experience gave me confidence that I can produce
custom and working PCBs. This, of course, was a very simple
project, but it took me through the whole process
end-to-end. It gave me a better understanding of the steps I
may need to take for more complex designs. I was
thinking maybe for the next one I should place the LCD, ESP32
and BME280 on a single board, hardwired. This one sounds
a bit more complex. How do you flash the chip? How do you
supply power? What is necessary and what is not? For
example, the Arduino Nano dev board has lots of components
on it. Are all of them needed? I have no idea. I will see
what I'll do next, but I enjoyed this experience.