Zaunkoenig M1K PCB

January 2020

The printed circuit board (PCB) of a gaming mouse is what makes a gaming mouse tick. You can compare it to the motherboard of your gaming PC. Our goal for the PCB of the Zaunkoenig M1K was to make the lightest gaming mouse PCB ever, because when you want to make the worlds lightest gaming mouse you have to look at every single component of the mouse.

Overview of the Zaunkoenig M1K PCB

Before getting into the technical details let us first take a look at what we are working with. The following picture shows six completely soldered M1K PCBs:

This picture shows the Zaunkoenig M1K PCBs with serial numbers 228 up to 333.

When you want to reduce the weight of a gaming mouse PCB you have two options. The first option is to make the PCB itself lighter. The second option is to choose lighter components (the parts that get soldered onto the PCB). Lets start by looking at the PCB itself.

Making the bare M1K PCB light

A bare PCB is a PCB without anything soldered onto it. It is common during PCB manufacturing to connect several of these bare PCBs with each other. The result is called a panel. The following picture shows a panel consisting of 16 bare M1K PCBs:

This picture shows a Zaunkoenig M1K PCB panel.

PCBs consist of a material called FR4. The FR stands for «flame retardant» and the 4 stands for glass fiber reinforced plastic. Fun fact: a PCB made from FR2 uses paper instead of glass fiber reinforced plastic. The M1K PCB is a two layer PCB meaning that it has circuitry on top top as well as the bottom side. See this «Strange Parts» episode if you want to see how the raw material for a two layer PCB looks like.

The specific gravity of FR4 is relatively high: it is at 1.85 g/cm³. That is almost twice as much as the density of Acrylonitrile butadiene styrene (ABS), the plastic most gaming mice are made from. Hence it is a good idea to make the bare PCB of a gaming mouse as small as possible: with its high density, even small amounts of FR4 increase overall mouse weight significantly.

When you are trying to make a PCB lighter you can either reduce the footprint, or the thickness. Lets start by looking at the thickness.

PCB thickness

While many gaming mouse PCBs these days have a PCB thickness of 1.0 or even just 0.8 millimeters, we chose a thickness of 1.55 millimeters, for that extra bit of stiffness. With thin PCBs you can run into problems during soldering. A thin PCB could also negatively impact click quality. We might experiment with thinner PCBs in the future though.

So far, so bad: the M1K PCB is thicker than the PCBs of most current gaming mice. That leaves us with saving weight through making a PCB that is less long and wide.

PCB footprint

The M1K PCB is just 46.0 millimeters long and 35.12 millimeters wide, making other gaming mouse PCBs look like giants; this tiny size is one of the largest factors in the lightness of the M1K PCB. There are several tactics we used to make the M1K PCB so small.

We saved a lot of PCB space by omitting every component that we did not deem essential for a gaming mouse. The M1K does not have a wheel, side buttons, sniper buttons or RGB LED. Components bring their own weight (0.6 grams for the sensor of the M1K for example, the PixArt 3360), but also need real estate on the PCB for both placement as well as traces leading to the component. Superfluous PCB components thus increase the total mouse weight twofold. This is why it is so important to keep a gaming mouse PCB as lean as possible.

Consider the following example: The switch we use in the M1K – the Omron D2F-01F – weighs in at 0.62 grams; however, the switch also takes up space on the PCB and consequently carries an additional weight penalty. Each Omron D2F-01F requires 12.8 by 5.8 millimeters of space on the PCB. Multiplied with the PCB thickness of 1.55 millimeters as well as the FR4 density of 1.85 the PCB footprint required for one single D2F-01F thus weighs 0.21 grams. The Omron D2F-01F will increase the PCB weight thus not only by 0.62 grams, but by 0.83 grams. If you count the extra solder, the extra traces and the the fact that around every PCB component there needs to be a little bit of empty space, one additional Omron D2F-01F will increase the overall PCB weight by almost 1.0 grams.

Even very small components like capacitors and resistors play a role in minimizing the PCB footprint. Take a look at two of the capacitor types we have on the M1K PCB:

This picture shows two types of capacitors (1608 as well as 1005) that are on the Zaunkoenig M1K PCB.

The three little things directly below the PixArt 3360 are capacitors with a footprint of 1.0 by 0.5 millimeters (metric code: 1005). The slightly larger thing to the left of these three is a capacitor with a footprint of 1.6 by 0.8 millimeters (metric code: 1608). Obviously these little things do not weigh much by themselves: one 1005 capacitor weighs in at 0.0016 grams and one 1608 capacitor weighs in at 0.0063 grams. Together, all 14 capacitors on the M1K PCB weigh in at 0.0412 grams; not even one tenth of a gram.

However when you look at the amount of space they take up on the PCB the calculation changes. All 14 capacitors occupy PCB space that weighs in at 0.39 grams: nine times as much as the capacitors itself weigh.

Another way to achieve a smaller PCB is to use smaller traces and vias. You can compare traces to streets: traces connect components like for example those four capacitors and the 3360 in the above picture: if you look closely you can see little lines between the 3360 and the four capacitors.

Vias can be compared to tunnels: they connect the top side of the PCB with the bottom side. In the following picture you can see around 40 of these vias. They look almost like little sandlion pits. Also in this picture is the MCU of the M1K, the Microchip ATmega32U2 (the square grey object in the middle):

This picture shows the MCU of the Zaunkoenig M1K PCB, the Microchip ATmega32U2, surrounded by traces and vias.

The traces on the M1K PCB have a thickness of 0.15 millimeters, and the vias have a diameter of 0.3 millimeters. That may not sound like much, but the quantity of traces and vias on the PCB adds up. As mentioned the M1K PCB is a two layer PCB, meaning that it has traces on the top as well as the bottom side. If all the traces were on the top side, the PCB would need to be bigger.

Choosing lightweight M1K PCB components

We were very limited in this regard. Take the sensor for example: a smaller sensor than the PixArt 3360 would weigh less, but it would have also have worse performance. Same goes for the Omron D2F-01F: sure, there are smaller switches, but the worse click performance and experience of smaller switches precludes their use. The Omron D2F-01F is big and heavy, but its worth it:

Various other PCB parameters

The most common way to separate PCBs is to use something that is more or less an industrial pizza cutter: very cheap, very stressful for the PCB, and this process results in very rough and imprecise edges. By contrast, all of the edges of the M1K PCB are precision cut on a CNC machine. Separating PCBs via CNC machine results in less mechanical stress for the PCBs, which in turn enables you to place components closer to the edge of the PCB. Thus, if you separate PCBs via a CNC machine your PCBs will be ever so slightly smaller and thus lighter. You can easily see how smooth CNC cut PCB edges look:

This picture shows the Zaunkoenig M1K PCB number 333.

Another thing that is obvious when looking at the M1K PCB is that it uses a gold surface finish. We chose gold for its superior planarity which improves solderability for even the smallest of PCB components. The standard surface finish for gaming mice PCB is tin, which has a less planar surface than a gold surface finish. A gold surface finish does cost a few cents more per PCB, but it is worth it.

As a side note, the color of the overall PCB is determined by the color of the solder resist. We went with black. Due to historic reasons green is the most common and thus cheapest solder resist color. Hence most PCBs will look green. Note that a black PCB has no performance benefit over a green PCB – it just looks better.

Each M1K PCB will have a serial number at the bottom left corner, burnt into the surface via a carbon dioxide laser. Serial numbers started at 001.

Made with modding in mind

The first prototype PCBs we used were PCBs out of competitors mice, that we heavily modified. From this experience we know how slow and painful it can be to mod gaming mouse PCBs. Hence, when we made our own PCB we decided to make it modding-friendly.

For this purpose there are five h4x0r pins under each switch: This picture shows the h4x0r pins of the Zaunkoenig M1K. You can use them to connect additional switches, for example.

See the following two pictures for information on how these h4x0r pins are connected to the MCU. This is how the five h4x0r pins left to the M1K MCU are wired up:

This picture shows the five h4ax0r pins on the right side of the Zaunkoenig M1K PCB. You can use these h4ax0r pins to connect additional switches, for example.

And this is how the five h4x0r pins to the right side of the M1K MCU are wired up:

This picture shows the five h4ax0r pins on the left side of the Zaunkoenig M1K PCB. You can use these h4ax0r pins to connect additional switches, for example.

You can use the h4x0r pins to wire up additional switches, or talk to other devices and peripherals, for example.

To make modding even easier we made our firmware open-source.

So, what kind of mods can you do with the Zaunkoenig M1K PCB? How about a capacitive scroll wheel with a haptic motor added for good measure?

This is how the above M1K looks on the inside:

This picture shows a heavily modded Zaunkoenig M1K PCB.

Closing remarks

To the untrained eye an M1K PCB can look like a monolithic object. In reality however it consists of lots of small parts, traces and vias. And each and everyone of these small parts, traces and vias plays a role when you are trying to minimize the overall weight of the PCB.

In case you have not read enough about gaming mouse PCBs today the following summarizes the core attributes of the M1K PCB.