How We Assemble PCBs (in 2021)
Ever wonder how our PCB assembly works behind the scenes? Well, in this article, we explain our current PCB assembly process and how we got here!
Hand Soldering Woes
Our Hand Soldering Setup
If you've ever tried your hand at soldering thru hole components on an unpopulated PCB, it might have been a rough first experience. Even though this class of electronic components are easily held in place since the leads enter one side of the PCB and exit the other, if it's your first time, you may have a lot of questions. What temperature should I use? How long do I hold the iron on the component for? What do you mean I made a "cold solder" joint!? For many, the first time soldering is a bit of a learning curve. But after a while, we come to love our thru hole components.
Now enter surface mount components. These only sit on one side of the board and are generally much smaller, meaning much harder to solder by hand. But it's completely possible! In fact, when we released our first board, which was the SNES Anti-Pad Hack Board, I personally soldered every resistor, capacitor, IC, and diode by hand. I was used to doing rework quite a bit when I worked as a Manufacturing Engineer for a power electronics company. So while it was tedious, it was definitely very doable.
A Slight Increase in Production
Player LEDs PCBs in Production
When the Player LEDs PCB was released, I personally soldered each one by hand. Typically, for many boards, going to a 3rd party to get them assembled was my go-to way of getting things done. But with the variety of options for LED colors, it would be a big investment and would have broke the bank. Other small makers were using manual pick and place techniques with solder paste and their own reflow oven. Since I had knowledge of the entire process from a past engineering role, I was completely familiar with everything about it..... but from a fully automated and professional setup.
I will admit, I wasn't convinced that manual assembly was going to be feasible, as I had little experience with it in the past. I have seen other people do it with much success but I dismissed the idea early on. But when I was searching for a way to decrease investment cost for everyone to enjoy these new boards, I bit my lip and gave it shot. Suffice to say, I was wrong, and happy I was!
We now hand solder paste and hand place most of our boards (Shurikens are the exception right now). The process involves a multitude of steps, each with their own challenges, but once those are overcome, the process becomes pretty easy to perform. For the rest of this post, I am going to show this process for a prototype board we are working on now called the Zero KO Board. It's the first time we are doing a board of this complexity so I figure it would make a great example.
Prepare For Solder Pasting
The very first step before placing any components is creating a little jig to hold our boards into place. We need to do this so that we can easily apply solder paste to each PCB.
Top of Zero KO Board
Bottom of Zero KO Board
For the jig, I can either use spare PCBs or right angle acrylic pieces to take shape around the PCB. Then those can be held in place with some masking tape.
Zero KO Board Setup for Pasting
Once secured, a PCB can be swapped in and out pretty easily. Next, the solder paste stencil is required. This will have solder paste applied over it and cover all of the component's pads to be reflowed and soldered to.
Zero KO Solder Paste Stencil
There are a few different ways to secure the stencil into place, but the cheapest way is to apply masking tape once its lined up over the PCB. Typically, you can place a "hinged" setup so its easy to swap boards in and out without losing much of the alignment
Aligned Solder Stencil
Applying the Solder Paste
Next is to squeeze the solder paste though the stencil's open areas. The solder paste we currently use comes in a syringe which is good for small and prototype runs. Simply apply some on the stencil and use a putty knife to apply.
Lead Free Solder Paste
Application of Solder Paste
Smearing of Solder Paste to PCB
Here are the results. There is some bridging on the pads but for other parts, it usually "fixes" it self. For now, we'll leave it be.
Close-up of Solder Paste on PCB
Pick & Place Components
In a large industrial run of boards, the process is fully automated with machines that can easily cost quite a bit. Maybe one day! But for now, the pick and place process is done by hand using nothing more than tweezers or a vacuum pen. Since I have designed the board myself, I have pretty much memorized where everything goes but I do have a cheat sheet for part locations if I forget.
Hand Placement of Crystal
I have to note that this was my first time placing a part with really small pin spacing. Not sure how it will turn out but let's give it a shot.
What's nice about this whole process is that everything on one side gets soldered at once (well at least for the surface mount components). The solder paste needs to be heated with a particular temperature profile. The rate at which the temperature rises and settles is really important to get the desired results. We use a little toaster oven fitted with a PID temperature controller, which just means that it's really good at doing the temperature profiles we want. And since we use lead free solder, which is harder to work with, this is really important!
Board Ready for Reflow Soldering
After going through the oven, everything soldered really well. Though, when I inspected for shorts, there were a few on the IC with the small pin sizing mentioned earlier. Reworking this to clear these shorts is possible but required a certain amount of dexterity, something I will need to gain more proficiency in. In the end, I went through the whole process three times before I got it right. As for the other two failed boards, I will place them in a bin for when I get better tooling and skills so I can rework them.
Solder Shorts on Pins
The bottom side is all ready to go, and usually that's it. We move onto thru hole soldering. But the process must be repeated for two small RGB LEDs on the top side of the PCB. Ran through this process quickly and got a great result.
RGBs LEDs on Top Side
Thru Hole Soldering & Testing
Since this is a proto, I do not have all of the headers on hand so I went ahead and soldered what I could. After that, I fired up the IC programmer and wrote a simple program to control the state of the RGB LED to show that I at least have the ability to program the board and get a desired effect.
So that's it! That is how we are able to build our boards and get them into final inventory (or in this case, prototype testing). In 2022, we do hope to have a manual pick and place machine / jig so that the placement of fine pitched parts is much easier to accomplish. This will get us by and will enable us to create more complicated products like the Double KO, Nostalgia, and Zero KO boards.
Thanks for stopping by!
If you are interested in learning about this process in more detail, here are some resources to get you better read on the subject. Also YouTube has a lot of videos on the subject, just search for terms like "SMT Assembly At Home" or "Assemble Your Own PCBs".