In this article, I demonstrate my at-home printed circuit board (PCB) assembly process. Many hobbyists and professionals shy away from soldering surface-mount components without the aid of machine assembly equipment. Though it’s possible to hand-solder these tiny components, it’s not necessary, as better assembly techniques exist. My process produces results that almost match the quality of machine placement.
Breadboarding: it’s something many of us have dealt with in prototyping. Placing and routing all those wire jumpers, and trying to get our circuit idea to function as we think it should, can be a painstaking process. The trouble is that when we want to convert our design over into something smaller, we need to move over to surface-mount components. Worse yet, perhaps your design requires a certain component that only comes in a surface-mountable package (like the ESP32-S3 I use so often for my designs). Sure, you can use a breakout board, but you still need to hand-solder it to convert it to a through-hole part. For many, hand-soldering surface-mount components may be too difficult to learn.
Luckily, assembling surface-mount components is not as hard as you may think. In this article, I’ll discuss the process that I have successfully used for hundreds of boards, both in prototyping and production.
Why surface-mount?: There are many reasons to go for surface-mount components. The most convincing ones are related to board area and part availability. To achieve a small and dense board, you have no choice but to use surface-mount components. I’m not saying there are no dense through-hole designs in the wild, but in my opinion, they’re much more difficult to design than a surface-mount board. Using surface-mount resistors, capacitors, ICs—and so on—will buy you lots of printed circuit board (PCB) real estate. Should you also require a specific component (say, the newest accelerometer or high-power MOSFET), it may only come in a surface-mount package. For situations like these, it’s good to know how to deal with what is available to you.
I save quite a bit of money assembling the board myself, as contract manufacturers can get expensive. If my design is in the prototype stage, I prefer to skip breadboarding altogether and jump right into a PCB design. Assembling my own prototype PCB assembly brings me closer to the final design faster, and I can wait until the design is ready before opening my wallet for someone else to assemble the boards in larger quantities.
The PCB Assembly Process
Assembling an unpopulated PCB has a distinct series of steps that remain basically the same whether done with a machine or by hand; the difference is the equipment used and how the assembly is checked for quality. For example, you probably don’t have an automated optical inspection machine lying around your workshop to check that all the solder fillets for each component look as expected. Nor do I expect you have a flying probe tester to check that the correct parts are placed within the correct locations. For tasks like these, your eyes do the checking.
Here are the general steps for PCB assembly in general (for parts on one side):
- Apply the solder paste to the PCB.
- Place the components into their specific locations.
- Reflow the solder assembly.
- Hand-solder the through-hole components.
- Rework the surface-mount components, if necessary.
- Clean the PCB assembly.
- Perform a final inspection.
- Do some final testing.
This is the back side of the PCB, where I will mount all my components.
You can choose to inspect your work after each step as well for increased quality control, but once you get the hang of it, you likely won’t need to perform an inspection until the end. For the remainder of this article, I use a PCB assembly I designed and sell for my small business. Figure 1 is the back side of this assembly, and Figure 2 is the front. I’ve hand-assembled about 200 boards so far, and the amount of required rework drops considerably with every batch. This is because as you assemble more units, you get a better feel of how to place the components and how to properly apply solder paste to a PCB. You may also start to realize which components give you trouble—at which point you can re-spin the board for a better design for manufacturing.
The top side of the PCB is for my customer, as it is their interface.
Step 1—Solder paste: This is arguably the most crucial step. Improperly applied solder paste dramatically increases the amount of rework needed, especially for tight-pitched ICs. Solder paste is a mixture of solder and flux made to feel almost like putty or spackle. When it gets heated to a particular temperature (and, ideally, heated at a particular rate), the paste transitions into solder for a solder connection. It’s worth noting that solder paste can expire, and it needs to be refrigerated to have a dependable shelf-life. I store mine in a mini fridge.
There are a few ways to apply solder paste to the pads of each component. For a board with just a few, large-padded components, you can apply solder paste to each pad with a syringe. But in general, I suggest using a stencil. Stencils can be purchased along with your PCB, provided the Gerber files you sent to the fabricator include the paste layers. PCBWay offers this, for example. If you have a design without a stencil and wish to buy only a stencil, there are companies like OSH Stencils that provide this service.
Once you have a stencil, you need to align the aperture for each pad with the PCB. Two problems often arise. The first is how to keep the PCB from sliding around. The second is how to keep the stencil from sliding around. The good news is that the solution to both problems is good old masking tape. To hold my PCB in place, I create a simple frame for it using older PCBs of the same thickness that are taped to a flat surface (Figure 3 and Figure 4). With the PCB which is under assembly placed in the center of this frame, I align my stencil on top and keep it in place with a piece of masking tape on one side (Figure 5). This way I can easily lift the stencil to remove the pasted PCB and insert the next one for pasting. If done properly, you won’t need to re-align the stencil for the next paste application.
I create a frame to hold the PCB being pasted with these spare PCBs. Masking tape keeps everything in place.
Here the PCB is placed within the pasting jig. Using spare PCBs helps create a plane of even thickness, making the stencil process easier.
The stencil is aligned over the PCB to be pasted. Masking tape on the top makes a good hinge to perform pastes on more than one PCB.
It’s important to note that before applying the solder paste, the PCB surface should be cleaned with a solvent like isopropyl alcohol. To apply the paste, I typically use a solder paste squeegee, but a putty knife or old credit card does the job as well (Figure 6).
When you’re ready to apply the solder paste, place a bit of it on the squeegee (Figure 7). Start from the top of the stencil and spread it downward with a consistent pressure and with the squeegee at a 45-degree angle to the surface (Figure 8). Once all the pads are pasted, you can clean excess paste from the surface by doing this same motion with the squeegee, but now perpendicular to the stencil. I use a lint-free disposable wire to clean the apertures for the tightly pitched components to decrease solder paste bridging, which can cause solder shorts after reflowing. I also tend to re-shape the solder paste on my squeegee after each paste to keep the spread and application consistent.
Clockwise from the top left are my lead-free solder paste, solder paste squeegee, and putty knife. An old credit card works if you don’t have a putty knife. The solder paste came from Jensen Tools (Kester 70-3213-0810), the squeegee from PCBWAY, and the putty knife from Lowes.
Solder paste is applied to the squeegee. This is more paste than technically required, but this is necessary to maintain an even application over the pads.
This is how I apply the solder paste onto the PCB. Notice the downward sweeping motion, with the putty knife at 45 degrees to the plane of the PCB.
When you’re done with a paste, lift the stencil gently to prevent smudging (Figure 9). Figure 10 is an example of a well-pasted PCB, and Figure 11 and Figure 12 provide microscopic close-ups of the board after pasting.
I clean any excess paste with the second motion as described, and gently lift the stencil to prevent smudging.
At first glance, the pasting looks well done.
To double-check the pasting, I use a USB microscope to perform a close-up inspection of the tightest-pitched component. The pads look well pasted for this micro-USB connector.
I check to see how the pads for the ESP32-S3 module look. They also appear to be well-pasted, with no evidence of solder paste bridging.
Step 2—Place components: This is probably the easiest step (except for very tightly pitched components). Simply place your components where they belong. Start with the largest parts first and work your way down in size (Figure 13). Large parts have more surface area, so the surface tension from the solder paste typically keeps them from moving around. Small parts have less surface tension and thus can easily get pushed around if the board is accidentally moved during the placement process.
Use vacuum tweezers or “regular” tweezers to pick up components. I prefer regular tweezers (usually with a curved tip) as I never had much luck with vacuum tweezers. The placement process only gets difficult once you are using ICs with tightly pitched pins that are hard to pick up and align. For one or two boards, it’s doable with a steady hand and patience (Figure 14). But if you are doing small runs of board assemblies, there are manual pick-and-place machines that do the job well. I personally use the PSE-20-AV model, sold by PickSoEasy.com. That has worked well for me.
Here I am placing the second component. I work my way from the largest component to the smallest.
All the parts were hand-placed. Some spots are empty as they are not required in my design.
Step 3—Reflow solder: Here is where it can get tricky. As mentioned previously, solder paste needs to be heated at a specific rate. More accurately, it has a specific temperature profile, and must be heated over a series of intervals in which the temperature is increased at specific rates. Figure 15 is a typical temperature profile for a lead-free solder paste.
This is a typical temperature profile for lead-free solder paste. The specs here are specific to the Kester paste that I use.
Since I sell the boards I assemble, I prefer to use a closed-loop, PID-controlled toaster oven to achieve this profile (Figure 16). The unit I use is sold by Whizoo.com. I purchased an oven ready-to-go (the most costly route), but there are cheaper ways to do this.
I will typically place more than one PCB in the reflow oven when doing a batch.
One common method is to instead use a small benchtop heat gun used for soldering repair and rework. These can cost around $50. Even though the paste needs a specific solder profile, you can still get the paste to transition to solder with a bit of practice with how much to heat to manually apply with the heat gun. Note that this method blows hot air onto the paste, and too high of a fan speed can blow smaller components right off the board. Extra care is required if reflowing in this manner. Sometimes I prefer this method over my reflow oven if the board is a one-off with only a few components. I’ve also seen people use hot plates for reflow soldering, with results that appear comparable to those from a reflow oven.
Step 4—Hand-solder through-hole components: Once the surface-mount components are all soldered, we can move on to the through-hole components. After the reflow process, it’s usually a good idea to perform a quick inspection of the solder-mount components before hand-soldering. In the event you need to rework (or, worse yet, scrap) a PCB assembly, it’s better to do so without through-hole components getting in your way. In my example PCB, I can see that all of my components appear to have soldered okay (Figure 17).
Here is my completed PCB. Through-hole parts are all soldered onto the PCB.
Step 5—Rework surface-mount components: I know I just said that rework should be performed in the previous step. But by now I’ve gotten comfortable reworking a complete PCB assembly with the boards that I produce, so I will perform this step toward the end of the process. And, after over 200 board assemblies, I know which failures to expect and have easy ways to fix them with documented rework instructions. For the PCB I have assembled for this article, I noticed there was a solder jumper that did not fully connect the pins. A quick swipe with my solder iron reworked this easily.
Step 6—Clean assembly: While not absolutely necessary, I highly recommend you clean now clean the PCB assembly to ensure its quality, even with no-clean flux. It gives the assembled PCB a professional appearance—something my customers appreciate. You can see the before-and-after yourself in Figure 18 and Figure 19. All kinds of solvents can be used to get a squeaky-clean PCB, but I generally use two methods.
This is my PCB before cleaning. Notice the flux around the through-hole components.
At last, here is the PCB after cleaning. All flux residue was cleaned off.
The first is using a disposable lint-free wipe and cotton swabs with isopropyl alcohol. This approach is simple and cheap, but it’s also time-consuming when cleaning an entire PCB—and of course even more so when you’re producing more than one. So, I reserve this method for small reworks or prototypes.
The second, preferred method is using an ultrasonic cleaner. I purchased a small one from sra-solder.com. My mixture is about 95% distilled water and 5% cleaner. The cleaner I use is Branson 100-955-914, and I find this works extremely well.
Other cleaners can work as well, but this product gave me the best outcomes when doing batches of PCBs. I also found that an ultra-sonic cleaner that is heated and left to scrub the PCB for 10 minutes produced perfect results. After cleaning, I dip the PCBs into isopropyl alcohol to remove the cleaning solution, then place them into my reflow oven for baking. The baking helps to ensure the PCB is free from moisture.
Step 7—Final inspection: Once the PCB is cleaned and dried, it’s time to see how it all looks. This is your last chance to find any assembly errors, such as reversed polarity components, or components that fell off the board due to improper soldering. My example PCB came out as expected.
Step 8—Final testing: For most of you reading this article, your PCBs will likely have a microcontroller, just like mine does. The first thing I do is plug the PCB into my computer via a USB cable, and check if the ESP32-S3 shows up as a serial device. If it does, I can be certain that it will flash okay. If it does not, I check to see if there are solder shorts on the USB connector on my PCB, or if my regulator is outputting the correct voltage. It also helps to check the in-line resistors with the data signals.
Once flashed, I reset the PCB and check if everything functions as expected. For my PCB, I check the digital inputs with tweezers, and check if I can pair over Bluetooth Low Energy (BLE). If that goes okay and my indicator LED changes to all of the appropriate status colors, the PCB is ready to be shipped to a customer.
Conclusion
Not all processes for PCB assembly are the same, but I hope that this article makes it easier for you to create your own. Even with tiny and tightly pitched components, you can assemble a professional-looking PCB in your workshop. If you have any questions about my process, feel free to reach out: Corleto.joseph@gmail.com.
RESOURCES
Espressif Systems: www.espressif.com
PUBLISHED IN CIRCUIT CELLAR MAGAZINE • FEBRUARY 2023 #391 – Get a PDF of the issue
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Joseph Corleto holds a Master’s Degree in Electrical Engineering. Aside from working as a full-time Electrical Engineer, he has a small business (Bit Bang Gaming LLC), which creates video game electronics hardware, and is actively pursuing the creation of a project-based video course using the ESP32. He describes himself as an Electrical Engineering handyman since he has skills in firmware, R&D, PCB design, PCB assembly, and automated testing. You may reach him anytime via his email: Corleto.joseph@gmail.com.
