How to Assemble a PCB With Tiny SMD Parts

I prototype PCBs from time to time, and I want to share my tips and tricks for DIY assembling of complex boards. Tiny IC packages may seem scary at once, but with some practice, you can confidently assemble PCBs of high complexity, including those with BGA parts, in your home lab.

Professionally manufactured PCBs and stencils are now cheap, which opens opportunities for makers to do some cool advanced electronics projects. Even for professional projects, DIY PCB assembly saves time and money and allows faster iterations with early prototypes before the board design is finalized.

I will guide you through the steps as I assemble a baseboard for Rasberry Pi Compute Module 4 and a display. Please follow our project on Hackaday, https://hackaday.io/project/176098-raspberry-pi-c...

A safety warning

Heating equipment is a potential fire hazard. Never leave any soldering equipment on unattended. Flux fumes are potentially toxic, and lead contained in soldering paste is harmful. Ensure you always work in a well-ventilated room, and preferably use a smoke absorber or local exhaust ventilation over your workspace. Carefully clean your tools and wash your hands. Never let the paste get in contact with food, especially if storing it in the fridge.

1. Your PCB board. When you order PCB, select immersion gold (ENIG) surface finish. It is slightly more expensive than HAL, but it provides flat pad surfaces that are easier to work with. I usually order bare boards from JLCPCB. Their prices are low, the process is all self-service, and they are swift. My 137 x 95 mm board cost me about $10 each when I ordered 5 of them.
2. Stencil. Better go with stainless steel stencil. You don't need the framework, and stencil size does not need to exceed the board's size by much. Usually, you can order the stencil together with the board. Mine cost $14.
3. Solder paste. The paste is essentially a mix of flux and tiny solder balls. There are leaded and lead-free pastes. Unless you know what you are doing and have a professional grade reflow oven, don't use lead-free paste, it is very hard to work with. Just make sure to carefully clean-up your workspace and tools from the lead-containing paste after you are done. I use Chip Quik SMD291AX no-clean solder paste. You can purchase it from DigiKey in syringes or jars of different sizes (like this https://www.digikey.com/short/4tp71q). A jar is preferable for stencil application. It lets you put the excess paste back. Don't buy the paste from retailers like Amazon, where storage conditions are not observed. The paste has a limited shelf life and needs to be kept refrigerated. Distributors like DigiKey take it seriously and send the paste with a cold pack.
4. Hot air soldering gun. There are many options on Amazon in the range of $40-60.
5. IR preheater, hot plate, or reflow oven. Let's stop here for a moment to discuss the pros and cons of different options.
   1. IR preheater. That is what I use. They are not very expensive, and if you are planning to do boards often, this may be a good investment. I have Aoyue Int 853A++ and pretty happy with it. It has a digital control and two external probes to control the temperature on the PCB's surface. You can get a newer version for $160, https://sra-solder.com/soldering-equipment/pre-he.... There are cheaper options too, like https://www.amazon.com/dp/B07QKBQ717.
   2. Hot plate preheater. You can get one for about $80 (https://www.newegg.com/p/04V-01GN-00007?item=9SIA...), or make your own. There are multiple DIY hot plate projects. The hot plate's good thing is that the heat is transferred through direct contact and better distributed. An obvious drawback is that it limits you to working on a single side of the board. The other side must be kept flat, free from components or through-hole pins. This may be fine for most projects but may be a problem for board reworks.
   3. Reflow oven. That is the most advanced choice. That is how most factory-made bords are produced (except, factories use advanced multi-zone conveyor ovens). Instead of heating your board with a hot air gun, you put it into a temperature-controlled heating chamber, and it goes through a programmed temperature profile over time. The whole board is heated as a whole, avoiding thermal gradients and local over or under-heating. This gives the fastest and most consistent results. It is worth considering if you are going to do serial manufacturing. The prices start at $300 for cheap Chinese options and stretch into thousands of USD. There are also many DIY projects of reworked kitchen toaster ovens. They are worth checking out. However, with reworked toasters, there is a problem that their heating elements are not good for even heating of PCB. Some components may stay unsoldered while plastic connectors are already melted.
   4. No preheater at all. You can use a hot air gun on its own without any preheater, but this puts your board and components under higher thermal stress due to temperature gradients from spot-heating. As materials expand and contract from heating and cooling, mechanical stress builds up and may cause joints to crack. This is less of a problem if your board is tiny and can be heated evenly by just moving the nozzle around.
6. Putty knife. I use a plastic knife. It does not unnecessarily scratch the stencil.
7. Lab spatula for applying the paste and removing the excess. Here is mine, https://www.amazon.com/dp/B079RGP3KT but any small flat stick would work as well.
8. Masking tape. A classic removable "blue" tape to keep things from moving.
9. Spare boards of the same thickness from some previous projects, or if not available, a pile of paper sheets may work.
10. Tweezers. Good tweezers are essential. Make sure they are convenient to hold and non-magnetic (not even slightly).
11. Soldering iron. You may need it for through-hole components and for spot-fixes.
12. Rubbing alcohol for cleaning.
13. Lint-free wipes. Wipes for nail polish removal work well.
14. Soldering flux. I recommend having syringe-packaged no-clean tacky flux, like Chip Quik SMD291 (https://www.chipquik.com/store/product_info.php?p...).
15. Wire solder. Similar to the solder paste, the best alloy to work with is Sn63Pb37 with a flux core.
16. Desoldering braid. Always handy for fixing any screwups.
17. (Optionally) High-temperature polyamide masking tape. Useful to protect some sensitive components like connectors. I recommend having it handy in different sizes, https://www.amazon.com/dp/B072Z92QZ2.
18. (Optionally) Sectioned boxes for electronic components.
19. (Optionally) Steel plate. I will explain below how it can be handy. I got a 12x12 inch plate from a local hardware store.
20. (Optionally) A few strong magnets. Those are great for the job: https://www.amazon.com/dp/B07N4JNGHR.
21. (Optionally) Inspection microscope. There are cheap USB-cam "microscopes," like https://www.amazon.com/dp/B06WD843ZM. The image is far from ideal, but it is still helpful.

Before you start:

1. Take soldering paste out of the fridge to let it stay at room temperature for a couple of hours to get to the right viscosity for application.
2. Go through the list of your components, a so-called Bill of Materials, and place the parts into sectioned boxes in the exact number and the order as they appear in the list so that you can map them easily. This will save you time when placing them on the board.
3. Clean up your workspace from anything you are not going to use along the way.
4. Make sure it is well-lit.

Place the board on a flat surface on your desk. Carefully consider the optimal orientation. You will need to be able to comfortably reach all components with tweezers when placing them.

Build a frame around the board that will keep it secured in-place and support the stencil around the board. It has to be of the same thickness as your board, so the stencil does not bend. The best option is to use other unpopulated PCBs secured to the surface with masking tape. To keep the stencil itself from moving, I came up with a trick of using a metal plate and a pair of powerful magnets. Another option would be to use masking tape again. Whatever you do, make sure you can later lift the stencil without sliding it along the board. If using magnets, it is useful to place something underneath each of them, so you can slide them to the side without moving the stencil. Experiment.

Make sure the board and the stencil are free from dust and any contamination. Wipe them with rubbing alcohol if necessary. Then, carefully place the stencil onto the board, align it with component pads and fix it in place.

With the lab spatula or some other appropriate object, put enough solder paste onto the pads or close to them on the side from which you will be moving your putty knife. Don't be afraid to put too much. You will be able to return the excess to the jar later.

Then use your putty knife to firmly push the paste into the openings of the stencil by moving it in one direction at an angle of approximately 45 degrees. You may need to add more paste. You may also need to remove the excess from the knife between the strikes. Make sure that all openings are filled with the paste at the end.

When done, carefully release the stencil and lift it up. This is the step that is easy to screw up. You need to be very careful not to displace the stencil to any side.

Inspect the result. There must be no paste bridges between the pads and no pads without the paste. Small issues may be fixed with a needle or other sharp object.

After the paste is applied to component pads, you should immediately proceed to the placement before it dries out in the open air. I recommend starting with small and most challenging packages, then placing all numerous regularly shaped parts, like resistors and capacitors, and finish with large components and connectors.

First, take a component out of the packaging, grab it with your tweezers, and carefully hover just above the footprint. If there is some extra space on the board close to the footprint, I found it handy to put the component there, in the right orientation, and then pick it up again before the placement. This way, when you grab it, you are already holding your tweezers above the board under the convenient angle, and the component is parallel to the board's surface. Now, when you are hovering the component above its footprint, hold the breath, and between the heartbeats, drop it down. I may be exaggerating a little bit, but precision is important. However, there is some level of forgiveness. The paste is viscose, and small components are light. They don't immediately stick. Use the tips of your tweezers to gently push the part from the sides until it is perfectly aligned. After that, give it a gentle push from above towards the board. Don't worry if there is still a little bit of displacement. Surface tension will finish the alignment during the reflow.

The next step is to heat the board. Check the recommended soldering profile provided in the solder paste datasheet. You will not be able to follow it strictly (unless you have a good oven), but it will give you a general direction. It consists of several stages: preheating, soaking, reflow and cooling. The idea is to slowly raise the temperature and let the heat distribute evenly. Then, keep a nearly-steady temperature for a little bit to let the flux get released from the paste, and let component pins soak. Then briefly raise the temperature above the solder melting point. Finally, gradually cool down.

Adjust the preheater to achieve about 160 deg C on the board's surface. If solder starts melting, you should reduce the setting. Don't let the board stay heated for too long, or the flux will evaporate, and you will get defective joins.

Use the heat gun to melt the solder by slowly going around the components, evenly heating all their pins. Try not to blow on them directly, and be especially careful with the plastic connectors that melt easily. You will see it visually when the paste melted. It changes the color from matt grey to shiny silver.

Check carefully if every joint is done, turn off the heat, and let the board cool down.

Now, inspect the board. The things to look for are: unmelted paste, lifted pins, solder bridges. Usually, all those defects can be fixed with a soldering iron.

In the very end, solder all the through-hole components. You must now be done.

You must now be done — time to start testing.

I hope you enjoyed this instructable and learned something new. Don't forget to like and follow the Hackaday project (the link is in the intro), and feel free to ask your questions in the comments.