In a previous article I describe prototyping an electronics project using through hole components and breadboarding. This is an effective method for quickly slapping together a simple circuit but falls short when you need to scale up or miniaturise your design.
Typically surface mount reflow is inaccessible to hobbyest, especially when manufactures expose their contact pads on the bottom of their components making it impossible to get a soldering iron where it needs to be.
In this article you will learn about the online services offering very affordable PCB printing and stencilling thus enabling the mere mortal the ability to put together a working circuit.
Prerequisites
The only prerequisite required is a PCB design. I find the most common tool to achieve this to be Eagle, and although it is a little daunting at first and not particularly user friendly, it is a great tool for circuit and PCB design.
The remainder of this article will not be a tutorial on Eagle. There are plenty of resources available on the web that could likely better introduce the software. I’m no expert myself.
Personally I used a PCB designed by Luke Weston, and apart from a few displacements of vias I made to conform to particular design rules, this is all his work.
Printing
In order for a PCB to be printed it must be represented in a format that can be read by a computed-aided machine. For PCB design this is the Gerber format. Eagle is capable of producing this format using its CAM feature.
Many Gerber files may be produced for a single PCB and represent the multiple layers that make up the board. For example a single PCB could contain the following: the bottom copper layer; bottom silkscreen layer; drill file layer; the top paste layer, etc… A list of other Gerber file extension can be found here.
When you have your Gerber files head on over to OSH Park. This is a great site that will print PCBs for a very reasonable price. I had 3 PCBs printed here for around £10 including international delivery costs with a total turnaround time of only 2 weeks. If this is too slow for you they do offer a much quicker turnaround time but at a much greater cost.
The best feature of the site, in my opinion, is the visual representation of what they think the board will look like after fabrication, giving you an account of what you should expect to come through the post.
The above image is the PCB I ordered. I am very happy with the quality of the print and will definitely use this service again.
Stencilling
When applying solder paste by hand it will be far easier for you to use a stencil rather than using the needle that often comes with the paste syringes. I learnt that through experience when hand pasting my PCB which had a couple of integrated circuits with very small pitch sizes. Poor application lead to bridging and a very unprofessional finish.
Presumably a sister company, OSH Stencils
produce the perfect accompaniment to a fabricated PCB. Using the same set of
Gerber files you created to print you PCB, OSH Stencils will laser cut a
stencil that can be used to apply solder paste to the PCB contact pads. The
important Gerber file in this process is the one with the .gtp extension.
This is the top paste layer defining the positioning and size of the contact
pads on the top half of the board.
The stencil can be made of polyimide film or stainless steel, the former being the cheapest. I bought the polyimide stencil because I was only creating 3 boards so forking out the extra money on a stainless steel one was less cost effective.
Applying the paste is easy. I followed this youtube video that describes the process very clearly:
Placement
Steady hands are required for this step. Using a cheap set of tweezers and a pair of helping hands I picked up from Amazon, it became fairly straight forward to carefully place each of the tiny surface mount components onto the solder paste.
The difficulty here is making sure you check the polarity is correct for components such as diodes and LEDs. In the case of LEDs you may be lucky enough to have a decent multimeter that will send a small current through the component when attached to each pole, meaning it will light up when you have it the correct way.
Another difficulty is ensuring that all the contact pads of an integrated circuit line up with the solder paste on the board. If they are off by just a little bit it could lead to bridging when the paste is reflowed. Using the magnifying glass on the helping hands is a great help here (hence the name I guess).
Conclusion
Here I have described an extremely manual process to build your own PCB. Of course this technique is not going to scale well, but if the goal is to create a small handful of boards then this method is fine.
It is worth noting that I have left this process deliberately unfinished and have yet to actually reflow the solder paste. I leave this for a future article where I describe a cheap and easy technique to perform the reflow.