Hand Assembly of PocketBone using the OSD3358

This article was guest written by Michael Welling, the developer of the KiCad PocketBone Design.  Michael is an Embedded Design Engineer with over 10 years of experience.  He owns an electronic design consulting firm, QWERTY Embedded Design, LLC.  He has an MS in Electrical Engineering, was an instructor at SIUC and a 12-year member of IEEE.  He is also a mentor for Beagleboard.org Google Summer of Code.

In the previous blog, the origin story and design process of the PocketBone were discussed. This edition talks about the process of gathering components and assembling the units at home by hand.

I put together a video showing the entire assembly process including hand placing the OSD3358-512M-BAS BGA and reflowing the boards in a toaster over.  Check it out below!  The rest of the article goes into more detail on the different steps and the equipment used.

Sourcing Components

First off, each component that is designed into a PCB needs to be ordered, including the blank PCB. These component are contained in the bill of materials (BOM) which includes the manufacturer part numbers and quantity needed for each part. The BoM is generated during the design process.  Typically you would search for the required devices through your favorite vendors (DigiKey, Mouser).  When considering a device to add to your BoM you want to make sure you pay special attention to the available quantity, lead time and life cycle status of the part. Components with long lead times, NRND and EOL status should be avoided if possible. Websites such as findchips.com and octopart.com are useful in this process.  If the device meets your needs, and availability and lifecycle look good, then add it to your BoM.  Once your BoM is finalized you can order the parts from your preferred supplier.The BOM for the PocketBone is available on the Hackaday.io page.

If Legacy parts are used in a design they should be replaced at the next convenient moment.  For instance, the PocketBone used a legacy part for the push-push MicroSD slot that is now EOL and should be designed out in the next revision.

The PCBs are ordered from a wide variety of sources. OSH Park and Seeed Studio are a few popular choices for small batches. The choices vary greatly in quality and price. Typically you find a PCB house that suits your needs and stick will them. OSH Park sponsored the board in this case so of course, this was a no-brainer for me.

Stencils and Solder Paste

The stencil for the PocketBone was ordered from OSH Stencils.

OSH Stencils provides stencils using polyimide and a stainless steel. For most cases, polyimide is good enough. I ordered one of the fancy stainless steel ones to avoid print issues with the BGA. The quality is great and the price was very reasonable.

Along with the stencils, I ordered the acrylic jig set to hold the board in place while applying paste.

The jigs are taped down to hold the board in place while applying the paste. With the stencil aligned and taped over the jig and PCB, the paste will be added above the PCB and dragged across the holes in the stencil to apply the paste to each surface mount pad on the PCB.

For solder paste, I ordered the MG Chemicals 63/37 No Clean, Leaded Solder Paste from Amazon.

I chose leaded paste because it has a lower temperature reflow profile than non-leaded.  Since I am using a toaster oven to do the reflow the lower temperature profile of the leaded makes life much easier.

I wouldn’t recommend using leaded solder in a production product as it isn’t compliant with ROHS and other international environmental standards.

Placing Components

Another thing that I ordered for this assembly was a pickup tool for the components. There are a number of how-to videos and guides on building one, as well as inexpensive professional units available on the internet, however the one I used was a custom made vacuum SMT pickup tool was from David Anders (prpplague).

This tool allowed for easier pickup and placement of the various parts. You could also use surface mount tweezers for most of the parts and it is good to have some handy for placement of certain parts and small adjustments. I also used a small SMT probe for fine adjustments during the process.

Here is an example of using the pickup tool to grab a 0402 passive component.

Assembly Time!

With all of the components and required tools in hand, it is time for assembly. As described above the solder paste is applied to the pads of the PCB using the stencil. The paste spreader should be held firmly at around a 30º angle as you drag across the pads. Make sure that the stencil is aligned and that it does not lift from PCB while applying. This takes some practice to get just right.

It should be noted that the PCB and stencil should be thoroughly cleaned with 99% isopropyl alcohol and dried before applying the paste.

Once all of the pads are covered and excess paste is removed from the stencil, the stencil is slowly lifted and the PCB is carefully removed from the harness.

Next, you slowly and carefully place the components onto the board. The placement of the BGA is by far the most critical as rework is very difficult. The BGA, connectors and buttons sometimes needed to be subtly adjusted for optimal alignment with the probe after placing. The smaller passives are less critical in terms of exact alignment as they will snap into place during reflow (the heating process) due to surface tension from the solder.

This step is where the benefits of the OSD335x System-in-Package really shine through.  Placing a single BGA instead of hundreds of discrete components makes this build much more approachable. Placing the equivalent discrete ICs and passives would require 10x the effort.

Also, the discrete AM335x SoC BGA and PMIC IC have much finer features than the OSD335x, making them much harder if not impossible to hand assemble. Furthermore, many of the passive component built into the SiP would be on the opposite side of the board requiring multiple passes through the reflow oven. Utilizing the Octavo Systems SiP instead of discrete components greatly increases the chances of a successful DIY assembly.

Here is a picture showing the final placement before throwing it in the oven.

Reflow

Up to this point, we have not discussed how the reflow process works. Each component and solder paste has a temperature profile which is a graph of temperature over time that should be matched within a certain tolerance in order to make a solid solder connection.

To meet the reflow curve for the leaded paste, I used a basic toaster oven with a Reflowster controller attached.

The Reflowster has the circuitry to power the oven on and off in order to the control the temperature. It uses a thermocouple to measure temperature and tries to best follow the reflow curve within the limits of the toaster. Once the reflow is complete, an alarm will sound and the toaster door is opened to let it cool. A well-ventilated area is recommended as the paste emits fumes during the process.

Once the boards have cooled, they are ready for visual inspection. Sometimes the smaller components will lift from one of the pads which are called a tombstone. Other times, finer pitch parts will bridge across leads. This is when a soldering iron or rework station will come handy.

After verifying the soldering, the boards are ready for their first test. I plugged in my SD card with a BeagleBone image on it and started it up.  All 5 boards I hand assembled successfully booted into Linux proving that they were fully operational.

I hope you enjoyed and are inspired to build your own 1GHz Linux based embedded system at home!