Transitioning to using SMT components

For several decades I have designed, built, and tested my own PCBs for several products/projects. During all of that time I made use of traditional “through-hole” mounting components. Beyond the schematic capture and PCB design software no special tools were required for this process other than a good soldering iron and some good quality wire snips to trim the component leads after soldering.

In recent years it has become increasingly difficult to source through-hole mounted components, particularly for the modern microcontrollers that offer the kinds of features new designs require. The time was right to transition to using Surface Mount Technology(SMT) components.

A while ago I began a new project, the design of a Wireless Debug Probe (WDBP) to connect a computer based debugger to and embedded system using either JTAG or SWD. This design, that will be open-sourced upon release, is based up the Blacksphere “Black Magic Probe” (BMP), an excellent open source debugging probe. The BMP itself is very, very small, using some of the smallest available SMT components.

The BMP is about 15mm x 35mm … REALLY small.

Since the WDBP is my first project using SMT components I decided not to use the smallest components so the PCB size is about 35mm x 90mm. While using larger components contributes to the size of WDBP, an additional factor was the Wi-Fi module with integrated antenna. Plus, I chose to add a footprint for the Tag-Connect TC-2050-IDC “bed-of-nails” connector for attaching my debug probe to the WDBP for debugging the firmware:

Here is a prototype of WDBP:

The WDBP prototypes have been built by a contact of mine who has been working with SMT components for some long time so I avoided the need to purchase any specialized assembly tools. However, during the testing there were a few issues that needed addressing, it was at this time that decided to investigate the minimum set of tools that would enable me to rework the prototypes.

The first issue when working with such small components is being able to visually inspect soldered joints. I tried a head-mounted magnifier, however the magnification was just not good enough to efficiently find any soldering issues. This led me to research and purchase of a microscope. The one I chose is from AmScope, model number SW-3T24Z, their Trinocular Stereo Microscope. I chose to get a trinocular ‘scope so that at some point in the future I could add a camera to the setup. Also, after reading the reviews of this microscope being used for SMT I purchased an additional lens to ensure the distance between the lens and the item under inspection was as great as possible to allow tools to be used on the PCB.

Apart from some small tools like anti-static tweezers the remaining tool investment was a hot-air rework station. This station permits a PCB to be held securely while hot air flows over it, allowing components to be removed, or simply reflowed to fix poor solder joints. Again after some extensive Internet searches I chose the Aoyue 866 rework station. This station comes with not only the base rework unit, but also a selection of nozzles for the hot air gun and a temperature controlled soldering iron. It is a good starter kit for anyone setting out on the SMT journey.

So far, the above tools have worked out really well in testing my prototype PCBS. The next phase of the WDBP project is the pre-production prototypes arriving in about 10 days. Having invested in this tool-set I feel confident that any minor issues with the PCBS can be addressed.

Sid Price