SMD Challenge: Keyring Edition
Portable keychain featuring hand-soldered SMD components from 1206 down to 01005, with transparent protective enclosure
Portable keychain featuring hand-soldered SMD components from 1206 down to 01005, with transparent protective enclosure
This project reimagines the popular SMD Challenge by MakersBox in a portable keychain format, creating a practical demonstration of advanced hand-soldering capabilities. Rather than designing yet another static display piece, I wanted something that could showcase technical skills in daily use while being robust enough for regular handling.
The keychain format addresses a fundamental limitation of demonstration PCBs: they typically remain on workbenches rather than serving as portable proof of capability. By creating a protected, functional device that can be carried and demonstrated anywhere, this project transforms a soldering exercise into a practical conversation starter for technical discussions.
For complete technical details, design decisions, and step-by-step assembly documentation, see the comprehensive development journal available in the project attachments.
The design centers around an STM32F030F4Px microcontroller in TSSOP-20 package, chosen for its balance of capability and compact footprint. The power delivery uses a dedicated 3V3 LDO regulator (XC6206P332MR) with proper decoupling, ensuring stable operation despite the miniaturized layout constraints.
A key design decision was implementing full USB-C compatibility rather than using a simpler power connector. This required careful attention to the CC pin configuration with 5.1kΩ resistors for proper USB-C to USB-C operation, demonstrating understanding of current USB standards while maintaining the compact form factor.
The front side implements a deliberate progression of component sizes, creating a structured soldering challenge:
This progression allows builders to gauge their skill development and provides clear milestones for technique improvement.
Rather than using conformal coating or permanent encapsulation, I developed a two-piece transparent enclosure system that maintains component visibility while providing protection. The design uses flush M2 screws to prevent snagging, with careful attention to wall thickness requirements for SLA printing.
The enclosure accommodates the USB-C connector while keeping the overall thickness to 7.5mm, ensuring the device remains practical for keychain use. Internal slopes and clearances accommodate solder joints and component tolerances without requiring perfect assembly precision.
The USB-C implementation follows proper specifications with 5.1kΩ CC resistors enabling USB-C to USB-C operation. The XC6206P332MR regulator provides clean 3V3 power with appropriate input/output capacitors for stability under the varying load conditions of the LED animations.
The STM32 setup includes an 8MHz external crystal with calculated load capacitors (16pF per side) based on the crystal’s 10pF load capacitance specification. Programming access is provided through dedicated SWDIO and SWCLK test points, though a VCC pad would improve convenience for future revisions.
An unexpected challenge emerged with the blue LEDs being significantly brighter than anticipated. Rather than redesigning the PCB, I implemented software solutions:
This approach demonstrated adaptive problem-solving while highlighting the importance of thorough LED characterization during the design phase.
The back side components (microcontroller, USB connector, power management) were reflow soldered using solder paste, while the front challenge components were hand-soldered using conventional techniques. The 01005 resistor required 23× magnification and proved to be at the practical limit of hand soldering—losing half a day when the first attempt damaged a pad.
The completed device successfully demonstrates all intended functionality:
All components from 1206 down to 01005 were successfully hand-soldered and remain functional. The 01005 resistor measures correctly at 10kΩ, confirming successful electrical connection despite its 0.4×0.2mm dimensions. The transparent enclosure provides effective protection while maintaining the visual impact of the miniaturized components.
The project demonstrates that advanced SMD hand soldering is achievable with standard hobbyist equipment, though the 01005 component represents a practical limit requiring significant magnification and patience. Future builds would benefit from revised resistor values for the blue LEDs and additional test points for programming convenience.
This project successfully bridges the gap between demonstration piece and practical device, creating something that can be carried daily while showcasing advanced assembly capabilities. The systematic approach to component size progression provides a valuable framework for developing precision soldering skills.
The integrated enclosure design methodology could be applied to other miniaturized electronic devices requiring protection while maintaining visibility. The software PWM implementation techniques proved valuable for brightness control in resource-constrained applications.
Planned improvements could include correcting the blue LED resistor calculations, adding dedicated VCC/GND test pads, and investigating scratch-resistant coatings for the enclosure. A more extreme version could incorporate 008004 components (0.25×0.125mm) or fine-pitch IC packages to further push the boundaries of hand-assembly techniques.
The concept demonstrates how modern prototyping capabilities—affordable PCB manufacturing, precision 3D printing, and accessible design tools—enable individual makers to create professional-quality demonstrators that were previously only possible in commercial environments.
