Learning to Love Surface Mount Technology (SMT)

SMD Kits


   
Lost in translation: Somehow ‘soldering practice’ becomes ‘welding practice’ in advertisements for certain small electronics kits from China. I first practiced ‘welding’ surface mount devices (SMD) with two Gikfun kits: flashing LEDs and infrared radar (proximity sensing), each costing less than $10. For the first kit I used a fine-tipped soldering iron and small diameter wire solder. The work was challenging, especially the soldering of integrated circuit components, but not impossible. (The photos above are from product advertisements—not my solder practice!) A circular pattern of flashing LEDs rewarded successful completion of the first kit (left in photo). In fairness, the kit makes good on its promise
of soldering practice, although as a finished product it would not likely sustain long term interest.

Reflow StationMelting solder    More hot air: Before undertaking the second SMD practice exercise I had acquired an Aoyue SMD Rework Station, along with paste type solder, which is applied to the PCB (or stencil) by squeezing from a syringe. Through trial-and-error I zeroed-in on a temperature and airflow combination that melted solder without blowing the tiny components into the carpet, provided the air flow nozzle was held at a suitable distance and angle with respect to the target devices. For what it’s worth, the experimentally determined temperature was 225 C. I cannot specify airflow rate meaningfully, because neither the Rework Station nor its instruction manual identifies units for the displayed air pressure numbers—Air flow was 15 something, units unknown.

    Then what? Without giving it thought I had imagined that after the solder practice stage, other SMT kits would be available, not just for learning to work with surface mount devices, but to make useful or interesting products on completion. I had in mind an end result that would be richer than flashing LEDs, not necessarily a big-screen TV, but somewhere between the extremes of solder practice kits and manufactured electronics. Far as I can tell, no such thing exists. Kits that include SMT components usually have those elements pre-installed, with only through-hole components or socketed IC’s left for the builder to install. It appeared for a while as if the Rework Station would gather dust, which from time to time could be heated up and blown away (its self-cleaning feature). —But then another thought occurred..

    Homebrew SMT: If kits of the type I imagined are uncommon, why not convert one of my previous projects to SMT, or create an SMT project from scratch. At this point I was unsure whether it would be possible to purchase surface mount versions of popular op amps or timers or TTL, etc., in small quantities and at reasonable cost. What if such components were only to be had in factory quantities? It would be crazy to consider buying a thousand 555's in order to make one timer. However, a quick Internet search confirmed that common SMDs can be purchased in small quantities from companies like Mouser or Digikey. Since such components are small, and almost lighter than air, shipping costs are also reasonable.

SMD version of Frequency Counter Project

    Hidden Advantage: From the amateur experimenter’s perspective, it is important to keep the cost of projects as low as possible. Companies that develop products for profit can afford to invest in advanced design software or sophisticated hardware; whereas amateurs or students are generally obliged to pursue their interests within stricter budgetary constraints. This thought led to an epiphany of sorts. You get more for less with SMD than with the older more familiar technology! The first such economy-of-scale poked through to my awareness in relation to the circuit board design phase. The non-commercial (free) version of Autodesk Eagle supports PCB layouts of up to 80 cm2, or nominally 80 mm 100 mm. As a guess, twice as much circuitry can be packed onto this small design area using SMD than using through-hole or socketed components. This comparison is based on circuits that are familiar to me—in other words, amateur designs. LSI and multi-layer boards would boost component density to a much higher level.

    Picking a project: I didn’t immediately think of a novel idea for a first homebrew SMT project. But one of my previous projects included six DIP ICs (TTL). That same project also involved a small number of resistors, capacitors and LEDs. If redesigned for SMD, the project would be significantly smaller, and should easily fit within an 8 cm 10 cm layout. Later I came to realize that the close spacing of solder pads for SMD IC’s, and the fact that they are all on the top PCB layer, made routing of connections more challenging. —Eagle has an automatic routing feature, but I have not tried it. Manual routing, or at least my manual routing is very non-optimal. It relies on a great many vias. For this prototype SMD project
I placed each part as soon as it was added to the schematic, and routed the traces (wires) that were possible to route at that time. I did not draw the full schematic and place all parts before starting to route connections. This one-part-at-a-time approach likely contributed to the large number of layer crossings (vias).

Revised schematic for SMD version of Frequency Counter

    Boards: I had ordered PCBs from JLCPCB before, and been entirely satisfied. CAM files were uploaded on Thursday and manufactured boards (quantity 5) delivered the following Wednesday, China to Eastern US in 6 days, including manufacture—Amazing! (This is not an advertisement.. I have no connection to JLCPCB except as a customer.) Components that were not on-hand (chiefly the TTL IC’s) were ordered concurrently from other sources.

Populated PCB

    Solder Practice Redux: It may be premature to claim having improved my SMD soldering technique through practice with this first homebrew SMD project, but here goes. After trying a few different approaches, one of which caused solder to pool under the chip—definitely not good, I hit upon a method that appeared to work better than others. First the IC was positioned over the bare pads, and in the correct orientation for soldering. With the chip in position I applied heat from a few centimeters height above one corner. This warmed the board in the area to be soldered. Then I just touched the heated part with solder paste—the heated board caused the paste to liquefy and flow along the two or three pads/tabs to be soldered. Heat was again applied from above, this time for long enough to melt the solder and cause it to flow onto the tabs, thus securing the IC at one corner.  Next I repeated the process on the opposite corner. By then the board was generally hot enough to liquefy the paste, without the necessity of applying additional heat. After the diagonally opposite corner was secured I returned to the first side, and similarly soldered the remaining pins, starting on the end away from the first application. Finally I soldered the remaining pins. All connections were shiny, with no excess or solder bridges. I used this method for all 6 ICs on the third board belonging to this project. An inset toward the end of the video demo (see link below) illustrates this process. It was by far the cleanest of several tries. No resoldering was needed.

    I should clarify that on a previous board I had applied cold paste in greater quantity, and afterward had to desolder several ICs, clean the board, and resolder them. Another practice board was fine electrically, but not as clean as the one soldered using the technique outlined in the preceding paragraph. Of course, proper equipment, including a stencil frame and oven would surely produce the same or a better result. However, when a hot air reflow station is the only tool available, it remains possible to solder SO-14 or SO-16 footprint ICs very cleanly—perhaps, as in my case, by substituting patience for experience.

Small Enclosure for SMD board

    Acknowledgements: Although the GPS Frequency Counter project in its original form has been described on a separate page, I want to reacknowledge key sources that contributed to the concept. First, the idea of using a GPS to define the counting interval came from the QRP Labs QCX transceiver kit, and subsequent correspondence with its designer Hans Summers G0UPL. The preamplifier circuit (upper left corner of PCB) came from the Four State QRP Group Frequency Mite kit, designed by Dave Benson K1SWL. I have included the revised schematic above (i.e., the SMD version of the project). For more detail please refer to the original write-up (non SMD version). Finally the microcontroller sketch for the project described on this page may be found here.

    Demo video: Learning to Love SMT



Projects Home







Project descriptions on this page are intended for entertainment only. The author makes no claim as to the accuracy or completeness of the information presented. In no event will the author be liable for any damages, lost effort, inability to carry out a similar project, or to reproduce a claimed result, or anything else relating to a decision to use the information on this page.