A enjoyable side-project that I labored on throughout my co-op time period right here at StarFish Medical was a custom-built reflow oven to solder floor mount parts onto printed circuit boards.
Why Construct a Reflow Oven?
More often than not, our PCBs are assembled at contract producers, however there are some circumstances the place it’s extra acceptable to assemble small prototype boards in-house to save lots of time. Up to now, we’ve used a board preheater and a sizzling air station to assemble them, however this has some drawbacks, similar to doubtlessly pushing parts previous their temperature limits or producing low-quality solder joints because of the lack of a correct reflow profile. A reflow answer that correctly implements a reflow profile is required if we need to tackle these points. Low cost reflow ovens that implement reflow profiles can be found on-line, however such gadgets typically require modifications to work nicely, so we determined to construct a {custom} one. Plus, it appeared like a enjoyable undertaking!
Making a Customized Answer
I constructed the reflow oven across the StarFish DAQ, one in every of our inside improvement platforms. The StarFish DAQ (Knowledge Acquisition System) is a perfect selection for a undertaking like this as a result of it means that you can shortly implement PID management loops with preset trajectories. This eliminates the event time that might usually be spent on implementing PID management, ADC enter, and PWM output on a microcontroller and a graph UI on a pc. It additionally supplies sufficient functionality to enhance on a undertaking with out requiring a platform change. Its excessive present outputs can run a fan motor for convection cooling or a servo to open the oven door on the finish of the cycle.
For the oven itself, I used the most cost effective toaster oven I might discover: a Hamilton-Seashore unit on sale for $27. An affordable toaster oven was most popular as a result of along with preserving undertaking prices low, toaster ovens typically have a quite simple electromechanical thermal management system. A bimetallic strip thermostat turns the heating components on if the temperature is beneath the setpoint of the thermostat and off if the temperature is above the setpoint. The thermostat will be bypassed by setting its restrict to a temperature past the utmost temperature of the reflow cycle and controlling the warmers with an exterior relay related to the oven’s energy inlet.
For the relay, I initially used an off-the-shelf system containing a mechanical relay that I might management with the StarFish DAQ related in between the toaster oven and the wall outlet. That relay was very sluggish and had a comparatively low variety of most switching occasions, which means that I couldn’t actually push the PWM frequency previous about 1 Hz. A better PWM frequency is most popular to scale back its impact on the temperature response of the oven, as decrease frequencies can produce artifacts within the temperature. A solid-state relay will be pushed at a better PWM frequency than a mechanical relay.
After proving the idea with the off-the-shelf relay, I designed a 3D-printed enclosure for a solid-state relay that additionally held the StarFish DAQ. The enclosure was an attention-grabbing design problem as a result of I needed to take into account electrical security surrounding the AC mains wiring and thermal security, because the relay might get too sizzling and soften the case. I included an isolation barrier between the excessive and low voltage sides of the enclosure that solely the relay might go, and an earth-grounded heatsink to passively cool the relay.
For temperature suggestions, I used a sort Ok thermocouple and an Adafruit breakout board. This thermocouple is related to the PCB being reflowed. The oven heats its contents utilizing infrared radiation, so the air contained in the oven (comparatively clear to IR) will be a number of levels cooler than the PCB itself (opaque to IR). Many low cost reflow ovens don’t have the thermocouple contacting the PCB and as an alternative measure the temperature of the air contained in the oven. This may result in the PCB temperature deviating from the reflow profile if the discrepancy shouldn’t be correctly accounted for.
Evaluating Efficiency
The temperature response of the oven was in a position to comply with the programmed reflow profile precisely, with just some damped ringing alongside the rising temperature within the soak interval and a little bit of overshoot through the reflow interval. These shortcomings may very well be improved by means of higher PID tuning, however they’re sufficiently small that the reflow course of works fairly nicely.
I examined the system on growth boards that we had made for an additional undertaking. These had been comparatively easy boards used to implement a modification to a different board that’s within the prototype stage. Their small manufacturing quantity made them unsuitable for skilled meeting. I hand-populated the parts on the board and reflowed it utilizing the reflow oven.
After some cleansing, the boards had excellent-looking solder joints. At first look, you may suppose that they got here from knowledgeable meeting service, quite than being assembled by hand and reflowed with a custom-made reflow oven! I used to be in a position to take a look at the reflow oven on different boards and it was in a position to produce good high quality solder joints each time. One board that was significantly spectacular was a board optimized for warmth switch, which is usually harder to solder by hand.
Studying From This Venture
This appeared to be a comparatively easy undertaking initially, however I discovered plenty of areas to dive deep on to enhance the system as a lot as attainable. I acquired apply with a number of expertise by means of this undertaking, similar to methods to safely wire a high-power 120VAC circuit, methods to apply and tune a PID management to a thermal system, and methods to design a 3D printed enclosure to satisfy electrical security and thermal necessities.
I’m grateful that I used to be in a position to work on attention-grabbing initiatives like this throughout my co-op at StarFish and study from the superb crew right here! StarFish Medical accepts work placements all year long for distinctive college students. Take a look at their Co-op Placements page for extra info.
Callum O’Riley is an Electrical Engineering student at UBC. His final co-op time period was a 4 month time period engaged on testing form reminiscence alloy actuators. He’s a part of UBC Rocket, an engineering pupil design crew, main a crew to design the electronics for a supersonic rocket.
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