Duke University Motorsports is a student group that designs and builds open wheel, single seat race cars to compete in the Formula SAE competition sponsored by the Society of Automotive Engineers. The team consists of Duke students from both Pratt and Trinity, in all classes. The purpose of the team is to provide students with a way to gain practical design and manufacturing experience in a fun and challenging setting.

Wednesday, February 15, 2012

Fan controller and SMT soldering

I received the PCBs for the fan controller a few days ago, and I soldered them up today.  The purpose of the fan controller is to take the PWM signal from the ECU (open source) and convert that to a PWM signal for the fan.  Since the fan only runs in one direction, there's only one MOSFET needed, and the circuit becomes pretty simple.

The circuit is simple - pull-up resistor on the input signal, one N-channel MOSFET to do the switching, a MOSFET driver to drive this high-capacitance FET, and flyback/transient protection.
The assembly process involves a few easy steps: 1) apply solder paste to the PCB. 2) place components on PCB. 3) Reflow solder. 4) Rework (if necessary).

Step 1: Apply solder paste
Apply solder paste out of syringe if you don't have a stencil.  For these large pitch parts, you can apply a dot of solder to each pad.  For smaller pitch parts (.5mm or smaller), you will probably have to lay down a line of solder paste.
 Step 2: Place components:
Place components carefully using tweezers or a vacuum extension
 Step 3: Place in reflow oven (in this case a Cuisinart toaster over - works great!)  Monitor the temperature using a thermocouple.  Typically I bring it up to 160C, let it soak for 60 seconds, then bring it to the reflow temperature (~200C) and check for visual confirmation of reflow.

Step 4: No rework necessary in this case.  With these large pitch parts, as long as you're careful with solder paste application, the yield is typically very high.

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