Wednesday, July 21, 2010

Surface Mount Board Reflow

I have been doing hardware design and prototyping using surface mount devices for quite some time.  I have tried many method of reflowing boards but I've found both the oven method and "hotplate" method to work quite well.

In the past I have been managing the oven and hotplate via a thermocouple and PID controller but I decided to move to a full ramping controller to make the process less labor intensive.

Old Reflow setup; digital controlled hot plate

I purchased the reflow oven controller kit from sparkfun, overall it works ok; the software is good but the hardware has some flaws.  Most notably is the driver for the relay; they're using a BJT which doesn't seem to want to drive the relay coil reliably (even after modding the base resistor).  I solder a mosfet in its place (note the uglyness in the pic below) I used a n-channel mosfet I had kicking around in my junk box.  Another flaw is their choice in relay; the one that was shipped to me didn't have blade connectors on the top (as pictured on the site) so I had to solder wires on the bottom (not very elegant).

The controller didn't work well for the hotplate; the hot plate has too much thermal mass and the control loop on the temperature controller is too simple to compensate.  The controller does work quite well for the oven (for which it was designed).

Oven with temperature control

The plan is to mount the controller in the oven chassis and run the LCD and buttons to a new front panel.

Results using this system have been good; I have been soldering the processor I'm using for a design (LM3S6965) in a TQFP-100 package; results have been good.  Minimal reword required; just wicking a few solder bridges.

TQFP 100 solder with the oven and controller

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