Computing stuff tied to the physical world

Smooth sailing

In Hardware on Nov 5, 2010 at 00:01

I’ve just completed three pcb panel reflows, based on my Reflow Timer progress. Here’s a panel of Pressure Plugs:

Dsc 2210

Perfect reflow, on all three panels. There is a small problem with a cold spot in the grill, but that’s a known (and predictable) issue with a specific spot on the panel. Nothing a soldering iron touch-up can’t fix.

Oh, and isn’t blue + gold just gorgeous? – I see that it’s being picked up / imitated by others as well, such as for the “USB BUB” and the “LCD Plug” – well, sort of… ;)

Here’s one of the (unretouched) corresponding reflow graphs:

Screen Shot 2010 11 04 at 18.27.00

A perfect profile on every run, as far as I can tell!

Note that the initial overshoot in the WARMUP phase is not very important. The key is to start off somewhere between 80° and 100°, with a fairly flat curve. How that point is reached turns out to vary a lot – but from then on, all reflow graphs I’ve seen so far look more or less the same. Which is all that matters.

The only improvement remaining, is to find a way to automatically come up with decent P, I, and D factors (I’m currently using 150, 5, and 700, respectively). I’m hoping that a calibration run which applies a step change will be able to solve this. The idea is to start at some temperature (below 40°C), then turn on the heater full blast until it reaches 120°C, then measure the slope at that point as well as the maximum temperature reached after the heater has been turned off. The slope says something about the heater power, the overshoot says something about the amount of “pent-up” heat stored between heater and thermocouple, i.e. the “thermal mass” of the system.

I’m delighted with my new setup, because it no longer needs to be chaperoned – I just turn the Reflow Timer on and walk away to start populating the next board. IOW, this reflow process is now a no-brainer and a time saver!

Onwards! :)

  1. Excellent, but what are the ham and cheese toasties like from it :-)

  2. The panel looks nice! I am curious about why there’s a cold spot. The aluminum plate heat spreader is not spreading the heat well enough? Heat is conducted away through a support point? Radiant heat from the top element is not uniform enough?

    • I think JC is suffering from one of the age old balancing acts… Thin plate reacts to changes faster, making it easier to control the temperature accurately, but unfortunately the thinner the plate, the less uniform the heat distribution will be.

  3. The picture of the Panel Plugs exhibits a strange optical illusion: when I scroll downward (using the wheel on my mouse) it looks like I’m zooming in. I had to roll up and down several times to convince my eyes of what I was really seeing. I suppose that any picture of a tesselated plane ought to do this (e.g. aerial photographs of cropland) – but maybe it’s a combination of foreshortening and the shallow depth of field? Both the rear and front rows of Pressure Plugs are slightly out of focus; that might account for it.

    By the way, clicking and using the vertical scrollbar destroys the illusion for me.

    I love this blog for many reasons; now I can add “slightly spooky photography” to the list.

    • Oooh, so it does! I feel dizzy now. It’s a combination of the perspective and the step size set for the mouse wheel. In my case it’s 4 lines in Firefox and the effect works.

      Reducing the browser window to a tiny letter box makes the effect even better!

  4. The cold spot is in the upper middle, where there is no heater (on both sides) – as you can see here. It’s no big deal, since it’s predictable. Sometimes I just turn the board around and run it through the reflow a second time.

    Maybe I’ll grow tired of it and get a bigger oven one day, but so far this little setup has served me well.

  5. If the cold spot is a non-problem, I guess I should stop thinking about fixes for it :-). Still, I wonder if increasing the temperature setpoint a few degrees would improve things. (I’m no expert, but have seen reflow values described up to 260 C.)

    • Which reminds me: I need to check what happens at temps 255 and up. There are some variables in the code which are stored as 8-bit values. The PID control is probably fine since it uses longs, but maybe there is some other issue lurking in there…

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