[TriEmbed] T-962 reflow oven notes

[Pete Soper]

Chip,
   I've been very busy lately and won't be able to publish some oven 
measurements for a while longer. But in a nutshell, the thermocouple 
probes are sitting in mid-air and do not properly track the temperature 
of the PCBs in the oven: they lead the heating and cooling such that the 
oven's notion of temperatures is wildy optimistic at the start and end 
of a cycle. For a goodly part of the middle period the PCB temperature 
is *exactly right* for a small amount of PCB in the oven. That is, for 
the point at which the PCB has heated up to the same temperature as the 
air surrounding the oven probes the accuracy seems nearly perfect (and 
then it goes to hell again when the cooling fan comes on). But for large 
amounts of PCB (two 4x4" boards) the situation is just hopeless: there 
is error all the time,  it's grossly off at the beginning and end, and 
*the boards do not get hot enough to reflow*. Other than that it's 
great. :-) So currently with a few small boards the profile is WAY 
longer in time than Kester calls for with the solder paste we've been 
using. The heating is too slow and the cooling is way to fast, with the 
board holding a high heat for very much longer than called for.
   But for a few small boards like we've been doing, when the cooling 
"overshoots" and dips badly that is not reality: the temperature is not 
dropping that fast, the temperature of the oven's thermocouples are 
dropping that fast. On the other hand, we did see uneven heating when we 
had a handful of little boards spread across the drawer. I think we can 
avoid that by making the boards physical relationship to the radiant 
heaters more uniform instead of having boards directly under them and 
other boards getting "late winter" illumination at a shallow angle. What 
I'm trying to say is we should keep the relative distance between 
heaters and board surface as uniform as possible. The drawer could 
perhaps be marked to show where the tubes sit.
   What I think is called for is arranging for the oven thermocouples to 
sit on the surface of the boards to be soldered. Gene did this at HP or 
someplace like that and they very accurately measured the board temps. 
(and this was the scheme for the measurements above: a thermocouple 
pressing right against a pad on a board about 2x2". I haven't pulled the 
oven apart far enough to establish whether there's any excess probe 
wiring that would support sticking the probes on the board surfaces or 
whether the wires would have to be extended. And with this scheme a 
little air movement might tend to arrange the convective cooling that 
seems so sensible at first blush. But it isn't clear the oven is capable 
of doing the right thing with a PCB that covers most of the drawer 
space.  What all this tells me is that there's a very wide lattitude 
with this kind of soldering: it's hugely more forgiving than one might 
think. (I agree with the notion that you could just watch a board in a 
toaster oven and solder it by the seat of your pants, while still 
believing the accuracy of this and frying pan techniques is probably off 
by a mile in relation to what the SMD component manufacturers want you 
to do.)
   A second best approach might be to heuristically determining a custom 
heating profile that intentionally tricks the oven into heating up and 
cooling off faster.  There would have to be multiple fake profiles 
depending on the total area of PCB inside. This is a total kludge, but 
doesn't involve messing with the guts of the oven. I'm not keen to screw 
the oven up and leave us wondering how to continue the sheer adventure 
of soldering the chips that cannot be hand soldered. :-)
   The mod that guy published that turns the main cooling fan on and off 
in concert with the heating tubes being on is, in my opinion, not 
worthwhile. In my opinion goosing the fan, waiting two seconds, then 
cycling it on and off while the heat is on will simply stress the PID 
control algorithm at best. I could see arranging for a small amount of 
*constant* air movement, but even that is very likely to exacerbate the 
spread between what the oven thinks the temperature is vs the real PCB 
temperature.
   Finally, the notion that the masking tape is creating the smell from 
this oven seems to be a red herring. The masking tape doesn't get much 
over 90F. When I opened the top of the oven and looked inside I became 
extra suspicious about this masking tape business, because 1) the tape 
is a couple inches from the control electronics (would they really want 
that to get chronically baked?) and 2) the tape looks and feels nearly 
new. It has nothing of the "is this tape or some kind of glue layer?" 
look of old, abused masking tape. There's no doubt at all that the 
vaporizing flux and stuff in the solder paste stinks to high heaven, 
though! (I got the killer ductless fume hood set up, by the way. 
Pictures coming when my schedule lightens up.)
   Also, I found this oven for $265 shipped from China, which is kind of 
amazing (that they could ship it for free). The larger T-962A that is 
much more popular is still commanding a much higher price.
   Gene Kahn helped me with some of these measurements and we used his 
amazing Fluke meter with a type K thermocouple. I cheated by video 
recording the oven with the meter next to it and then fast-forwarding 
the recording to quickly jot down elapsed times and temps at intervals. 
I recommend this to folks who would go stir-crazy watching the oven for 
a dozen eight minute runs. :-)

-Pete