[sorry for my latter incomplete message(s) sent, it seems Outlook has tur=
n
crazy today...]

Hi Steve,


----- Original Message -----
From: "Steve Baldwin" <steve@tla.co.nz>

| > I am not even planning to read and plot temperature curves.
|
| I did and that gives me enough information to manually control the
| oven to get a near perfect temp curve. I have a thermocouple on the
| same plane as the board being soldered and use that to control
| what I turn on/off and when.


A good idea is to build a thermal datalogger PCB with several
thermocouples over it, obviously soldered with high-temperature solder, t=
o
record the profile in several points of the board. That's the way the ove=
n
'tuning' it's done on industrial reflow linear ovens (Siemens SIPLACE one=
s,
for example).


| Turn on the oven with both elements on full and the fan on.
| When the temp reaches 150=B0C, turn the elements off and leave the
| fan on. I know that the temp will overshoot by 5=B0C and won't fall
| back down below 150=B0C within the 2 minutes preheat time.
| After the 2 minute pause I turn the fan off and put the top element
| on full. In just over a minute, it will reach 200=B0C. When it reaches
| 215=B0C I turn it off, knowing that it will overshoot to a max of 225=B0=
C.
| At that point the fan goes on and when it drops to about 190=B0C, I
| open the door.
| The whole process takes about 10 minutes which is a welcome
| break and gives your eyes a chance to refocus on things further
| away than 6".
|
| I have a pneumatic paste dispenser which is way faster than a
| manual syringe but if I was going to improve the process anywhere,
| it would be making a solder screen. It would reduce the paste
| application stage to a few minutes, regardless of the board
| complexity. I can't see the point in messing round with an x,y
| applicator when a screen will use a lot of the same techniques and
| equipment as making a PCB.
|
| Steve.


I've done a little research on this topic, mainly on industrial equipment=
s
available, from a practical point of view. Here at our School we need to
build such an oven for development purposes (fine-pitch DSP's...).

About the screen, the guys i've met said that the industry standard is a,
LASER made, stainless steel stencil (=3D $$$...). They never saw a screen
doing the job. However, we'll do some experiments with a screen, mainly d=
ue
to economical reasons.

We've seen the "Toaster Oven " page from the Seattle guys, and their
amazing results with only an "On/Off" control law over a first order
plant.
We'll try with some feedback. We've just recorded our plant's (grill
oven... :o) temperature step response (2 thermocouples hanged). The data
obtained fits very well an exponential response, with some delay
(verified w/MATLAB). The main control problem is the plant's delay.


Best regards,

S.-

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