I've recently purchased a popular T962A SMD reflow oven. The reviews are split on this oven - some find it to be quite acceptable, while others find it completely useless. After much researching, I decided to give it a try.
(Feel free to skip this part if you already have the oven.)
T962A is an inexpensive IR (infrared) SMD reflow oven. It has a microcontroller to control temperature and timing (profile), so you can basically put populated PCBs in, push the button and simply wait for a few minutes. Then the PCBs are done.
There are some issues with this oven, but my experience is rather positive overall. For my purpose this oven works well.
However the problems are:
1. The oven produces horrendous smell.
I'm not taking about the smell of solder paste melting. The oven itself produces the smell even without anything in it.
2. The heat distribution is not very even.
This is not an issue with leaded solder, but becomes a major problem with lead-free solder. As lead-free solder requires much higher temperature to reflow, the extra temperature needed to ensure the solder reflow on all parts of PCBs can burn the parts in the hot spots. Even though I'm lucky enough to not fry any PCBs with this oven, some parts did come out looking burned.
I want to share what I did to improve on those issues. Step 1: Why So Smelly?
[url=]Show All Items[/url]
Like any oven, I expected some odor/fume to come out on the first few runs. But this oven is no ordinary oven in that respect.
Upon its first run, it produced something that I have never smelled before. It was so toxic, that it gave me a bad headache. And the smell did not go away even after a few runs.
So I decided to open it up and see what's causing the odor.
Opening the hood was quite simple, just pull the drawer out, and remove a few screws. Then the top is open.
Then there was the source of the problem - masking tape! Masking tape is used to prevent electrical shorts between the aluminum foil used to cover the insulation material and the controller PCBs. Even though there's heat insulation material beneath it, the tape gets hot enough to produce that nasty odor. So I removed all of the masking tape, and replaced it with Kapton tape, which is heat resistant.
(Sorry I forgot to take the picture of the masking tape before removal. You can see some pics here though.)
This mod took care of the odor completely.
Step 2: Improve the Heat Distribution
[url=]Show All Items[/url]
Looking at the inside of the oven, got me thinking - is there any way to improve the uneven heat distribution?
Infrared reflow oven has an inherent weakness - because the infrared heat is absorbed at different rates depending on the surface color, black ICs get heated up quickly, yet metal parts heat up slowly. Temperature profiles are used to give some "soak" time to let the temperature even out between different parts of the PCB. However that can only help so much.
Modern reflow ovens mostly use convection technology. Convection ovens use moving air to deliver the heat. This method distributes heat much more evenly then IR method.
T962A has a cooling fan in the back. This fan sucks air into the oven to cool down the inside. The heated air comes out of the bottom of the oven, which has many slots.
So I thought, why not use this fan to give some air movements during the heat up, to help distribute the heat?
Looking at the controller closely, it is revealed that:
Heating elements are controlled by a SSR (Solid State Relay) via 5 V logic voltage from the controller.
Fan is control in the similar mannor - logic signal controlling a triac via an opt-coupler.
Which means that I should be able to sense whether the heater is on or off, and turn on the fan as I wish.
I connected four wires to get the necessary I/O out of the controller board. and routed the wires out. Connecting the switch to the fan controller line proved that it works.
(If you are doing this please see comments on the photos - there are some importent information in there.)
Arduino to the Rescue
Now I got the signal that tells me whether the heater is on, and a line that lets me turn on the fan when I want. I believe by blowing just the right amount of air when the heater is on would improve the heat distribution. So I took out my Arduino, and wrote up a simple code. The code basically senses if the heater is on, and if it's on runs the fan at a low speed. To run the fan at a low speed, I chose to do super low frequency PWM, which ended up being turning on the fan for 0.1 to 0.2 seconds, and turn off for 0.8 - 0.9 seconds. I did it this way instead of using Arduino's PWM output, because the fan controller may not respond well to the signal going on/off hundreds of times a second. I'm using 15% duty cycle to run the fan.
By running the fan slowly enough, the hot air inside is moved around to help distribute the heat. The heater is strong enough to still heat up quickly.
The result is an improved heat distribution, and seemingly more accurate temperature reading. I think the air movement helps the thermocouple (temperature sensor) inside the oven to sense the air temperature.
More testing and tweaking might be needed to get the most out of this modified oven. But so far I'm very happy with this.
