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In the past, I used a hot plate for reflow, but because it was so small, I could only reflow a small amount and the temperature did not rise easily.

So, I decided to use the oven for reflow.

Selection of models

Convection Oven

The most important thing is that it is a convection oven with a blower inside the oven.

In a normal oven, the area near the heater is heated strongly and the area far from the heater is weak, resulting in uneven temperature.  If you put multiple substrates in, you may end up with overheating or underheating depending on where you put them.

The convection oven can reduce such temperature unevenness.  Convection ovens have a blower function in the oven that circulates the air in the oven while the heater heats it.  This air circulation has the effect of heating evenly regardless of the location.

In order to reflow the substrate at a uniform temperature, I decided to select a model from the convection oven.

Fine temperature setting

 

Temperature settings are also important.  As much as possible, I want to heat the solder paste to match the temperature profile when it is heated.  A model with fine temperature setting increments is preferable.Furthermore, it must be a type that can change the temperature setting during heating.  Convection ovens are more expensive than ordinary ovens, so it seems that many models have high functionality.  However, due to their high functionality, some models do not allow you to change the temperature after the heating has started.Hitachi Convection Auburn Toaster HMO-F100 cannot change the temperature setting during heating, so you have to stop, change the temperature, and start heating once.

Tescom oven selected

I checked various manufacturers such as Hitachi, Iris Ohyama, Tiger, Delonghi, and decided on TESCOM's low temperature convection oven TSF601 because of the fine temperature setting and the possibility of heating and drying at low temperatures.

Purchase & check operation

Buy

This oven can also be used for salad chicken, roast beef, and yogurt because of its ability to cook at low temperatures.  If you use it for reflow, you can't use it for cooking, so it's a bit of a waste, but it's unavoidable.

It's a pretty good design.

You can also choose the temperature.

Two heaters, one above and one below

Two at the top.

Two at the bottom.

It is better to have a lot of heaters so that the temperature can be heated evenly from place to place.

Temperature unevenness check

Now, let's measure how much the temperature is actually uneven with a thermocouple thermometer. Set the temperature to 100°C and measure the center of the oven, the back right side and the front left side of the oven.

I set it to 100 ℃, but it's about 15 ℃ higher. There is little temperature unevenness, and the temperature unevenness is ± 5 ° C. Isn't it good?

Find the settings that make my profile look good

Now, the above graph shows the temperature profile of the solder paste used for reflow. Low-temperature lead-free solder paste with a melting point of 138°C.

I tried several different temperature settings so that the temperature inside the oven would change to match this curve.  As a result, 90 degrees 180 seconds, 140 degrees 60 seconds, the door opening and closing, and cooling with the air blower felt good.  It becomes like this when the temperature change at that time is plotted.

Light blue is the temperature measured at the center of the tray, and green is the temperature at the far right.  There is a temperature difference of about 10 degrees depending on the location, but it is roughly the same as the temperature profile of the solder paste.

Try to actually reflow

Let's try reflowing. I'm going to reflow the board of the kit FCAV that can convert a Nintendo Famicom (NES) to stereo and video output.

Mount only one regulator on this board.

Applying the paste

Apply the solder paste to the pad with a solder paste gun. Here are the details of the solder paste gun 👇👇👇.  It's a very useful tool that you won't find anywhere else.

Mount the regulator

Place the regulator in the correct position with an electric vacuum pick. Click here for details on the electric vacuum pick.  This tool is very handy and is definitely recommended for those of you who are mounting multiple parts by hand.

Heat at 90°C

Heat at 90°C for 180 seconds.  The temperature in the center rose to about 120 degrees.

Changed to 140°C

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The 140°C setting actually only went up to about 150°C, so it was heated at the 160°C setting.  The target of 160°C was reached.

Cooling

Open the door and use a fan to send air to cool it down.

I was able to mount it perfectly

 

I was able to mount 4 cards at the same time.  In terms of space, you can have 8 sheets at the same time (about 20 cm x 20 cm).

Perfect mounting.

Built-in temperature sensor

A thermocouple thermometer was fixed to the wire mesh so that the temperature sensor would not move during heating.

I will measure the temperature in this area in the back.

I bought a nice oven!

The temperature setting will need some trial and error for a while, but it was a pretty good convection oven. Since it can also be heated at low temperatures, I think it could be used to dry filaments for 3D printers.

This convection oven seems to have a good reputation, and at the time of this writing, there doesn't seem to be much in stock. Check it out if you see it.