Why do we need a reflow soldering oven?.
Reflow soldering is simply unavoidable if we want to place lots of tiny components in a really small area. Nearly all commercial components today are designed to be small and soldered in ovens, so if you want to make high tech pcb designs you have use ovens. For example if you want to use BGAs (Ball Grid Arrays) you really don’t have the choice, these components don’t have any accessible pins to solder!.
How it really works?.
Reflow soldering is based on baking a component following some temperature ramps and phases. So it is not only a question of heating the component until the solder melting temperature, you can obviously do that but you risk destroying the component or not melting completely the solder.
Components suppliers will normally recommend the profiles for temperature you have to keep. They will recommend maximum heating/cooling ramps a reflow temperature and maximum times for every phase to avoid damaging the component.
How can I make this?.
You can make pretty decent reflow soldering machine using inexpensive baking ovens. The only problem with baking ovens is that the temperature control is just a ON/OFF control system, not really accurate in temperature and where you can not control at all the heating/cooling ramps.
That’s not really a problem, with our controller you can just modify a standard oven to have a nice and accurate temperature control and ramps.
Our controller will have its own temperature sensor inside the oven and it will control heating by connecting and disconnecting the oven power supply really fast (several times per second) with different times for ON and OFF. This is called PWM (Pulse Width Modulation) and it is used all over the world for temperature regulation and for motor speed control.
In the other side our controller is equipped with a high performance 16 bits microprocessor that will decide the width of the pulses of energy to the oven with a PID controller (Proportional, Integral, Derivative). This kind of controller has been the standard for temperature/speed regulation for the last 25 years. This kind of controller is adaptive, meaning that it reads the temperature and the speed the temperature changes and makes decisions on the next pulse width to maintain the temperature stable.
So our installation to make a reflow toaster oven will look like this
The simple re-flow oven is composed by the parts you can see in the previous diagram:
- OVEN: The oven can be a self-made one, see here for more information, or a simple commercial oven. If you choose a commercial toaster oven try to find one that can reach temperatures over 250ºC and if possible that have infrared quartz lamps. Other ovens have quartz solid heating elements but you will not reach really nice heating ramps with those, they will be much slower on heating and cooling. If you don’t find ovens with infrared lamps you can use solid heating quartz elements but the results will be worst. If you find a oven that can be fixed to over 250ºC you an use it without disconnecting the build-in thermostat, only by fixing the thermostat and the timer to the maximum.
- TEMPERATURE SENSOR: For the temperature sensor we have chosen K type thermocouples. The advantage of these sensors is that they show a really fast thermal response which is necessary for an oven with ramps of 6ºC/sec. Another advantage is that they are easy to find, and they can withstand really high temperatures, depending on the materials of the cable they can measure temperatures over 1000ºC.
- SOLID STATE RELAY (SSR): We use the solid state relay to switch the power supply of the oven at high speed. The reason is that conventional relays have moving parts so you can not switch them too fast or they will fail. Solid state relays don’t have any moving parts so they can switch really fast. The relay you should use is a 230/120Vac (if you are in the States) relay in the load side and it must be possible to control with a 5Vdc signal. Solid States Relays are sold in many sizes depending on the Amps they can handle. Your SSR should be dimensioned depending on the power of your oven, one really important thing, nominal Amps capacity of SSRs are given always considering that the SSR as a big heat sink attached, so if you don’t want to burn things please follow the indications given in the SSR supplier datasheet concerning heat dissipation.
- CONTROLLER: This is our small circuit, he will control the SSR switching with a 0-5Vdc signal and modify the power sent to the oven depending on the temperature. See more information here.
- COMPUTER: Our controller is not “stand along” so you will need to connect it to a computer to manage it. The computer is also used to visualize and store the heating graphs, and configure the controller. Any computer with windows and a USB port will be OK.