Created on Sunday, 16 October 2011 12:59
Last Updated on Monday, 09 January 2012 20:55
This article, is part of a set of articles, that demonstrates the conversion of a compact electric oven, popularly known as a toaster oven, into a surface mount technology (SMT) reflow oven. In this article the measurement of the temperature ramp rate, of the toaster oven, is undertaken. The measurements, which are done upto a temperature of up to 250 degrees Celcius, are performed to help characterise the oven under different combinations of loading and insulation.
Figure 1 : Compact Oven Parts: A compact or mini toaster oven consists of quite a few parts that can impact on the temperature ramp rate these are (1) the grill rack, (2) the glass door, (3) the baking tray and (4) the heating elements.
The temperature ramp rate of the toaster oven converted for use in the SMT reflow process (Figure 2) could be critical to the success of the project as too high or too low a temperature ramp rate could mean that the solder will not reflow correctly. A consequence of soldering devices with an inappropriate soldering reflow profile could mean that the soldered components could exhibit erratic results due to internally cracked solder joints or other soldering anomalies.
Figure 2 : Temperature Ramp Rate: Depending on the sources you read and the type of solder used, the temperature ramp rate of the soldering temperature profile could be between 0.5 and 3.0 degrees Celcius per second.
The temperature ramp rate has been measured crudely under different loading and insulating conditions by noting the measured temperature of the PID controller every 5 seconds when the set temperature is 230 degrees Celcius. The PID controller has been set to act in P mode only (I = D = 0). The loading and insulating conditions are as follows:
- Baking Tray : Only the baking tray has been placed in the oven and the glass door has been left uninsulated.
- Aluminium Sheet and Baking Tray : An aluminium sheet, that covers approximately 85% of the glass door, has been placed inside the toaster oven and in front of the glass door to act as insulation. The baking tray was placed in the oven.
- Aluminium Sheet, Baking Tray and Soldering Mat : An aluminium sheet, that covers approximately 85% of the glass door, has been placed inside the toaster oven in front of the glass door. The baking tray has been placed in the oven. In addition a single layer of soldering mat has been placed on top of the toaster oven to prevent heat loss from the top of the toaster oven.
- Tin Foil and Baking Tray : Tin foil has been used to cover the glass door to act as insulation. A baking tray has been placed in the oven.
- Tin foil and Grill Rack : Tin foil has been used to cover the glass door to act as insulation. A grill rack has been placed in the oven.
- Grill Rack : Only the grill rack has been placed in the toaster oven and the glass door has been left uninsulated.
Figure 3 : Measurement Results: The result of insulating and loading the toaster oven under different conditions. The best temperature ramp rate result has been achieved (~1 degree Celcius/s) when the glass door has been insulated and the baking tray is left in the oven. The worst result is achieved (<0.5 degrees Celcius/s) when the glass door is not insulated and the baking grill is left in the oven.
I intend to analyse the results more comprehensively in the characterisation article of the SMT reflow oven series however, it is safe to say that the results suggest that too much heat is being lost through the uninsulated glass door. The temperature ramp rate tends to be higher too when the baking tray is left inside the toaster oven compared to when the grill rack is left in the oven.
Figure 4 : Insulation Material : (1) Reflect-a-gold is considered to be the Rolls Royce of insulation material and almost has a price tag to match! It is described as " .... state-of-the-art polymer laminated glass cloth is extremely light weight and capable of handling continual operating temperatures up to 850º F, reflecting 80% of all radiant heat while offering a weight savings advantage versus other types of reflective materials". (2) Aluminium tape is commonly available and much cheaper to buy and it should also provide adequate insulation for the SMT reflow oven project although not as efficiently as the reflect-a-gold material.
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