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SMT (Reflow) Toaster Oven - Construction

 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. This construction has been inspired by the one described in  Conversion of a toaster oven into a SMT reflow oven .

0000194Health and Safety Message: Most websites that explain the toaster oven to reflow oven conversion process have some type of health and safety warning attached to them so I'd better do the same! In the U.K the mains voltage is 240V and like the mains voltage in most  countries can be extremely dangerous. Modifying a toaster oven could involve encountering a voltage that  could cause you or your nearest and dearest great distress. This conversion should be performed by a trained and certified electrician that knows what they are doing. [N.B Plugs in the U.K are fused which is not the case in some countries. The conversion process demonstrated here uses a fused plug.]



 

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Figure 1: Major Components: This figure shows the major components required to convert a compact oven into a SMT reflow oven. (1) Well, you need a compact oven (aka toaster oven,  electric oven etc). The capacity of the oven required could depend on the size of PCB's that you anticipate assembling. For this project I choose to use one of the smallest sizes - a 6 litre one. (2) PID controllers as you can imagine come in a variety of types and sizes. 

The one used in this project is an Universal Digital PID Programmable Temperature Controller that is sold with a "free" 0 - 400C K-type thermocouple. (3)Terminal assortments are extremely useful especially when you consider the health and safety message in Figure 1. (4) Soldering mats are used to provide thermal insulation for the instrumentation compartment. Other types of insulators could be used too.

(5) Solid State Relays (SSRs) are characterised by the maximum amount of current that can flow through them. For this project I decided to be on the safe side and use a 40A one. This could be considered to be an over kill but health and safety took precedence when I chose this part! (6) RTV heat resistant silicone sealant and  adhesive.

 

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Figure 2: Major Components Costs : The pie chart above shows the cost (16-Feb-2010) of the major components used in the project when purchased from our favourite online store (.co.uk) The darker shade of the same colour shows the average postage and packaging cost for an item shaded in the lighter colour. 

The darker shade suggests that if you live close to a hardware store it could be beneficial to pay it a visit as quite a considerable percentage of the total project cost could be saved on postage and packaging alone. You could also attempt to purchase the items second hand if possible especially the toaster oven as the conversion process will almost definitely invalidate its warranty.

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Figure 3: Compact\Mini\Toaster Oven : The compact oven used was one from Signature that has a rating of AC 230V, 50Hz and 600W (1) with a capacity of 6 litres. It turned out to be quite a good oven to use for this project as it was fairly straight forward to disassemble.

The triangle head tamper proof security screw (2)  should provide no opposition for a trained and qualified electrician! This oven has two heating elements (3) one at the top and one at the bottom. The top control knob (4) is to turn the top, bottom or both heating elements on or the appliance off. The other control knob (5) is a timer that allows the oven to be turned on for up to a maximum of 15 minutes.

For this particular application I decide to remove the front panel that hosts the knobs completely. If I hadn't have done this it would have meant cutting through the metal panel to mount the PID controller.  In some SMT toaster oven conversions, described on other websites, the timer and upper control knob are used as part of the final control circuitry. However, in this case I decided to rely on the PID controller for all control (see below).

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Figure 4: A Tale of Two Compartments :  A toaster oven typically consists of two compartments. One for baking sausage rolls or PCB's with SMD devices (1) and another that could be considered to be the component or instrumentation compartment (2).

The aim of the toaster oven to SMT reflow oven conversion project is to replace or augment the basic analog control circuitry with a sophisticated embedded computer  system to control the reflow solder process.

The instrumentation compartment, of the signature oven, has convenient mounting points (3) (which were used to secure the SSR mounting bracket - see below). The heating elements in this unit used a convenient screw and nut feature (4) for securing terminal leads and did not  employ a more sophisticated tamper proof securing method. This was quite convenient as the latter offer more resistance to change even for the trained electrician! 

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Figure 5: Solid State Relay and Mounting Bracket:  A bracket to mount the SSR relay was cut from aluminium  sheet metal (1). The bracket (5) is used to mount and conduct heat away from the SSR. The SSR could be considered to consist of a control part (4) that accepts, in this case, a 3 - 32VDC voltage and a switching part (3) used to switch the heating elements in the oven on and off. Terminal connectors (2) are used to connect the switching part of the SSR to the heating elements. The switching part of the SSR can accept voltages of between 24 and 380VAC. 

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Figure 6: Insulating the Instrumentation Compartment:The instrument compartment should be insulated to ensure that any instrumentation placed within the compartment operates within an environment that is tolerable, temperature wise. The compartment is insulated by using soldering mats which can typically resist the high temperatures attained during the SMT reflow process. The mats that I bought can resist temperatures of up to 600C.

Annoyingly though, one mat at 300mm by 240mm was just too small to fold in half to create the two layers of insulation required, so I had to use two mats cut to shape. Dabs of RTV silicone adhesive (1) were used to secure the mats (2) in place.  The  SSR mounting bracket (3a) was tethered in place using M3 screws and nuts (3c).

The mounting bracket  ended up being in a convenient location as it is coincident with the air flow  provided by the air vents (3d).  RTV silicone sealant was also placed around the electrical contact (3b) of each heating element to insulate the contact from any electrical conductivity associated with the soldering mat. 

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Figure 7: Completing the Story: A K-type thermocouple (1) rated at 400 degrees Celcius was included for free with the PID controller (3).  The PID controller has been connected to the thermocouple and temporarily mounted on an acrylic sheet (6) cut into the shape of the front panel. A hole has been drilled into the side of the instrumentation compartment and the thermocouple has been passed though to the heating chamber (4). The SSR (2) is mounted on the aluminium bracket to complete the construction of the instrumentation compartment. 

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Figure 8:SSR Terminal Configuration: After the cover to the instrumentation compartment was fitted back into place, the toaster oven was turned on. To my surprise a negative temperature was reported on the front panel of the PID controller (3). After going over the problem for a while, well a few hours actually, I decided to revisit the connection of the thermocouple to the PID controller.

Initially the thermocouple was connected to the SSR as in (1) which produced the negative reading. I then decided to swap the thermocouple leads around (2) such that the +ve terminal of the PID controller connects to the -ve thermocouple lead and vice versa which solved the mystery, although I am still baffled about what thermocouple colour codes actually mean! 

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Figure 9: Result!:The now working SMT reflow oven appears to give excellent results and the PID controllers seems to be remarkably accurate.

 

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