SMT Oven

Up ]


Building a Surface Mount Oven
for Under $400

Supplies Needed and Approximate Cost

  • Tabletop size convection oven1 ---$74.50
  • Temperature Controller that is Ramp & Soak Programmable2 (CN1501-K) ---$219.00
  • Temperature sensor for controller, we used a type K thermocouple but other thermocouple or RTDs would also work.  (CN1501-K)
  • Thermocouple- self adhesive type K although any will work (SA1-K)  $60 for pack of 5
  • Panel mount jack for thermocouple (RMJ-K-F)  < $5.00
  • High Heat Paint from hardware store- flat black ---$8.00 per can
  • Power Cord
  • Duplex 110V AC power outlet, 15A or greater
  • Metal Case (must fit the temperature controller and power outlet)
  • 20A minimum solid state relay (SSRL240AC25) ---$30.00 (or less surplus)
  • Soldering Iron & Solder
  • Screwdriver Set
  • Needle-nose pliers
  • Painter's Tape

1Any convection oven should work.  The oven we chose was the Sanyo SK-VF7S Digital Convection Oven, Stainless Steel Finish.

2The temperature controller we chose was the CN1501-K.


Assembly Instructions

Remember from basic physics that there are three main types of heating:  Conductive, convective, and radiative.  We need to change the properties of the oven you bought to balance these three types of heating.

Maximizing Convective Heating

  1. Completely disassemble convection oven.
  2. During this process be sure to note the different styles of screws and the parts of the oven that they belong to.  This will come in handy when the oven is reassembled.
  3. Although the oven's heating controls will no longer be used, it is important to identify the connections to and from the dials and heating elements.
  4. Adjusting the oven fan:  Convection ovens have a small fan in the back.  The purpose of this is to have hot air flow over your board to convectively heat all the components evenly.  The more air flow the more even and faster the heating.  Though these fans circulate the air within the cavity well, there is a way to improve the circulation with a minor adjustment to the fan blades, at least for the oven we purchased.
  5. Readjust the blades of the fan by bending them from the 90-degree angle to an angle of around 45 degrees.
  6. Connect the fan back to its motor and turn it on.  If there is no noticeable difference in circulation of air you may want to use a different fan blade.  If you do, remember that the fan must be able to endure high temperatures!

Minimizing Radiative Heating

Most ovens are stainless steel or aluminum inside the cavity.  This is an almost perfect reflector for heat.  Since the reflectivity of solder and the copper traces on the board is very high, and the reflectivity of the components (particularly black encapsulated IC's) is very low, the components will get hot much more quickly than the board which is bad.  To fix this we make sure there is not direct line of sight from a heating element to the PC board.

  1. The first step is to spray the oven interior black using flat black heat resistant paint.  Spray only the oven interior black.  Do not spray the entire oven black…just the cavity!  Make sure you have removed the heating elements- don't paint them.
  2. The next step is to cut from thin gauge metal two covers for the heating elements.  Most toaster ovens have covers with lots of holes over the heating elements already.  We just cut two strips of 1/16" aluminum about an inch wide and wired them on the existing covers.  Paint these flat black too.  The point of this is to make sure there is no direct line of sight from the heating element to any part of the PCB
  3. Most ovens have a glass panel on the door.  The inside face of this glass must also be painted black however you must leave a section unpainted to be able to view the PCB.  Place a strip (or two) of painter's tape on the inside of the door.  Make sure you remove it before turning on the oven!
  4. Follow the instructions on the spray can and make sure to apply at least two coats of paint.

One disadvantage of the black paint is that the sides of your oven are going to get a lot hotter than they normally would.  Make sure when you operating you don't touch the oven!

Conductive Heating

To accomplish conductive heating, an aluminum plate 1/16'' thick replaced the oven rack. This surface is where the PCB will be placed.  The aluminum plate was painted black in order to reduce the reflection of heat and make sure it got hot as quickly as possible.

 

Putting it all together

Rewiring the heating elements

The side panel of the oven will no longer be used.  Instead, the heat elements will be controlled by the temperature controller.

With a soldering iron, connect one wire from the heating elements to the hot wire (black) of the power cord and the other to the neutral (white) wire..  The elements should be wired in parallel, not series.

Connect the ground wire from the power cord to the case for safety.

This power cord will then plug into the temperature controller which is connected according to the diagram below.

There will be two cords coming from the oven.  One is to control the heating elements, the other operates the fan.  You want the fan to run continuously, even when no power is applied to the heating elements.

Use the diagram above to wire the temperature controller and thermocouple.

It is important to mount the thermocouple DIRECTLY onto the surface on which the PCBs will rest.  Not doing so will result in the thermocouple's measurement of ambient temperature and not the surface temperature of the PCB.  Run the wire out through a hole in the oven and provide some strain relief.

 
This is a picture of the controller and relay placed in a box.



Here is the completed temperature controller

 

Programming the Temperature Controller

We use the manual from the temperature controller to program the ramp and soak profile.  The specific values for the ramp and soak profile we developed can be seen in the table below.
  • Stage 1:  Preheat to 50 °C in three minutes
  • Stage 2:  Rapid ramp from 50 °C to 170 °C in five minutes
  • Stage 3:  Slow ramp from 170 °C to 180 °C over two minutes
  • Stage 4:  Rapid ramp from 180 °C to 220 °C in two minutes
  • Stage 5:  Five minute cool down to 25 °C

 
Graph of measured temperatures when characterizing oven

 

Does it Work?

Yes, very well.  The photos below show boards we milled using our T-Tech PCB mill and ran through our home-made oven

A board with solder paste prior to cooking. The same board after placing parts and cooking.

Other SMT Oven Projects

 

 


Important Links
Sharepoint ] Catalog ] Desire 2 Learn ]

Search for:


The material appearing on this web site is copyright © 2004-August, 2009 by the School of Electrical Engineering at Oklahoma State University. All Rights Reserved.