SMD Reflow Oven Project

liyf

SMD Reflow Oven Projectadr1an — Fri, 04/11/2011 - 2:23am


BackgroundThis project is to convert a fairly ordinary (read below - there is some caveats) Toaster Oven into an SMD Reflow Oven. To accomplish this, I've decided to combine an Arduino Mega (I have a few idle Rev A 1280's and need the I/O Ports for…) , a 3.2“ TFT Touch Screen interface, some Thermocouples + Associated Hardware and the Australian Robotics Smart Relay Shield .
I will update this page throughout the Build, but for the moment, heres some information and pictures of the early stages.
I also should really say upfront that you should not try any of this at home unless you are familiar with working with 240V AC and high currents. Do not mess with AC unless you are sure of what you are doing. And never leave this plugged in unattended. Who knows what bugs might be lurking in the code and what might happen. In fact - I'd suggest just walking away now and not even bothering building this project yourself - but do read my build log ..


Selecting an OvenWhilst you could approach this from 'any old oven will do' - and many do - however I decided I wanted to approach this from the best possible standpoint. When researching many other SMD Oven projects I discovered that there is a lot to be said for Oven Selection. Some of the considerations are:

• Size
  
  • The smaller the oven, the lower the “ramp up” time and less opportunity for issues with “hot spots” (sometimes). You are limited though to the size of the board thats possible to be reworked or reflowed. However thermal control can often be an issue.
  • The larger the oven, obviously the bigger the boards you can do. But this comes at the price of having a greater potential for “Hot Spots” and uneven heating profiles. This can often be a non-issue depending on the number and type of elements in use.
    
• Element Type and Number
  
  • Top Element Only ovens offer a cheap and fast route to building a reflow oven, but they can often cause issues with plastic components as the single element is often on for longer providing direct heat to the PCB substrate and can also 'cook' the fibreglass
  • Dual Element ovens are the middle of the road. With the addition of a lower element, you can often switch the two seperately - but you must research the model you plan to convert as often the elements will be wired in series - meaning individual control will require some extensive rework of the oven elements
  • Quad Element ovens are the bees knees. With any luck you'll have the must common setup of 2 top, 2 bottom, with both pairs wired in series sets - giving ultimate control over the heating elements in the oven (and thus combined with PID and multiple thermocouples the most optimum heating conditions)
  • IR type elements are the ones you want - almost all modern ovens use IR elements.
    
• Fan-Forced or Non-Fan-Forced (Normally Aspirated ? ;) )
  
  • This was a tough call. Fan-Forced ovens offer the fastest possible ramp time, elimination of Hot-Spots and ensure a smooth thermal profile across the entire oven and with some hacking, potentially a nice support on the cool-down cycle. But they are sometimes also responsible for basically blowing components around inside the oven! I initially, in the interests of costs, have opted for a non-Fan-Forced oven - but in hindsight… i slightly regret it
    
  
  

Electronic Components in Project

• **Arduino Mega 1280** . As mentioned above, whilst you could easily accomplish this with an MHVBoard or other smaller Arduino, I am opting to go for the whizz-bang touch screen display which means I need the extra I/O the Mega has to offer. Plus, I have a few old Rev A's I want to burn up.
• **Australian Robotics Smart Relay Shield**. This is a brand new product designed by one of MHVs own and sold through another one of MHVs own    An Arduino Compatible Shield featuring 4 channels of 10A/240VAC Relays, an ATTiny2313A providing a Serial or Standalone operation, XBee Footprint allowing easy Wireless operation and a large prototype area - its one mother of an Automation Shield. 2 Channels too many for this particular project - but the opputunity to be an early adopter for an otherwise perfect solution for my needs meant its in the project!
• **MAX6675 Cold-Junction Compensated K-Thermocouple-to-Digital Converter.** This is a Rail-to-Rail OpAmp combined with a Tempreature Sensor to do the conversion and cold-junction compensation required from a K-Type Thermocouple (which can produce signals as weak as 50mv). Its technically EOL now, however, supplies are readily accessible and its replacement, the MAX31855is not quite so prevalent yet. I originally was doing this the 'old school' way with LM358's (bad, but on hand) and LM335's for temp sensing (again not best choice by long stretch). Apathy set in regarding bad results, so have moved to the MAX6675 so I can start shaving other yaks.
• **Thermocouple Type-K Glass Braid Insulated**. There are two of these to provide both seperate 'zone' sensing as well as combining the two sensor readings for a mean average to assist with any calibration issues/margins of error.
• **3.2" TFT LCD Touch Screen + Shield**. This will be the user display and interface. I plan to show the Reflow profile in use along with plotting the current tempreatures and time in the cycle etc. Control Buttons (start/stop, profile selection etc) will also appear on this
  

The SoftwareThis bit isn't written yet beyond nasty hacking - I'll put something up on the MHV GIT shortly - however the basic flow is pretty simple:

• Power On and ensure all Relays are off
• Offer Ramp-Soak-Spike or other built in profile types (stored on SD card)
• Alternative Manual Profile with entered Set Points
• Wait for “Go” Button
• Commence the selected profile
• Switch Relays off and on using either element bank as appropriate and measure against the thermocouples and use “PID Magic” and setpoints to manage. Average the two thermocouples to cancel any reading errors.
• Once cooldown is reached, sound buzzer and power off relays
• Reset Cycle
  

The Oven WiringWell I've succesfully stripped down the oven to commence taking it over (muwhahahaha). I will be ditching virtually all the mechanical controls present! It uses a mechanical switch that then goes to a 3 pole selector (used for bake/grill/toast etc to control element selection via a dial) and a standard bi-metal strip 'tempreature sensor' *cough*. So will be junking all of that. The only bits that will remain will be the actual elements wired in series as they are - and probably the two 240V neon lamps used for the power and thermostats lights - or they might go to and be replaced with some LEDs.
The front slightly convex panel that the dials sit on will also go - to be replaced with a newly fabed panel for the 3.2” TFT - which will probably sit flat for ease of construction/mounting etc. This modification is fine, as the surrond has quite a deep lip on it.
Here are some images of the existing oven pre-modification. I stupidly didn't snap any the other night before stripping the shell - so you'll have to live with a Promo Shot for that one
I've ended up going with the Sunbeam BT5300 BakenGrill Oven. Heres their promo shot:

And heres some of the images I took of the initial tear down

The left hand side of the oven simply contains the links for the two element banks. One is insulated because of its proximity to the screws for assembly I assume.

The current Bi-Metal Strip Thermostat that is in place. GONE! it will be

Here you can see the mode selector switch and the wiring running off (or back?) from the top elements. The darker red wire coming from camera-right is the neutral that connects the far side of both banks in series returning to the thermostat.

The convex control panel seen from the rear. All those current knobs will be gone. Top to bottom the Bi-Metal Strip Thermostat, The 3 Mode Selector Switch and finally a combined Manual On/Timer Switch with builtin bell.

Here you can see the interior. The tray sits quite nicely in the centre of the oven. I'm going to shield over the elements to prevent direct heating of the PCB which can lead to scorching.

And all the original wiring seen slightly wider. Nearly all of this will go except for the wiring on the Elements. You can see the Blue wire linking neutral to the top element bank before running back to the thermostat.
Full size images should be visible in a set on flickr
STAY TUNED FOR MORE INFO AS THE BUILD PROGRESSES!