A Program to Control SMD Soldering Using a Toaster Oven
Contents
1. A Program to Con Soldering Using a Toaster e d 7 Ever want to try SMD surface mount device soldering without using a magnitying glass and super steady hands Production houses use ovens which cost thousands but it s possible to do a reasonable job with a regular toaster oven You just need to control it correctly Enter the SparkFun Electronics Retlow Toaster Controller with reflow soldering SMD devices at home but it came with only rudimentary software As a learning project it had many features to explore including a PIC16F88 MPU thermocouple temperature sensor using the A D converter control buttons for inputs an LCD readout and a serial port with boot loader for quick program changes A personal goal was to get back to low level near assembly code programming after decades of being out of touch Many months and 1 500 lines of C code later this little controller can really do a lot including programmed modes and data logging to your PC Ts controller looked like a great project for experimenting Getting Started The controller hardware kit is a snap to build well 1 This project involves line voltage connections which are potentially fatal if mishandled Double check all wires before applying line voltages 2 Using the Controller for reflow soldering can result in operating a toaster oven at extremely high temperatures which can burn fingers or cause fires 3 Solder materials whet2. you want to apply it to other things The original SparkFun software which does have many useful basic code examples in it has been heavily modified and expanded to get the resulting code described here The 4K memory limit of the PIC makes life really tough for getting sophisticated with the program Every extra feature or line of code was scrutinized Heavy use of functions subroutines was required to eliminate redundant code The fact that only 200 memory locations are left out of 4096 is an indication of how tight it is The user interface is further hindered having only 2x16 characters on the display and just three control buttons there is a fourth button for reset only lt 2 5 C Sec Temperature C however a lot can still be done as seen in Figures 4 and 5 The top line contains the settings for the current mode of operation and the second line shows the functions for the three buttons marked up down and select on the PC board Each button can have two functions depending on whether a short push or long push greater than two seconds has been made The slash mark separates short long functions A User Manual has been prepared which details all the capabilities and operating procedures The modes are described briefly as follows x none 1 Main Menu This comes up first after the splash screen and allows the user to select one of the five operating modes This is t
3. Time constant helps the control system compensate for the lag time in heating up the oven elements In addition the controller program has a data logger function which will output the time set temperature actual temperature and relay on off conditions to your PC Data points are sent once per second It uses the built in serial port on the board a nice to have feature and is directly readable by HyperTerminal or similar programs The data can be easily copied into Excel and analyzed or plotted as seen in some of the plots in this article Software Tools Selection Every programmer has to select tools and it s never an 70 NUTS2VOLTS June 2008 easy choice Cost complexity including the learning curve and capability are all factors In selecting BoostC over CC5X which seems to be the standard compiler in most articles the cost differential of many 100s was important As this was my first PIC programming effort and not knowing whether would like it or be successful or ever want to do it again opted for a very low cost entry into the process It turns out that this compiler behaves very well the cost is incredibly low and there is excellent user support through the forums could often get a response to a question back in 24 hours from the creator of the compiler didn t come across a single bug either although a few features could be improved such as error messages It also integrates right into the
4. just enough for regular solder to melt but not for lead free RoHS work found that by adjusting the little metal tab on the temperature dial inside the oven cabinet could raise the limit to 250 C which is enough Of course unplug the oven before taking screwdriver in hand The second attempt then FIGURE 6 Toaster oven heating curve Manual Mode looked like Figure 6 Notice there is a time lag while the elements heat up then the slope increases nicely The slope of the curve at any point is the degrees per second that the oven can heat This is important since the slope of the desired curve needs to be able to match the soldering curve requirements If you need more heating rate than the oven can produce you re out of luck and need to get a more powerful oven My oven is a few years old luckily and has 1 550 watts so it can get up to about two degrees C sec Many newer ovens come with only 1 200 watts and may have commensurately lower heating rates although their insulation may be better My suggestion is to go to yard sales and or Craig s list and look for one with a high power rating The toughest part of the curve to meet is at the top when the oven is already quite hot but where it still has to climb quickly from about 180 degrees to 215 lead based solder or 235 RoHS degrees in just 30 seconds but the heating capability is at its lowest least slope Following the initial test then
5. C port pins to be sampled and converted to a 10 bit digital value The result is converted into a three digit display and sent to the LCD readout Board buttons labelled Up Down and Select are sensed by other PIC port pins and are used by the software to control the relay temperature and timer functions One helpful feature of the 16F88 is that you can turn on weak pull ups on its ports This means the button inputs will read five volts when not pushed and zero volts when pushed without having to put extra oull up resistors to the five volt supply It s no wonder the board looks so simple The power supply regulates the five volts for all the ICs A MAX232 IC converts the O to 5 volt digital signals to and from the PIC to the standard 12 volt RS 232 levels for interfacing to the PC Something not supplied with the kit is the AC outlet box seen in the picture The relay wires are run to this box to switch the hot side of the outlets An alternative that you may consider safer is to not mount the relay on the Status LET Control R7 10uF Vv CS IK c 10uF TXIOUT 4 ion Tih T2 Out T2Ih RlIn R1Out R2In R20ut 68 NUTS2VOLTS June 2008 circuit board but put it inside the outlet box itself and just run the low voltage control signal back to the controller board A significant amount of additional information on the hardware the build kit and boot loader software is available from the S
6. Microchip MPLAB IDE too for a consistent programming environment For those of you who want to tweak the code with another compiler there are several things which must be changed globally including using small letters for all the register names changing the way binary numbers are entered and of course pragma usage The boot loader from SparkFun proved to be more problematic It worked perfectly over the serial port during the early days of program development when was making tons of stupid mistakes during the learning process However when the program size became larger than 2K which is half of what the PIC16F88 can accommodate the process went tilt There are some peculiarities in the way the PIC programs jump to addresses above 2K that are incompatible with the SparkFun boot loading scheme Luckily by that point my programming skills had improved and could use my ICD2 clone to directly burn in the program changes to the PIC It was not as onerous as had originally thought taking only about 30 seconds for a complete remove burn re install cycle Cooking Right Along To start suggest trying out the oven in Manual Mode just switching the relay on and leaving it there for perhaps four minutes Use HyperTerminal to collect the serial data output and plot the heating characteristic First try ran into a problem before the temperature had peaked the oven s thermal cutout safety switch clicked out at 220 C
7. ff period at the end of the heating cycle starting at about 230 seconds Toaster ovens appear to have enough insulation usually through a double skin of the oven itself that they don t cool off very fast According to the FIGURE 7 Program Mode temperature tracking Program Temp E Actual Temp experts the cool down rate is important to get solder joints that are not too brittle One way that can work here is to open the oven door in a controlled manner while watching the actual temperature fall Yes But Does It Work The moment of truth had come took an old PC memory stick which has many surface mount chips on it with close leads 0 68 mm spacing and removed one of them using a heated air de soldering tool The back of the board had new unused pads on it to which applied liquid flux from a pen Then smeared on a thin layer of real solder paste evenly across the pads to be representative of what could be done at home without a custom cut solder paste stencil put one of the chips back in position on the pads and ran the controller program in the oven The photo in Figure 8 shows an excellent solder joint with no bridges at all The solder paste didn t stick to the masked and fluxed areas between the pins probably could have applied a little more solder paste This is not to say the process is foolproof yet but it looks like it can be made FIGURE 8 Close up of actual SMD solder
8. he basic home screen and always available when exiting one of the other modes The only way to exit the Main Menu is to turn off the unit 2 Manual Has a stop watch and relay control plus displays the current temperature The user can manually start stop reset the stop watch and turn the relay on or off directly This mode is FIGURE 5 Program Mode LCD screen Main Menu Screen Current mode MM Main Menu up button function Program Screen Step Number P1 Program P2 Program2 up button function R run short S stop short R reset timer long Peak Temp 208 230 C Reflow Zone _ 45 75 sec max 60 sec typical Soaking Zone 2 0 min max 60 90 sec typical Pre heating Zone 2 0 4 0 min max 120 150 180 Time sec useful for calibrating the oven or making toast 3 Semi Automatic The user can set a temperature and the oven will track this temperature and turn the relay on and off automatically This can be used for very long operations such as chemical bath e g etchant temperature control for several hours FIGURE 4 Main Menu LCD screen Top line Mode Select Manual Semi Auto Program 1 Mode Manual Program 2 M x x Nx Sel Setup select button function Nx next mode short push Sel select this mode long push down button function x none Program temperature C or F T
9. her lead based or lead free can be toxic if ingested The toaster oven you use for this project should be permanently dedicated to electronics and no longer used for heating food products RS 232 to Computer optional PLEASE USE CAUTION WHEN OPERATING THIS CONTROLLER AE am oer ait P E P FIGURE 1 Completed controller project 120 VAC from Box Wall Outlet Controller Power Cable from Wall Wart by Kit Ryan packaged components and no SMDs It even includes a power supply All you need is a serial cable to hook it up to your PC for programming as shown in Figure 1 A feature of this kit is that the PIC16F88 already comes loaded with a boot loader so a real programmer isn t immediately needed but more on that later The kit powers up with the LED flashing and the LCD readout showing Spark Fun Very reassuring that everything works The circuit shown in the schematic in Figure 2 is designed to control a relay that can switch 120 VAC for regulating the temperature of an ordinary toaster oven How it works is that the thermocouple output is amplified and linearized by the Analog Devices AD849 IC to within 2 degrees F No adjustment is needed and found it to be within one degree of the reading compared to room Relay Control Cable to Outlet June 2008 NUTS2VOLTS 67 OvenFlow 1 0 thermometers The analog signal is between the range of O to 5 volts and fed to one of the PI
10. imer 0 000s 000C 000C Actual E P1 R R Jmp Ed Ex select button function Ed Go to edit mode short Ex exit back to Main Menu long down button function Jmp jump to next step short or long June 2008 NUTS2VOLTS 69 OvenFlow 1 0 4 Program 1 and 2 This is the most capable mode of the software permitting the controller to follow a time temperature profile with up to 10 steps Each step is programmable by the user and both the setting and the actual temperatures are displayed in real time The software comes with a built in profile similar to the Kester solder curve to make getting started easier It can of course be modified A nice feature of the programmer is that the steps for up to two programs are retained in the EEPROM on the onboard PIC16F88 after the unit is turned off The LCD display for this mode shows how much information can be crammed into just 32 characters 5 Setup Five variables can be modified to tailor the controller to user preferences including e Temperature units Celsius or Fahrenheit Time temperature increment during programming the amount of change for each button push can be varied between one and 10 seconds or degrees LED on with relay gives a visual indication that the relay has been switched on kind of a safety feature e Clock calibration allows a 2 change to the built in system clock to get the one second timer as accurate as desired
11. ing s 5 ES arati seab RRAS S J A 7 ry y 1 A ath ie 4 FP 5 2 ee PA ear s A a Poe roe ABAREREL ERE ANY ary June 2008 NUTS2VOLTS 71 72 NUTS2VOLTS June 2008 OvenFlow 1 0 to work with a little effort Do It Yourself The hardware is obtainable from SparkFun Electronics and saves a lot of time compared to assembling your own circuit from scratch If you do go it alone just be sure to follow the schematic exactly and absolutely use a PIC16F88 MPU because every PIC is slightly different and the software will likely not work with another MPU Add the outlet box and then download the software from the Spark Fun site I ve included two versions of the program there for your enjoyment a hex file that can be burned directly into the MPU using a programmer and a C file and associated header files in case you want to play around with the program yourself Do a search for reflow to find the controller info and files Sorry but there s no easy way around using a programmer unless someone comes up with a tiny bootloader that s compatible with the large program size This particular MPU is actually designed for in circuit programming but that feature is not built into the SparkFun kit and will probably require some circuit modifications to make it work Even so a programmer would still be required Also posted on the SparkFun website is a complete User Manual for the p
12. parkFun website 1 Simple is Okay Complicated is More Fun Toaster ovens can get up to over 500 degrees F hot enough for SMD soldering But heat alone is not enough It takes the right time temperature profile to melt the solder without destroying the parts or getting solder bridges in the process A typical SMD heating profile is shown in Figure 3 courtesy of Kester solder systems Having a required profile was what drove the need for a orogrammed mode Since the PIC16F88 can write its own internal EEPROM memory to store a program all the pieces were there for a really capable circuit The schematic is reproduced with permission from SparkFun Electronics The BoostC header file is provided with permission from BoostSource Technologies The RA2 AN2 VREF RAIANI RA3 AN3 VREF RAO AN0 RA4 AN4 RA7 OSC1 RAS VPP MCLR RA6 OS VDD RB7 AN6 PGD RB6 AN5 PGC Contrast FIGURE 2 Controller schematic author is not affiliated in any way with either company Controller Software FIGURE 3 Standard reflow solder profile Fees rA g EGANI CZI EANN LY f ELENA mt Se a oe AAN E ar OSX AN 4 f OvenFlow 1 0 Standard Solder Paste Reflow Profile for Kester Paste Containing Alloys Sn63Pb37 or Sn62Pb36Ag02 Preheat gt Soak gt Reflow lt Cool Down gt The software was developed to be more flexible than just controlling a toaster oven should
13. rogram that explains all the screens and control functions That manual includes a more in depth discussion of the software too Be aware that the PIC16F88 has only AK of program memory and the program described in this article uses 95 of it If you want to add more features you ll likely have to cut something out NV 1 SparkFun Electronics controller amp software sparkfun com 2 Kester SMD soldering time temp profile kester com 3 BoostC C compiler source boost com
14. tried out the Program Mode using the built in settings that approximate the Kester curve in five steps If you allow for the warm up period which takes about 20 seconds the tracking is quite good see Figure 7 pink curve vs blue curve The plot actually shows the result of much tuning of the relay control algorithm compared to my original simplistic turn on relay if the temp is below the program turn off relay if it s above approach The final algorithm looks ahead about six seconds to where the temperature needs to be called the set_temp and compares it to where the actual temperature appears to be headed the projected_temp The weighting of the two values the current temperature error and the projected temperature error can be adjusted to allow good tracking with different ovens OvenFlow allows for a time constant variable which the user can adjust to help with the projected temperature function The initial warm up time lag is best adjusted by inserting a first program step of around 15 25 seconds at room temperature before trying to track the soldering curve One interesting aspect of control loops such as the one in the program is their tendency to oscillate around the desired set point Oscillations are clearly visible in the pink curve However since it s only a few degrees off at any point it appears adequate for the SMD soldering function of interest Another thing to be noted is the cool o
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