Easy Reflow
When designing high-speed applications, working with SMT components and soldering by hand can be tedious. Robert’s H8/3687-based SMT Reflow Oven Controller transforms a conventional infrared toaster oven into an effective reflow oven that ensures thermal control.
For years surface-mount technology (SMT) has been the manufacturing technology of choice, particularly because it’s less expensive than classic through-hole processes and because of the reduction in board size. I’m sure you have noticed that SMT is now mandatory for prototypes, even if a project isn’t space-constrained. Many new and exciting components are only available in SMT packages. High-speed designs (using high-speed digital or radio frequencies) won’t work using large through-hole packages.
It’s possible to solder the majority of SMT components by hand, but it’s painful and time-consuming. The components must be soldered individually with either a small soldering iron or a hot-air pencil. But small packages like 0603 resistors are likely to fly away when soldered with hot air and stick to the soldering iron at the worst time.
A solution to this problem is to use reflow oven technology just like the manufacturers do. A reflow oven is a well-controlled oven that allows to you cook a PCB with its components and solder all of the pads simultaneously with solder paste. Great! The only issue is that reflow ovens cost a fortune, ranging from $2,000 to $1 million. The ovens cost so much because they need strictly controlled thermal profiles to ensure good soldering and also to limit the thermal stress on the components.
In this article I’ll show you how a homemade controller built around a Renesas evaluation board can transform an inexpensive toaster oven into a reflow oven. First, I’ll cover how reflow ovens work and explain which toasters on the market are best suited to mimic them. Then, I’ll describe my controller’s hardware and software. I’ll finish up with some tips on how to assemble your own SMT boards using the reflow oven. Let’s get hot.
REFLOW OVEN
The key to good reflow is a precise multistep thermal profile, as illustrated in Figure 1. The preheating profile allows you to slowly bring the PCB to a temperature high enough to dry the solder paste (approximately 100°C) while minimizing the risk of thermal stress on the components in the reflow phase. After thermal stabilization, the board is heated as quickly as possible to the reflow temperature (approximately 250°C). The board stays above the reflow temperature for a set period of time (usually around 30 s) before it’s cooled down in a controlled fashion.
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What are the key characteristics of an industrial reflow oven? Quick heating and cooling is important (the quicker the better) in order to reduce the thermal stress on the components. (The pads and solder paste have a lower thermal latency than the components themselves.) Maintaining a homogeneous temperature inside the oven is also important, especially when you’re working with large PCBs. And, last but not least, regulation is a must in order to guarantee that successive reflows follow the exact same profile. As you can imagine, I will handle the regulation with a microcontroller-based solution, so let’s focus on the low-cost oven you’ll use to replace a real reflow oven.
In order to have a quick heating time, some industrial reflow ovens use infrared heaters. Why not just buy an infrared-based toaster oven like the one shown in Photo 1? I bought mine on the ’Net for less than $150. It has two quartz heating elements on the top and a classic resistor on the bottom (a total of 1,100 W).
I used a thermocouple affixed on a small PCB to take the first measurements. The temperature got up to 250°C in less than 4 min., which is definitely enough for this application. The only real drawback was the lack of a cooling system. I decided to use a manual cooling method: I open the door when the controller asks.
HOME-BUILT CONTROLLER
As for building the controller, I used the Basic Micro EVB87 Renesas evaluation board, which is fitted with an H8/3687 microcontroller (see Figure 2). This inexpensive microcontroller is exactly what I needed for this project. It has a low pin count, high RAM and program memory for using a simple high-level language like BASIC, and on-chip ADC. The evaluation board already provides an LCD and push buttons, so all I had to do was add two small circuits using the prototyping area. The first was a thermocouple interface. I used a K-type thermocouple and an Analog Devices AD595 monolithic thermocouple amplifier with on-board cold-junction compensation directly connected to the H8/3687’s ADC. More than simple, isn’t it? Lastly, I needed a way to drive an external relay to switch the oven on and off. It took nothing more than a BD235 transistor and a free-running diode. Of course, you could use your favorite microcontroller (e.g., Basic Stamp’s) for this design, but this evaluation kit speeds up the process (see Photo 2).
On the firmware side, I decided to try to the BasicATOM programming language because this application isn’t managing microseconds. This was easy thanks to the user-friendly development environment provided by the board in addition to the resident boot-loader. It took only 4 h to write and debug the full software, even though I wasn’t able to use the debugger for some reason.
The application, for which the source code is available on the Circuit Cellar ftp site, is extremely easy to read. It is structured into two independent sections: a configuration section and a state machine section. The former allows you to manually modify all profile parameters with the LCD and a couple of push buttons. The latter manages all of the successive steps of the thermal profile and reads the actual oven temperature in real time. Let’s look at the details.
First, a welcome screen is displayed. Then, a static screen displays the current temperature in real time and allows you to select either Configuration mode or Run mode.
In Configuration mode, the LCD successively displays each of the nine key profile parameters (preheating slope, drying temperature, drying duration, heating slope, reflow temperature, reflow duration, cooling slope, thermal hysteresis, and differential coefficient). It also allows you to modify the profile parameters with the plus and minus buttons. (It’s easy to store these values in EEPROM, but it isn’t done in the current version of the firmware.) In Run mode, the LCD shows the current actual and preset temperature, as well as the current state and the remaining time to be spent in the state (see Photo 3).
Seven steps are managed by the state machine in order to achieve a thermal profile that’s as close as possible to the theoretical one. The first step involves preheating. The preset temperature is linearly increased from the ambient temperature up to the drying temperature. Next, the controller waits for the actual temperature to be equal to the drying temperature. The third step involves drying. The preset temperature stays constant during the drying time.
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Heating follows this. Basically, the preset temperature is linearly increased from the drying to the reflow temperature. Then, the controller waits for the actual temperature to be equal to the reflow temperature. Next, the preset temperature stays constant during the reflow time. Finally, the preset temperature moves linearly down to the initial ambient temperature, and you open the oven door to help the process.
In addition to state machine management, a thermal control loop is implemented in the software using a PD-like algorithm. An estimated future temperature is calculated using the actual temperature plus a multiple of the differential of the temperature. The estimated future temperature is compared to the preset temperature in order to switch the heater elements on or off. This allows you to account for the thermal latency of the captors and heaters and to get a more stable temperature. A small hysteresis factor is also used in the comparisons in order to extend the life of the heater. Figure 3 shows the actual thermal profile compared to the preset values generated by the controller.
OVEN IN USE
Now that you know why you need a reflow oven and how to build one, how will you use it? First, you need to get some additional equipment. You’ll need a good magnifier (a video magnifier would be best) and a syringe of solder paste. It’s easier if you have a pneumatic fluid dispenser (which is easy to find on the ’Net, but you will also need shop air) because using the syringe can be difficult. You will also need a flux cleaning spray, a small classic iron (15 W or less), and, ideally, a hot-air iron for repairs. Don’t worry, the Internet is a good source for low-cost equipment. Even if you don’t have a video magnifier, you can be successful if you have a good optical lens and good eyes!
A clean blank PCB is extremely important. I know from experience that it’s nearly impossible to achieve good reflow results with home-etched PCBs. Solder masks are really helpful in keeping the solder paste on the pads. I don’t know how to get solder masks on home-etched PCBs, so find a good PCB manufacturer and order your PCB with solder masks.
You have to put solder paste on the pads with the syringe either manually or with the fluid dispenser. Then, you must place each component on the PCB on the solder paste spots. These two steps are time-consuming, but things speed up with experience! Remember that it is far quicker to use solder paste than to try to solder by hand. After all of components are placed, you can put the board in your reflow oven and press the Start button. After 10 min. or so, your PCB will be cooked. Beware, it can still be quite hot. Use flux removal fluid and a toothbrush to clean the board (see Photo 4). Refer to the Circuit Cellar ftp site for a detailed illustration of the process.
By the way, you can use this process for boards with components on both sides if you put light components on one side. Reflow only that side first, and then flip the board and repeat the process: apply solder paste, place the components, and then reflow. The components on the bottom side will usually stay soldered thanks to the surface tension of the solder.
Finally, you should check the electrical connections and make the necessary repairs. If you follow the process closely, you shouldn’t have too much trouble. My last 160 mm × 100 mm board had more than 300 fine-pitch components on both sides, but I had only a couple of bad joints and one solder bridge on a 0.65-mm pitch IC.
THREE-DAY SOLUTION
This project proves that SMT technology is neither boring nor too expensive for the occasional prototype circuit maker. With a $150 toaster oven and a low-cost controller, you can achieve reflows with good results. The only other things you need are a good lens and some basic equipment that’s readily available on the used market. And, yes, you’ll need some patience too.
This project also shows that BASIC is efficient language for low-speed, real-time applications, especially those that are associated with microcontrollers with plenty of RAM and program memory in order to fully use the power of such languages (e.g., the Renesas H8/3687).
Last but not least, this project demonstrates that embedded development doesn’t necessarily have to be a huge undertaking to give good results. It took me only three days to finish this project. I spent one day drawing the concept and algorithms on paper. The last two days were spent building the prototype, writing the software, and optimizing it.
The next step will be to experiment with BGA packages, but that should work too. Happy reflowing!
PROJECT FILES
Download the code and additional photos – Lacoste-168.zip.
RESOURCES
Analog Devices, Inc., “Monolithic Thermocouple Amplifiers with Cold Junction Compensation: AS594/ AD595,” rev. C, 1999.
Basic Micro, “Renesas 3687 Tiny Series Starter Kit User Manual,” ver. 2.1, 2003, www.basicmicro.com/downloads/ Renesas/3687/3687RSKUsersManual.pdf.
———“3687 Starter Kit: Quick Start Guide,” 2003, www.basicmicro.com/downloads/Renesas/3687/QStart.pdf.
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D. Kubin, 3687 Evaluation board schematic, 2003,
www.basicmicro.com/downloads/Renesas/3687/Schematics.pdf.
Research International, “Reflow Technology Handbook,”
www.research-intl.com/reflowtechbook.htm. (No longer available)
SOURCES
AD594/AD595 Thermocouple amplifiers
Analog Devices, Inc.
(800) 262-5643
www.analog.com
EVB87 Renesas evaluation board
Basic Micro, Inc.
(248) 427-0040
www.basicmicro.com
H8/3687 Microcontroller
Renesas Technology Corp.
(408) 382-7500
www.renesas.com
PUBLISHED IN CIRCUIT CELLAR MAGAZINE • July 2004 #168 – Get a PDF of the issue
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Robert Lacoste lives in France, between Paris and Versailles. He has more than 30 years of experience in RF systems, analog designs and high-speed electronics. Robert has won prizes in more than 15 international design contests. In 2003 he started a consulting company, ALCIOM, to share his passion for innovative mixed-signal designs. Robert is now an R&D consultant, mentor and trainer. Robert’s bimonthly Darker Side column has been published in Circuit Cellar since 2007. You can reach him at askrobert@lacoste.link.
