July 2004, Issue 168

Easy Reflow
Build an SMT Reflow Oven Controller

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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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Figure 1—In order to achieve reliable solders and to minimize thermal stress on components, a precise thermal profile must be used with four successive steps.

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).

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Photo 1—You must choose an appropriate toaster oven. For the best results, find one with infrared heaters on top that allow for a quick temperature rise. This one, which was made in France, costs less than $150. The thermocouple wire is on the left.

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.