The Secret to Even Heating in Your Reflow Process

Introduction

The most common misconception about solder reflow is that the hardest part of the reflow process is reaching the peak temperature. It’s hard to build an oven capable of reaching lead-free reflow temperatures of 255°C (480°F). It’s harder to design an oven to achieve the proper rate-of-rise without damaging components. To complicate matters, heater duty cycles are only a small part of the equation. The type of heater and its placement also affect the final results. In this article we'll walk you through how we balance these risks to build the amazing reflow ovens.

Importance of Following a Profile

Lots of problems can arise if reflow profiles aren’t properly followed. 

Overshooting target temperatures can cause a variety of problems, including damaging components and delaminating PCBs. Prolonged exposure to high temperatures will foster intermetallic growth in ICs, resulting in a shortened mean time before failure. Extreme temperatures can ruin the internal dies altogether.

Similarly, the proper rate-of-rise is critical to a successful reflow. Heating too quickly can thermally shock components (e.g. capacitors and ICs), cracking or otherwise damaging them. Contrarily, heating too slowly can cause the flux to activate prematurely and exhaust itself. Exhausted flux can’t keep oxidation at bay during the reflow phase, resulting in problems like graping (shown below) and “head-in-pillow” defects. 

Lastly, not reaching reflow temperatures gives you a second chance to reflow the board and damage the components. Prolonged high temperatures can cause damage to components, even if they’re within the component’s rated reflow temperatures. Components are usually rated to reach reflow temperatures for short periods of time. The same can be said of PCBs: multiple reflows increases the chances of PCB delamination.

Using Infrared Heaters

Infrared heaters are a perfect choice for reflow because they’re quick to heat up and cool down. They respond quickly to requested changes in temperature without substantial overshoot. This gives our toaster reflow ovens great inertia scores. When used with care, infrared heaters can be an effective replacement for more modern convection-based ovens.

Unfortunately, the defects described above can all result from poor use of infrared heaters. These heaters can produce hot and cold spots when used improperly. This is commonplace in cheap Chinese ovens like the T962. Such ovens have extreme hot and cold spots, resulting in some parts of the board reflowing while others never do.

The diagram below depicts a microcontroller being reflowed under an unshielded infrared heater (as in the T962). It illustrates a core problem with infrared heaters - the spots under the heater are hotter because they receive more light. It’s hard to reflow the entire board without overheating components under the heater.

Infrared heaters also have a tendency to heat dark components faster than light ones. The diagram below depicts infrared light hitting a PCB with various components on it. The large black microcontroller will absorb more infrared light than the passive components on the board, causing it to heat up faster. Successful infrared reflow without overheating dark components is difficult. This is why IPC’s J-STD-020 requirement 3.2.2 states, “It is required that this equipment use IR to heat only the air and not directly impinge upon the SMD Packages/ devices under test.”

Heat Distribution with Controleo3

Expensive commercial ovens use infrared heaters to heat the air and blow it onto the board with fans. Reflow ovens used by Controleo3 balance cost and complexity by eliminating the fan and using quartz infrared heaters that emit less IR. Our ovens take several precautions to mitigate the uneven heating caused by infrared heaters. 

• Toaster ovens typically have shielded heaters (shown below). This prevents a large part of the infrared light from reaching the board.

• Our build guide recommends more power from the bottom of the oven than the top. There’s a reflow tray separating the bottom heaters from the PCB. The aluminum tray and insulation absorb infrared light and disperse the heat before it reaches the PCB.
• Controleo3 biases the power towards the bottom heaters. Our recommended starting bias has the top heater on 20% less often than the bottom heater. This can be changed by the user depending on the application.

Heat Distribution with Other Methods

Reflow ovens should work to give you precise temperature control and consistent temperatures throughout the oven. Unfortunately, most low-cost solutions available today don’t properly address heat distribution. They’re like wildfires: hot, aggressive, and hard to control. 

Cheap Chinese Reflow Oven (T962/T962A/T962C)

Although relatively low-cost, these ovens are plagued with poor design choices. Firmware and hardware upgrades for these ovens are typically highly recommended. However, these upgrades still don’t address the underlying issues with the T962.

Their hardware has many unaddressed problems that are left for the user to discover. Remarkably, the oven uses masking tape inside the chassis. This will burn and release a bad smell. It’s a miracle if the manufacturers haven’t noticed. The window is also held in place by hot glue.

Most notably, there are four unshielded infrared heaters right above the PCB tray. None of the heat comes from the bottom of the oven. This means that the top of the PCB receives direct infrared light. As explained above, this causes uneven heating and can damage components. As all the heaters are topside, not much can be done to address this design flaw.

Ovens with One On/Off Control

This approach has lost popularity since the introduction of Controleo3 - for good reason. Switching all the heaters with one relay means that there’s no way to control the direction the heat comes from. The top heaters will run as much as the bottom ones. This promotes uneven heating and can damage components. Moving all the heaters to the bottom isn’t an option. The oven would be a glorified hot plate.

Running the heaters on simple control systems with a single relay also causes large current spikes. If you’re not careful, this can easily trip a circuit breaker.

Hot Plate Reflow

Born out of necessity, hot plate reflow is one of the cheapest methods available. Simply put, it involves placing a single-sided board onto a hot plate or frying pan and cooking it like an egg. This keeps me up at night.

Most importantly, all of the heat comes from the bottom. PCBs are great thermal insulators. This means that the bottom of the PCB will get hot while the top is hardly warm. This puts the PCB at a very high risk of delamination. It also means that high temperature lead-free reflow isn’t possible.

Lastly, how do you follow a reflow profile on a hot plate?

Conclusion

It’s clear that there’s lots of low-cost reflow methods. However, it’s also evident that they can put your PCB and components at risk of heat damage and other defects. How much is your time worth? Your components? The opportunity cost of waiting for replacements? Do yourself a favor and build a great reflow oven. Do it once and do it right.