Printed Circuit Board Assemblies (PCBAs) are often viewed as commodity items. But consistently producing them in volume can take time to perfect. Electronics manufacturing services (EMS) providers may well have their own preferences when it comes to their choice of machine type and brand. But, broadly speaking, the process steps that they go through to produce PCBAs are the same. There is, however, one specific step that can make all the difference.
When it comes to quality and consistency, creating the Surface Mount (SMT) reflow oven profile can have a notable impact. Optimising the reflow oven profile is crucial for ensuring high-quality and consistent solder joints in SMT assemblies, as a well-defined thermal profile not only enhances product reliability but also minimises defects such as tombstoning, solder balling, and voiding.
Why is SMT reflow oven profiling important?
The reflow oven is used primarily for the reflow soldering of surface mount electronic components to Printed Circuit Boards (PCB).
If you fail to profile the oven correctly, then all of the hard work and effort that has gone into loading feeders, programming the machines, optimising the build and then running the production line will be wasted.
So what should your EMS partner be doing to perfect their reflow oven profile? Let's find out.
Working on the baseline
The New Product Introduction (NPI) engineer will usually start the process by selecting an oven profile they have used before and that is stored on their system.
Over time, some EMS providers may develop a range of "baseline" profiles, which can then be selected depending on the PCBA they are working on. The time taken to perfect the baseline profile will pay dividends as most assemblies will then fall within one of them.
Baseline profiles may include:
- One dedicated to densely populated assemblies with elevated temperatures.
- One dedicated to "standard" assemblies.
- One dedicated to assemblies that contain sensitive parts and require lower temperatures.
Using their skill, judgement and experience, the NPI engineer will select the profile and temperature settings that they believe are best suited to the new assembly.
Requesting a sample PCBA
Ideally, the OEM will have provided their assembly partner with a populated printed circuit assembly. If one isn't provided, then the NPI engineer may request one for use during the set-up process.
The NPI engineer will attach thermocouples to multiple locations across the sample board. Accurate placement of thermocouples on the PCB is vital for obtaining a representative thermal profile. These should be attached to a range of components with varying degrees of heat dissipation, such as a transformer, Ball Grid Array (BGA) and passive devices.
Depending on the size of the PCBA, it is recommended that between three and six thermocouples be attached to critical components and solder joints. They must be connected properly to monitor temperatures effectively.
Quite often, EMS providers will tape these in place, but, unfortunately, there is a risk that they will lift during the process, which can result in the air temperature being measured rather than the board and lead temperature.
For a secure connection, we recommend applying:
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High temperature solder to secure the thermocouple to the component lead.
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An adhesive, such as Chipbonder epoxy, to ensure residual glue is not found between mating surfaces.
Special attention should be given to BGAs. For the best results, a thermocouple should be placed underneath the device, which makes direct contact with the terminations and enables a small hole to be drilled through the underside of the board. However, this can be a destructive process, so it will require a sample PCBA from the outset.
The PCBA will then be run through the reflow oven with thermocouples attached to a data logger. It's important that that the data logger is positioned correctly so that it doesn't affect the movement of the PCBA through the oven.
Once the PCBA has passed through the oven, the results from the data logger will be downloaded into the chosen oven profiling software. And this is where the first real analysis of the profile begins.
Key phases of the reflow profile
Each solder paste manufacturer recommends a baseline profile with plus (+) and minus (-) tolerances. Where possible, the EMS provider will try to achieve a result in the middle of these tolerance bands. However, certain component requirements may require them to work at the upper or lower limits of these tolerances.
The NPI engineer should be paying close attention to the following four areas (based on the assumption that a typical lead-free alloy with a melting point of 217 degrees Celsius (°C) is used):
Ramp zone
The ramp zone phase preconditions the PCB assembly prior to actual reflow by gradually increasing the temperature to remove flux volatiles and reduce thermal shock.
The rate of temperature increase is measured in °C per second and should be between 1 and 3°C per second. This will be more noticeable at the beginning of the profile when the board temperature changes from ambient to 120°C. Failing to comply with this could lead to component damage, and this is one area component manufacturers typically specify.
Soak zone
The soak zone normally occupies 33-50 per cent of the total heated tunnel length in a reflow oven. It exposes the PCB to a relatively steady temperature that allows the components of different masses to be uniform in temperature. The soak zone also allows the flux to concentrate and the volatiles to escape from the paste.
Reflow zone
The reflow zone is where the temperature peaks, allowing the solder to melt and form joints between components and the PCB. This is the area where the temperature is elevated to between 230 and 250°C. A critical measure is the time above reflow, which typically lasts between 45 and 90 seconds. It's important to check component specifications for maximum temperature and control the peak temperature and time above liquidus to prevent component damage and ensure proper soldering.
Cooling zone
In the cooling zone phase, solder joints solidify and prevent thermal shock to components. A controlled cooling rate is crucial to form a fine-grain solder joint structure that enhances mechanical strength. A typical cooling rate is around 4°C per second.
Best practices
Here are some best practices to keep in mind when it comes to SMT reflow oven profiles:
1. Profile selection: Based on the assembly's complexity and component sensitivity, the EMS might choose between a ramp-to-peak or ramp/soak/reflow profile. A ramp-to-peak profile involves a linear temperature increase to the peak with minimal soak time—good for simple assemblies. In contrast, a ramp/soak/reflow profile includes a soak period before reaching the peak temperature—ideal for complex, high-density PCB assemblies.
2. Oven maintenance: Regular maintenance of the reflow oven ensures consistent thermal performance. This includes cleaning flux residues, verifying zone temperatures, and calibrating conveyor speeds.
3. Data analysis: The EMS provider should analyse the profile data using thermal profiling tools and software to ensure it meets the solder paste and component manufacturers' specifications.
Fine-tuning the profile
Depending on the data logger output, the NPI engineer may decide to "fine-tune" the profile further. Using the oven profiling software, the NPI engineer has the ability to amend certain oven parameters (like speed or temperature) through the oven profiling software and then view the "predicted" results.
In our experience, we have found the prediction tool to be highly effective. However, it does rely on ensuring the oven is accurately represented within the software. To do this, the software will need to understand the oven make and model, the number of zones, and the length of each zone.
Providing the NPI engineer is happy with the predicted results, they will then make physical changes to the oven parameters and store the profile under a unique reference number. The reference number will typically consist of the item code, the revision level and whether the profile is for the top or bottom side of the PCBA.
Conclusion
So there you have it, these are the key steps that your EMS provider will follow to create the perfect oven profile and ensure that your products are built to a consistently high standard. By meticulously developing and monitoring the reflow oven profile, manufacturers can achieve optimal soldering results, leading to reliable and high-quality PCB assemblies.
If you’re interested in learning more about how an EMS provider sets up and controls its reflow ovens, feel free to contact the ESCATEC team. Alternatively, you can download our guide to quality, consistency and delivery for more insights.
Editor's note: this post was originally published in May 2019, and updated in February 2025 for relevance and accuracy.
