Introduction to Solder Reflow
Solder reflow is the process of heating solder paste to melt the solder particles and form permanent solder joints between electronic components and PCB pads. Precise control of the solder reflow temperature profile is critical to achieve reliable, high-quality solder connections.
The eC-reflow-mate is a specialized reflow soldering oven that provides excellent control and repeatability of solder reflow profiles. By optimizing the oven parameters and solder profiles, you can enhance the performance and yield of your reflow soldering process using the eC-reflow-mate.
Solder Paste Characteristics
To develop effective solder reflow profiles, it’s important to understand the characteristics of the solder paste being used. The key properties of solder paste include:
- Alloy composition (e.g. Tin-Lead, Lead-Free SAC305)
- Metal load (percentage of solder particles by weight)
- Particle size distribution
- Flux type (e.g. no-clean, water soluble)
- Viscosity and printability
- Melting point
The solder paste manufacturer should provide a recommended reflow profile that specifies temperatures and durations for the critical stages of the reflow process. This is an excellent starting point when developing a reflow profile for a specific solder paste.
eC-reflow-mate Oven Parameters
The eC-reflow-mate offers several adjustable oven parameters that allow you to optimize the reflow profile for your solder paste and PCB Assembly:
| Parameter | Description | Typical Range |
|---|---|---|
| Zone Temperatures | Temperature setpoints for each heated zone | 25-300°C |
| Conveyor Speed | Speed of PCB transport through oven | 20-200 cm/min |
| Recirculating Fan Speed | Speed of fans that circulate hot air | 0-100% |
| Nitrogen Flow Rate | Flow of nitrogen gas for inerted atmosphere | 0-20 L/min |
By adjusting these parameters, you can control the shape of the reflow profile and hit the time and temperature targets specified by the solder paste manufacturer.
Developing a Solder Reflow Profile
A typical solder reflow profile consists of four main stages:
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Preheat: The PCB assembly is gradually heated to evaporate solvents and activate the flux. Typical preheat temperatures are 150-175°C.
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Thermal Soak: The temperature is held steady to allow the PCB and components to all reach an even temperature and the flux to fully activate. Thermal soak is typically 60-120 seconds at 150-200°C.
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Reflow: The temperature is quickly ramped up to melt the solder particles and form solder joints. Peak reflow temperatures are typically 220-250°C for lead-free solder.
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Cooling: The assembly is cooled to solidify the solder joints. Controlled cooling at 2-4°C/sec is desirable to create a fine-grain solder structure.
Here is an example solder reflow profile for a lead-free SAC305 solder paste:
| Profile Stage | Target Temperature | Duration |
|---|---|---|
| Preheat | 150°C | 60-90 sec |
| Thermal Soak | 175°C | 60-120 sec |
| Reflow | 245°C peak | 30-90 sec above 217°C |
| Cooling | < 6°C/sec | As fast as possible |
To hit these target temperatures and durations in the eC-reflow-mate, you would set the zone temperatures, conveyor speed, and fan speed to shape the reflow profile. Thermocouples and profiling software can be used to measure the actual temperatures experience by the PCB assembly and fine-tune the oven parameters.
Optimizing Reflow Profiles
Once you have a baseline reflow profile that matches the solder paste specifications, you can further optimize the profile for your specific PCB assembly. Factors to consider include:
Component Thermal Limits
Make sure sensitive components will not be damaged by the peak reflow temperature. Refer to component datasheets for maximum temperatures.
PCB Thermal Mass
Large, thick PCBs require more time to heat up evenly compared to small, thin PCBs. The thermal soak duration may need to be extended.
Nitrogen Atmosphere
Using nitrogen gas in the reflow oven reduces oxidation of the solder for a shinier, more reliable joint. Nitrogen also enhances wetting of the solder. The tradeoff is increased cost and equipment complexity.
Void Reduction
Solder Voids are a defect that can weaken the joint. Voids can be minimized by using a longer thermal soak to allow volatile flux materials to fully outgas before the solder melts. A rapid cool down also helps minimize voids.
Reflow Defect Troubleshooting
If you encounter solder defects or quality issues after reflowing PCBs in the eC-reflow-mate, adjusting parameters in the reflow profile may resolve the problem:
| Defect | Potential Cause | Profile Adjustment |
|---|---|---|
| Solder Balls | Excessive peak temperature | Reduce peak temp by 5-10°C |
| Grainy/dull joints | Insufficient peak temperature | Increase peak temp by 5-10°C |
| Tombstoning | Temperature ramp rate too high | Decrease ramp rate to reflow by 10% |
| Voiding | Insufficient thermal soak | Increase soak duration by 30 sec |
It’s advisable to only change one reflow profile parameter at a time so that you can determine the impact on the defect rate. Proper PCB design and solder paste printing are also major factors in reflow soldering success.
Frequently Asked Questions
What is the maximum PCB size that can fit in the eC-reflow-mate?
The eC-reflow-mate can handle PCBs up to 18″ wide by 20″ long.
How long does it take for the eC-reflow-mate to warm up?
Warm-up time is approximately 15 minutes from room temperature to a 200°C setpoint. The oven can be programmed to complete an automatic warm-up sequence.
Can I use leaded solder paste in the eC-reflow-mate?
Yes, the eC-reflow-mate is compatible with leaded and lead-free solder pastes. Develop the reflow profile based on the paste manufacturer’s specifications.
How much nitrogen does the eC-reflow-mate use?
Nitrogen consumption will vary based on the reflow profile and PCB size. As a rough estimate, one 230 cubic foot N2 cylinder will last for 300-600 reflow cycles.
How often does the eC-reflow-mate require maintenance?
The only regular maintenance is to clean flux residue from the oven interior when it begins to accumulate. The frequency depends on the flux type and volume of PCBs processed, but is typically every 1-3 months.
Conclusion
Developing an optimized solder reflow profile is essential to achieving a high-yield, high-quality reflow soldering process. By understanding the characteristics of your solder paste and PCB assembly, you can adjust the parameters of the eC-reflow-mate to hit the target profile temperatures and durations.
Include adequate preheat and thermal soak, control the peak reflow temperature, and cool quickly to produce reliable, aesthetic solder joints. Always refer to component datasheets and solder paste specifications as a starting point when developing reflow profiles.
Systematically optimize the reflow profile and oven parameters to minimize defects like voiding, solder balling, and poor wetting. The eC-reflow-mate gives you the tools to maintain precise, repeatable control of the reflow profile. With an optimized reflow process, you’ll be on your way to consistently manufacturing PCB assemblies with the eC-reflow-mate.
