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Let’s continue from the previous article: How Temperature Changes Solder Your PCB
Troubleshooting RTS works the same way as RSS: adjust temperatures and timing until results improve. You’ll need some trial and error –nudge temperatures up/down and check what happens.
Below are common RTS soldering problems and how to fix them:
Problem: Solder Balls
You see many tiny balls of solder stuck in the residue around the joint after soldering.
- Cause: The oven heated up too slowly.
- Why it happens: Slow heating makes the solder paste’s cleaning chemicals (“flux”) burn out too early. This lets the raw solder metal rust (oxidize) before it melts properly.
- Fix: Slightly increase the heating speed in your temperature profile.
Solder Beads
People often mix up solder beads and solder balls. Solder balls are a small number of large balls, usually found near chip capacitors and resistors.
While solder beads are often caused by too much solder being applied during printing, you can sometimes fix the problem by adjusting the temperature profile.
Like solder balls, solder beads that happen on the RTS temperature curve are usually caused by the temperature rising too slowly. This slow heating creates a “capillary action” during melting (reflow). This action pulls unused solder from where it built up down to the bottom of the component.
While the solder is melting, its surface tension pulls it towards the circuit board. This force squeezes the solder out to the side of the component, forming solder beads.The fix for solder beads, just like for solder balls, is to make the temperature rise faster until the beads disappear.
Not Enough Solder Not enough solder usually happens when the heat isn’t even or when things get heated up too fast. This makes the component leads get too hot and start soaking up the solder. After reflow, the leads might look like they’ve lost their solder and appear thicker, with less solder left on the pads. Slowing down how fast you heat things or making sure everything gets heated evenly can help stop this problem from happening.
Poor Wetting
Poor wetting often happens because of how long the solder paste is heated and how hot it gets. The special chemicals in the solder paste that help it flow (made from organic acids) break down over time and when exposed to heat.
- If the heating process takes too long, the solder joints might not wet properly.
- The RTS heating profile keeps these helpful chemicals active longer than the RSS profile. This makes poor wetting less likely with RTS.
- If you still get poor wetting using RTS, check the first two-thirds of the heating process. Make sure the temperature stays below 150°C (302°F) during this part.
Keeping it cooler at the start helps the paste’s chemicals last longer, leading to better wetting.
Tombstones in Solder Joints
Tombstones happen when uneven forces during soldering make small parts stand up on one end after reflow. This issue is less likely to occur when:
- The heating process is slower, and
- The circuit board is flatter.
To fix this problem, try slowing down how quickly the assembly heats up, especially when the temperature is below 183°C (the melting point of solder).
Voids are small air pockets or gaps inside solder joints, often found during X-ray or cross-section inspections. These bubbles can be trapped air or leftover flux.
Common Causes:
- Peak temperature too low – The solder doesn’t melt properly.
- Reflow time too short – The solder doesn’t fully flow and trap gases escape.
- Heating stage too hot – Rapid heating can cause gas bubbles to form.
Since the temperature ramp rate in the RTS (Ramp-to-Spike) profile is tightly controlled, adjusting these factors can help reduce voids.
RTS Temperature Profile: Benefits and Considerations
The RTS (Ramp-to-Spike) temperature profile isn’t a universal solution for all reflow soldering issues, and it may not work for every oven or assembly. However, it offers several advantages:
- Lower energy costs
- Higher production efficiency
- Fewer soldering defects
- Better solder wetting
- Simpler process control
This doesn’t mean the traditional RSS (Ramp-Soak-Spike) profile is outdated, or that RTS can’t be used in older ovens. Still, engineers should know that newer, more optimized reflow profiles–like RTS–are available and often perform better.
Above are some interpretations of the RTS curve issue. If there are any inaccuracies or deficiencies, please feel free to point them out. At the same time, we sincerely invite you to inquire about the price.
