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SOLUTIONS

Reflow Profiling for New Users

We are often asked the question “How do I develop a temperature profile” for my Sikama oven. Conduction heat and dwell, two unique features that set Sikama ovens apart from the competition seem very foreign to some new users. Temperature and time (dwell) are the factors that make a good profile.

Why do you stop or dwell the part in the Sikama oven when other manufacturers require continuous movement through the oven? We stop the part in each zone to allow heat to rapidly transfer from the conduction heat platen to the part. Conduction heat transfer is faster and more precise than convection heat transfer so by stopping the part (dwell) for a short period of time we actually speed the process. We use convection and radiant heat to enhance the profile.

Would my part be damaged from excessive heat if I stop it in the oven? No, the part will not see a temperature higher than the set point of the hottest zone and it will not be exposed to the temperature longer than the dwell time set by the user.

Typical dwell times range from 30 seconds to 3 minutes or higher, depending on the mass of the part or fixture being processed. Dwell time for a wafer or ceramic substrate might be 30-45 seconds ranging up to 2-3 minutes or more for higher mass applications. Higher mass means more time is needed for the part to soak the heat.

Where do I start with temperature settings? First, get the melt/liquidus temperature of the solder from the solder supplier. You will want to set the temperature in the Reflow Zone 10 to 30 degrees higher than liquidus as a starting point. Higher mass parts and fixtures will require more heat than thin ones. Once you have determined a start temperature for the Reflow Zone you can work backward to a start point for Zone One.

An example for a 4 Heat Zone oven might be as follows:
The liquidus temperature of Sn63/Pb37 solder is 183° C.
1. Set the Reflow Zone (zone 4) at 210° C
2. Set zone 3 to 170° C
3. Set zone 2 at 140° C
4. Set zone 1 at 100° C.
Temperatures can be adjusted based on results from a sample run. Using a temperature recording tool with thermocouples attached to the part at the solder joint will greatly help in establishing the correct peak temperature setting.

What is the purpose of nitrogen in reflow soldering? Flux is the medium that binds the solder spheres into a paste. Its’ purpose is to hold the spheres together and shield the solder as it changes from a solid to a liquid, back to a solid, preventing oxidation in the process. Flux does a good job but often can use help from a cover gas to provide an inert atmosphere for a perfect solder joint. Nitrogen or a forming gas must be used when soldering gold tin applications where flux cannot be used.

I hope this explanation will remove some of the mystery of reflow soldering and give the user confidence to start down the path for a perfect solder joint or sphere.

Phil Skeen
Sales Manager
Sikama International, Inc.