Lead-free processes propose new requirements on reflow soldering quality

What role will Reflow soldering equipment plays in lead-free SMT process actually? what new requirements does lead-free process proposed for reflow soldering quality?

• How to obtain a smaller horizontal temperature difference?
Lead-free soldering process window is very small, lateral control of the temperature difference is very important. Reflow temperature is generally affected by three factors: 
(1) transfer of hot air reflow
Current mainstream lead-free reflow oven take 100% full of hot air heating. In the development of the reflow oven, infrared heating, but due to the the infrared heating presence of different color devices infrared absorption reflectance and shadow effect is due to the block adjacent to the original device, both cases are caused by temperature difference leaving the risk of lead-free solder out of the process window, infrared heating technology in the reflow oven heating has been gradually phased out. In the lead-free solder, need to focus on the heat transfer effect. Especially for the large heat capacity of the original device, if you can not obtain a sufficient heat transfer will cause the rate of temperature increase significantly lagged behind the small heat capacity of the device and result in a lateral temperature difference.

(2) reflow chain speed control
The chain speed control will affect the horizontal temperature difference of the circuit board. Conventional, lower chain speed, large heat capacity of the device will get more time for heating, so that the horizontal temperature difference decreases. But after all the settings of the temperature curve depends on the requirements of the solder paste, so unlimited reduce chain speed is unrealistic in the actual production.
(3) reflow wind speed and air volume control
We have done such an experiment, keep the reflow furnace conditions set constant to only reflow furnace fan speed reduced by 30%, the temperature of the circuit board will be reduced to about 10 degrees. So we see the importance of wind speed and air volume control to temperature control.
In order to achieve the wind speed and air volume control, you need to pay attention to the following three aspects:

A. Fan speed should be implemented variable frequency control, to reduce the effects of voltage fluctuations it;
B. minimize pumping equipment exhaust volume, because the central load pump drainage is often unstable, easily affect the flow of hot air in the furnace.
C. the stability of the reflow soldering equipment
Even if we get an optimal reflow temperature curve settings, but to achieve repeatability and consistency of the equipment, it still need to be assure the stability, especially lead-free production. if the temperature curve drift slightly ,it would be easy to jump out of the process window causes cold or damage to components. Therefore, more and more manufacturers have requirements for stability testing.

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