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Case Study: Voids in Solder Joint - Preheat Effect in Saddle Reflow Profile

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A void is a cavity in the solder joint. Three factors contribute to generation of the voids: air bubbles, de-wetting of solder and poor discharge of air bubbles. The effect of these factors can be reduced using the following methods.

 Air bubbles - reduce the amount of bubbles generated while solder is molten 

 Solder non-wetting - improve solder wettability

 Poor discharge of air bubbles - improve discharge of air bubbles  

Voids reduce cross-sectional area of solder joints, lowering their thermal and electrical conductivity. It is also desirable to minimize voids because they reduce strength at solder joints, affecting their reliability.

This time, preheat effect in saddle reflow profile is being studied.

< Occurrence of voids >
Using a short preheat time at low temperature tends to increase the generation of voids in components with bottom-side terminals such as LEDs even with saddle type reflow profile.

Void Generation Mechanism
Even with the saddle type profile, low preheat temperature and short preheat time is not enough for full volatilization of flux (solvent and activator components) during preheat. The remaining flux volatilizes while the solder is molten, producing more voids (because the bubble generation increases).
*Since the profile design relies heavily on flux composition and the used components (large components that are soldered at the bottom side such as BGA and power transistors, or small components that form solder fillet such as compact chip components, this tendency does not necessarily occur in all reflow soldering processes.

Saddle type reflow profile with different preheat temperature and time was compared for the occurrence of voids.

Countermeasures against Voids
Preheat parameters affect void generation. When a large number of voids is generated in components with bottom-side terminals such as LEDs, it is recommended to set the preheat temperature and preheat time as high as possible.
*It is necessary to be careful, because if the preheat time is set too long, voids may be generated due to solder de-wetting.

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