August 11, 2022

Benefits of Spray-in-Air Cleaning Technology

Just as it is important to choose the right cleaning chemistry, it is also important to choose the right technology. So which is the most effective?
Main changes in electronic assemblies 
In the early days of SMT, the components were mostly only leaded, with quite big gap between package and PCB, chip sizes were not under 0603. After soldering, gaps under components were mostly not completely filled with flux residues.
Today, small chips, bottom terminated components (BTC), have gaps between body and PCB smaller than 50um (usually up to 10-20um), totally filled with flux residues. That makes cleaning more challenging. Additionally, not all types of flux residues are easily soluble. In practice, solder paste choice is done preferably according to soldering properties, not according to cleanability.
 
Pic.1: BGA – gap 200um – easy to clean
Pic.2: QFN – gap 30um, cleaned
Pic.3: 0201MLCC, human hair
Therefore, not only optimal chemistry choice plays substantial role in cleaning effectivity, but mechanical support. “Digging” flux residues from the tiny, long and narrow gap under component body, or between terminals of BTC needs high kinetic energy.

How to bring that kinetic energy most effectively directly to the spot? There are several possibilities.
 
Ultrasonic – mechanically, far best option, however local pressure in thousands of Bars and temperatures of hundreds °C in the microcavities after their splashing makes even today, many electronic manufacturers uncertain about bad influence on components reliability.

Spray-Under-Immersion – turbulent movement of the entire bath with immersed assemblies seem to be efficient, because the contact time between chemistry and assembly is 100%. But the energy from liquid movement cannot reach the assembly surface effectively. Some liquid layer on the surface stays without movement, or with only slow movement. (Laminar layer) Thus, a direct mechanical effect in gaps under components is very low. 

Spray-in-Air – liquid flow (round or flat beam) with high-speed hits the assembly. The laminar layer on the assembly surface is much thinner. In practice, spray in air can form a liquid stream even in the gap 50um thick! 

Direct Spray-in-Air – this is the most efficient spray configuration for cleaning under components. The spray beam hits the surface of PCB under 90°, cleaning liquid flows along the surface, all directions. Scanning of such beam makes cleaning conditions very uniform, eliminates shadows.
 
Pic 4: Direct Spray-in-Air
Pic.5: Progress in cleaning flux residues under chip 0805 - intervals 2,5 min – total cleaning time 7,5min, Direct Spray

Today, Spray-in–Air is the most spread cleaning technology for many applications. From cleaning small batches of very complex boards to the mass production of reliable electronics.
Thanks to this approach, users have much wider process window in cleaning using only one type of cleaning chemistry for different soldering materials and different soldering profiles. 
 

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