Selective Soldering under Nitrogen  [ Return towards  Applications  ]

Even with the predominance of Surface Mount Devices (SMDs), selective soldering technique is still commonly used. The necessary use of connectors in many products today and the overall cost effectiveness of this process are some of the factors that contribute to its predominance in the industry.

In many cases where only a few connectors are used and board real estate is crucial, the use of special wave soldering process - selective soldering - may be advantageous. Selective soldering is equivalent to performing a wave soldering process with a mask. It allows SMDs to be present on both sides of the printed circuit board, and hence only a few PTH connectors are required for their mechanical strength. Selective soldering is now gaining rapid adoption in the electronics components and packaging industry.

There are a few types of selective soldering methods. Equipment can be manual, in-line and robotized to adapt to modern PCB assembly lines requirements. Some equipment use laser diodes while the majority uses a mini-wave of molten solder. Similarly to wave soldering systems, the mini-waves are subjected to creating dross. However, due to the nature and precision of the wave, a dross-free mini-wave is essential for a high-yield process. The need for nitrogen is therefore essential in mini-wave soldering. In addition, the inert atmosphere improves wetting while increasing surface tension results in higher quality joints. The flow of nitrogen is generally very low at 100 scfh and its low cost is easily offset by the gained benefits. 

Rework of PCBs with Hot Nitrogen  

Every effort is made to eliminate defects in PCB assembly. However, due to process variations and the complexity of soldering, defects are always present in PCB assembly. Because of the hi-cost of PCBs and components, it is often better to correct or rework these small defects.

Rework is typically performed with the help of rework station. These are generally manually operated and hot gas is used to melt, move, and resolder the defective joints. The hot gas jets provide localized heat to only the portion of the substrate that is to be reworked on, without damaging the other components.

These rework stations can either use compressed air or nitrogen. Using nitrogen to rework is more effective than using compressed air. Nitrogen allows the pulling of components more easily while increasing wettability and the aesthetic appearance of the joints. The rework process is a precise and sensitive process and using nitrogen increases the quality and yield of this process. This is a cost-effective method since the required flows of nitrogen is minimal.

If your rework equipment is not configured for nitrogen, a slight equipment modification might be required.