Fluxing and Cleaning consideration in soldering

Fluxing is one of the major items you may consider when soldering. Flux is used in the conjunction with solder during the soldering process. Choosing the right flux and using it correctly are critical to proper soldering. Fluxes have varying degrees of activity or ability to remove contaminants from device terminals and PCB pads.

The main characteristic of fluxes is the solubility. The solubility of flux determines which cleaning processes can be used to remove the flux. In the past, many circuit assembly facilities used active, non-water-soluble fluxes. Active fluxes can remove surface oxides from device leads or terminals to form even and smooth solder joints with excellent wettability. However, active fluxes are highly corrosive. When active flux is not completely removed, solder joints are more susceptible to corrosion, and it affects the long-term reliability of the device. Thus, active flux must be removed from the PCB. The removal process is called cleaning or washing. Many materials that are highly effective at removing flux, such as Freon and trichloroethane, were subject to environmental legislation years ago and are now illegal in environmentally conscious countries. As a result of these concerns, two trends have emerged:

  1. Some circuit assemblers consider using low activity fluxes. Today there is a type of fluxes known as “no-wash” “leave-on” flux which has low activity level and is no need to remove from the PCB. However, low activity flux often leads to wettability problems. Wettability is a measure of how well the solder connects the device leads or terminals to the printed circuit board (as Figure 1).

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Figure 1. Good Wetting in a soldering joint

Two important factors in a low-activity flux process are the cleanliness of the device leads or terminals and the plating thickness of the leads or terminals. If the terminals of the device are not clean enough, de-wetting may occur. If the plating thickness of the device is insufficient, terminals will cause non-wetting.

  • De-wetting: Fluxes with lower activity levels are not as effective as active fluxes at removing oxides and contaminants from the device leads or terminals. De-wetting can occur when contaminants are not properly removed from the device’s pads and/or leads. De-wetting is characterized by the appearance of irregular and dispersed solder droplets on the joint surface, often separated by a thin layer of solder.

  • Non-wetting: If the plating thickness is too thin, the solder may not adhere to the device leads or terminals. This problem especially applies to axially leaded devices because the solder must bridge the via from pad to lead (as Figure 2).

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Figure 2 Non-Wetting in a soldering joint

  1. Because of the above-mentioned problems associated with low-activity fluxes, the electronics industry has developed water-soluble, highly active fluxes. These new fluxes offer the performance of highly active fluxes with the water solubility for easy cleaning.

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SRA #80 WATER SOLUBLE FLUX - Superior fluxing ability with Instant wetting

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Chip Quik Liquid Flux Water Washable in 16oz Bottle - Water soluble and Highly Activated Organic Acid (OA)