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Flux Residue & Reliability of Conformal-Coated Boards

Solder flux residue on printed circuit board assemblies can cause long-term quality problems. Boards that will have a conformal coating need extra attention in this area.

Most boards these days are made using “no-clean” flux, and electronic manufacturing service (EMS) providers follow specific IPC standards for how much and what type of residue is acceptable on the finished assembly.  But in certain cases “water-soluble” flux is preferred. In these cases the boards need to be washed with water, and the process must be carefully controlled. Knowing which process your EMS partner is using will help you understand potential quality issues.

First, what is flux?  Flux is a tacky, slightly corrosive material that is essential for soldering. For surface mount technology (SMT) assembly, tiny beads of metal solder are mixed into a flux to create a solder paste. The flux has two functions. One, it serves as a “glue” to temporarily hold things in place for soldering.  Two, it ensures a good solder joint by cleaning the metal traces and pads on the board and preventing them from re-oxidizing during the solder process. Click on the video below to see more about the process:

We use a stencil to put the solder paste on the board. The stencil cut-outs precisely match the board's metal areas (traces and pads). The flux in the solder paste is mildly corrosive and cleans the metal.

Next, the pick-and-place machines gently insert SMT components into the solder paste. The flux holds the components in place until the assembly goes through the solder reflow oven. In the oven, controlled heat melts the solder beads and the solder flows along the metal on the board. As the solder cools it creates a mechanical bond and electrical connection between the components and the traces on the board.

Some of the flux evaporates in the process, but some is left on the board. 

For no-clean flux, the heat of the reflow oven renders it inert. It's safe to leave this residue on the board, provided it meets IPC standards for the class of product. (Generally, it can't be excessive or overly tacky and must not interfere with visual inspection or electrical test points.)

No-clean flux helps manufacturers save quite a lot of time and money by eliminating the cleaning step after solder reflow. But there are still some cases when no-clean flux can be a problem, particularly when we'll be using a conformal coating on the finished board assembly. The no-clean flux residue can flake off and leave gaps the conformal coating, which defeats its purpose.

Some EMS providers clean the no-clean boards before conformal coating. But no-clean flux residue can be very difficult to remove, and it can make more sense to use water-soluble flux instead.

Water-soluble flux is comparatively easy to wash off. But it's extremely important that it be washed off completely, because it remains corrosive after reflow. If left on a board, it can lead to electrical shorts months or years later.

We use an orbital aqueous cleaner to wash water-soluble flux residue off PCB assemblies. The cleaner directs sprays of hot, de-ionized water to thoroughly clean all areas of the board, even underneath large surface mount components.

For process control, we use ionic contamination testing.  A board is submerged in an alcohol solution, and ions in any flux residue will dissolve into the solution. The test instrument detects these ions by measuring the change in conductivity of the solution. This indicates how much flux has remained on the board after washing.

In a well-controlled process the results will be consistent over time, and below the maximum threshold.

Ionic contamination testing instruments have been around for a long time, but newer models offer improved process control by storing all test data electronically. This allows manufacturers to continuously monitor the process and review consistency of the results. It also provides an automatic record of board cleanliness for each product over time, giving a complete production and product history.

Ionograph

Ionograph

If your PCB assemblies will be conformal coated, it makes sense to talk with your EMS partner about whether they will use no-clean or water soluble flux, and what process controls they have in place to ensure the long-term reliability of your assemblies.

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