Cleanliness: you get what you ask for

The key to maintaining cleanliness is asking more from your suppliers than you probably currently are. Remember, if you ask for nothing, you will likely get it.

It’s been over three years since we’ve written a post related to incoming cleanliness and effective cleaning processes. Unfortunately, it’s not because everything is now clean within the electronics industry. As a reliability laboratory, we typically see multiple projects each week where contamination or residues cause field performance problems.

Sources of Contamination:

• PC fab process

  • Etch residues: sulfuric acid, cupric chloride, ammonia, tap water

• Component cleanliness

  • Same residues as with PC fab process

• Connector cleanliness

  • Plating/tinning of leads

  • Mold release

• Housing cleanliness – plastic and metal

  • Particles from manufacturing process

  • Mold release

• Receiving and storage

  • Packaging materials

  • Storage areas

There are other sources. In this article, we focus on incoming boards and components.

Solutions

The good news is that most of the sources can be cleaned (receiving and storage being the exceptions). Cleanliness of incoming materials is most important when using a no-clean flux with no intention of removing it. When no-clean is the flux of choice, every part of the PCBA brings its own possibility to create a failure with process residues. Worse yet, there is no opportunity to remove it with a subsequent wash process. There is also the risk of multiple marginally clean parts combining to create electrical leakage and/or electrochemical migration related issues in the field.

The key to maintaining cleanliness is asking more from your suppliers than you probably currently are. Remember, if you ask for nothing, you will likely get it.

Bare boards are the base for all PCBAs, so it is important to test every lot of boards for cleanliness in some way. ROSE testing is the historical method, but Foresite recommends ion chromatography, which features a more effective extraction method, as well as the ability to identify specific ions. Bare boards can be effectively cleaned in an in-line cleaner using a saponifier, but this is a process that should be done by your PC fab supplier and not by you. Bare board residues are normally from the etch process and can be some of the hardest to deal with as they are mostly strong acids that can create multiple failure modes. When etch residue is left behind, you can see issues with solder mask adhesion, solderability problems, internal CAF failures, and more. You must know you are starting with a clean base for your bare boards to help reduce the risk of failure going forward, since your assembly process adds even more residues.

Component cleanliness is as important as any other part of the assembly due to the fact that, if there are residues present, they are going to be in the areas most likely to cause electrical leakage. This can happen at the board level between leads or even across the component body with minimal amounts of available moisture and voltage differential. Testing components for cleanliness using ion chromatography is rarely performed in terms of lot acceptance; unfortunately, we do use it commonly after a failure has occurred.

Connector cleanliness is like any other component since there is normally a plating process used for the leads. Every plating or lead tinning process uses various chemistries that must be removed before installation into your product. Connectors also add mold release to the mix of chemistry that can cause electrical leakage and dendrite growth inside the body of the connector that doesn’t need any outside residues.

When looking at cleanliness of the housings that your product goes into there are concerns with mold release, like the connectors, but there is an increased risk of more concentrated residues transferring to the PCBA surface. This happens when the product goes through multiple condensation cycles and the moisture collects at the lowest point of the housing. When there is enough moisture to drip down onto the surface, it has effectively collected residues from the entire surface of the housing and is now very concentrated.

Receiving and storage areas can contribute to assembly issues, but are different than the areas that we have discussed to this point. One of the most important things to consider is the control of humidity in storage. If boards or parts are exposed to excessive moisture, there is a chance for oxidation, which impacts solderability. Another parameter to control is air quality. We have seen storage areas being open to the receiving areas. Every time the dock doors are open, there is an opportunity for introduction of atmospheric contamination. If the parts or boards are exposed to dust or debris, there can be an impact on solderability or even conformal coating adhesion.

In general, you must remember that if your product doesn’t go through an effective wash after assembly, you are exposed to the cumulative effects of the potential contamination from every item on the BOM. Holding your suppliers accountable for incoming cleanliness is critical to protecting yourself from costly cleanliness-related failures down the road.

Figures 1 and 2: Component as received, and component after epoxy removal