Why Cleaning Still Matters

Estimated reading time: 16 minutes

Las Marias: Right now, in mission critical applications, you just have to clean your board even though it's no-clean.

Solis: But to what level do we clean the boards? That's another question. How clean is clean? How clean is clean for a mission critical board? The level of cleanliness I need for that environment is going to be different for manned space flight, as it is for military missile operations, as it is for public transportation, as it is for communication devices. The level of cleanliness and reliability is different because the environments are different in which they operate in. You need to start at the end service environment. The more reliable it needs to be, the cleaner it needs to be, and we're not even talking about conformal coating considerations yet.

It's a more complex question, but it's been reduced down to “no-clean.” It is not that simple. The new engineering generation hears the word "no-clean" and they think, "Oh. Okay. I don't have to clean." "Why don't I need to clean?" is the question that needs to be answered. Why do I not need to clean? Why is it no-clean?

Las Marias: It depends on the manufacturer and what they want to do.

Solis: The manufacturer and the end service environment it needs to operate in.

Las Marias: So Foresite also does failure analysis. How does that process work?

Solis: It depends on the failures we’re addressing. If we're looking at a metallurgical issue, then it could be as simple as we’re looking at reflow profile from the SMT assembly, checking to see how much energy and time was used in the process. Did they have a proper ramp rate? Did they have a right activation energy and temperatures for the type of flux they are using? Did they reflow at the right temperature? Or did they over-temperature during reflow? What metal system are they using? What are the reliability needs of the assembly and did they properly process and reflow that solder for that part? Is it the most appropriate alloy and grain structure for the application? Does it have an inner metallic, a continuous and contiguous inner metallic? Is it a lead-free system? Does it have a large amount of copper precipitate or copper solubility into the metallurgy? It depends on the failure. We help them get rid of the factors in the process. That's just one. Metallurgy’s just one disipline.

If we’re working toward a chemistry failure analysis, say they didn’t use enough thermal energy in the reflow and they haven’t rendered their flux activators benign. In this case we help them through thermal dynamics studies to see where they’re missing the thermal energy requirements and what should they have done to the board to properly cross-link the flux residues so that they are benign.

Las Marias: In the SMT process, where is the defect more evident or where does it come from mostly?

Solis: From our experience, it’s process excursions. You have to really look at terms like “touch-up”, “rework”, and what do they mean? To some companies, they’ll add a solder heat alone and call that “touch-up”. Some companies when they add solder or flux they can call it “rework”. Some call this “touch-up” and only call it rework when they take off a device and re-solder in a new one. These rework processes are excursions from the standard engineering of the assembly process. If the company doesn't have the ability to own traceability of the rework and understand the amount of cross-contamination possibilities that are contributors in each step of the rework processes, I don’t advocate reworking without material review activities for that product (MRB).

If I’m adding a cored solder to a rework process, I need to make sure I know what type of flux is in the solder and how to activate the flux completely. If I’m adding liquid flux to that board, I need to make sure I completely activate that liquid flux and I didn’t allow active flux to cross-contaminate the adjacent components. If you haven't educated your rework crew or your production crew on what matters, they could be dry brushing solder balls with a dirty brush and relocating contamination from PCBA to PCBA (just as an example). A little bit of rework and a little bit of touch-up and they think it's value added. The process of taking a brush and brushing the backside of a printed circuit board, which may have flux contamination, and now that same brush goes to brush the next board, and I transfer the contamination from the first board to the second board. Anything that touches the board or goes into the board, you really have to consider what is the possibility for adding cross-contamination?

If I rework it, how am I changing the original design of the material and the chemistry? So, I consider all rework as a process excursion and risk to the product. Unless I understand what rework really is and its contribution to my product. From my perspective, that's why I get called to walk assembly lines. To see where the opportunities are for cross-contamination. Once it comes out of SMT, that’s when it really starts. When operators start touching the boards, they can start transferring contamination. Most engineering communities aren’t aware that rework causes cross-contamination.

Las Marias: What is your advice for the industry on this front? How do they avoid that?

Solis: Back to one of my first statements, when we approach our suppliers we need to get the best quality we can afford and not the cheapest thing we can find. The PCB itself is the second most integrated part of a PCB assembly besides the ICs. After ICs, the PCB is not a commodity like a resistor or capacitor. It is a complex assembly. It needs to be treated like a complex assembly. That is one of my largest recommendations. Buy the best that you can afford. It starts with the PCB. If you don't start with a clean PCB, then you're not going to end with a clean assembly.

Las Marias: Is the cleaning for PCB different then the way that you do it for PCBA?

Solis: That depends on the quality level you want to achieve. It comes down to “How are you washing?” Some PCB manufacturers use tap water in some of their processes and depending on what your architectures are (that may be fine for a toy that gets placed into a fast food meal). Once you get into higher reliability, you have to look at the quality of your water and the positive and negative effects of washing. Do you need to use chemistry to clean up that circuit board? What are the reliability requirements? That will drive how clean you need to be. If you have microvias and through-hole vias and you are trying to use cold DI water, you’re not going to get them clean. If you use hot DI water, you might get them clean. If you use hot DI water with chemistry, you'll lower the surface tension and clean all this architecture that you couldn't with DI water alone. But now that will add cost to your assembly. It's really, what is the end use environment? What is the end quality needed? You’ve got to work backwards.

Las Marias: Paco, do you have any final comments?

Solis: When it comes to the PCB assembly process, the different departments of organizations need to work together. Your quality assurance, your procurement department, your supplier quality department, they all have to work together to understand cleanliness. Every part that goes into that PCB assembly is going to affect its final cleanliness and its reliability in the field. It’s not just the job of one department much less one engineer. Reliability engineering must work with quality assurance, with procurement, with subcons, with suppliers, and all with a good knowledge of all disciplines which go into the assembly. But, they have to be educated on again what the end use requirement are. The breakdown of the process creating a reject could come down to something as simple as a dirty glove. A dirty glove or a chemical that gets substituted because it’s believed to be cheaper and equal (as an example). Replacing a material in the process without really looking at what effect chemically I am doing to my assembly can be a very large expensive mistake. A small change in a process, a different foam, a different sealing agent, a different adhesive tape, a finger cot, a solder, or a flux, the engineering and the education must be there.

Las Marias: It's the skills of the operators as well.

Solis: Very true! You must have a trained and informed workforce at all levels. From my experience most operators do as they have been trained and believe what they are trained to do has added value. If they have not been trained for cross-contamination avoidance they will still operate as if their process is not generating rejects. From Janitors to Managers and all steps in between. All must understand their contributions to cleanliness and reliability and help to decide if cleaning be required? It comes down to education, training, and communication.

Las Marias: Thank you very much, Paco, for that.

Solis: Thank you!

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