Posted on 08 Oct, 2015 by Russell Poppe
Although the IPC standards give pretty clear guidelines, the cleanliness (or otherwise) of a PCB assembly is too often a subjective and even contentious subject within the electronics manufacturing industry.
So when you’ve chosen to outsource your assemblies, how do you decide what to specify?
Firstly let’s ascertain what "clean" actually means. I think we’re on safe ground to assume that lumps of unwanted debris - or "particulate matter" - mysterious white residues and corrosion of any kind are obviously not allowed. Such things not only make the assembly look untidy, but could be damaging the functionality or long term reliability of the product. So, most of the time, when we’re talking about "cleanliness", it’s all about flux residues.
It is possible, to an extent, to measure cleanliness. For example, if an assembly is cleaned, you can measure the residues in the cleaning process cycles until the point that the desired maximum has been achieved. However, for many - perhaps most - applications, it’s more about the cosmetic finish or other process considerations.
As you might imagine, when we start to talk about solders, fluxes and cleaning agents there is quite a bit of chemistry going on. But let’s neatly avoid all that and have a look at the practical issues surrounding whether an assembly should be cleaned or not. So, why clean?
Obviously, there are cosmetic considerations. If a "no clean" flux is used during the SMT process the assembly should look perfectly fine, as most of the flux gets burned off during the process. However, if there is through-hole soldering afterwards - particularly hand soldering - it can show small areas of flux that make it all look a bit untidy, due to the extra flux that is typically applied and the reduction in both heat and time associated with hand soldering, compared to SMT ovens.
Perhaps most importantly, any residues mustn’t interfere with visual inspection. If there is anything left that is obscuring solder joints, for example, then it must be cleaned off, either by hand or using one of the automated cleaning machines available.
If the product is to be tested using test probes, for example in-circuit test or flying probe, then flux can cause unreliability if the probes can’t access their test points on the PCB. This is usually fine if a pin-testable flux is used, but any excesses may need to be cleaned off.
Get the "Design guidelines for in-circuit testability" technical paper
If the assembly is to have a conformal coating, then all flux residues must be removed before application, so the PCBA will have to go through a cleaning process.
So why not clean? Well, some fluxes are designed to be cleaned off, and some aren’t. Again, neatly side-stepping the chemistry here, if the flux is designed to stay on the PCB then trying to clean it off generally isn’t a good idea. There is an argument that the flux can provide a harmless protective layer over the solder joints and removing it can expose the PCB finish to problems later.
Naturally, there is cost to consider. Cleaning is an additional process, so it’s going to have a value attached to it. And there is also the impact on the environment with additional water or chemical supplies needed, which then have to be disposed of correctly.
It’s also worth bearing in mind that certain types of components can’t be cleaned, as the cleaning agent can get inside the body of the device and damage it. So, if cleaning is required it can complicate the build process - for example, having to fit components after the main clean, then individually clean those components, or by adding extra drying stages. Again, extra processes mean extra cost.
So what are your options when it comes to cleaning? Well, there’s a few, which include:
- Localised cleaning, which can be achieved with a brush and cleaning agent such as IPA. However, care must be taken to ensure the residue is cleaned off and not simply spread thinly across the assembly.
- There are automated solutions as well; some that use water (very similar to a dishwasher) or other cleaning agents, depending on the flux used. These cleaners tend to be relatively small in terms of physical footprint and the waste can usually be disposed of down existing on-site drains.
- Ultrasonic cleaners, on the other hand, tend to be larger in size, cost more and, due to the chemicals used, will require specialist handling of the waste that is produced to ensure it is disposed of safely. However, there are still certain components that can’t be cleaned using this equipment.
Many electronics manufacturing services (EMS) providers now standardise on a "no clean" process. So, if you do want something different, it is worth weighing up the options, the pros and cons, and letting your EMS partner know what your requirements are to make sure that they have the equipment, skills and processes in place to meet your demands.
Image by Judy
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