To Clean or Not to Clean
To clean or not to clean?
I like to start with the rather philosophic question. This is because very commonly, cleaning of electronic assemblies is considered as a non- productive operation, which only increases the costs of the product. As a cleaning machine manufacturer with almost 25 years of experience we can confirm this standpoint at many production sites. Frequently, the reasons for cleaning are like “our customer will like it (requires it)”, not so frequently the information about final application and environmental condition for assembly life is taken into account and only very few of customers really know, why they NEED to clean and to which extend.
I would like to explain some myths about cleaning process coming from the 90-ties when the NO- CLEAN soldering technology was considered as universally applicable.
It is always interesting, to remember a little history. Cleaning begun in and of 50-ties last century, more precisely during Korea war, due to new experiences with malfunction of some electronic in tropical conditions. Since this time, until and of 80- ties, all important assemblies was obligatory cleaned
Than the quality of surfaces before soldering was so stable, that it allowed to apply only very low activators in vary dissolved resin flux base. NO CLEAN was born...
The term “NO CLEAN” in real assembly life conditions is often misunderstood. People feel safe, if they use such fluxes and even they often think, such fluxes must be not cleaned. Both is an improper, very risky thinking.
NO- CLEAN flux is not a technical definition, it is only a commercial term of sales people. More exactly defined, it is a low solid, low activated flux. The base can be modified rosin or resin or even other organic compounds.
It offers the possibility to seal ionic active residues (which are present on the surface of soldered joint after reaction of organic acids from flux activators with surface oxides.). Such sealing (i.e. insulation from environmental moisture) is only so good, like the quality of reflowed resin allows. Some are less, some more resistant to water absorption and dissolving. Almost all are, however, brittle and do not like temperature variations. It crack after some time. And the way to ionic active residues becomes to be free.
It is necessary to add, that ion migration (similar like in electric battery or accumulator) needs three things:
Free ions - there are present inside of flux residues
Bias – it is present in every assembly in function
Moisture – it is present during every big temperature changes (remember good cold drawn beer glass)
Hence – unless we are not sure, that our assembly is never exposed to these three factors simultaneously, we should be worry.
Statistically the leak current and ion migration are responsible for big portion of reliability failure of electronics.
Now a pragmatic question. What is a projected lifetime of my assembly which I build?
This question is equal to the definition of reliability of our board. Widely accepted IPC 610 standard defines three product classes, which (not exactly) are defined as:
1: assemblies where function is essential (consumable electronics)
2: assemblies where function and reliability is required (industrial electronics)
3: assemblies, where function, reliability in harsh environment is required and human health or life can be affected by failure)
So, for manufacturers, which produce the two upper class equipment, the question about cleaning decision should be obligatory.
And to produce Class 3 equipment without cleaning process at the end can be really risky.
One should not forget that ionic active residues are not only danger. There are other:
FOD (foreign object debris from the production steps – it may be spill from PCB singularization process, solder balls, fibres from cloths, skin fractions. Grease and salt (usually from handling – even if it is properly done)
Organic residues – rest of resin. If you make “conformal” coating, you should have PCB surface of certain surface tension. Simply clean surface from things which does not match with coating layer.
Imagine at home. If you paint wall or door, or any other surface you always clean the base. Maybe electronics is different? The word “conformal coating” comes from very ancient time of electronics, when coat was still made of rosin based material and therefore was dissolving rest of rosin based flux on the board. But the assembly was much simple and never on such exposed places like now. Today, it is really a very misleading name!
Beside of that, there are some processes, where cleaning is unavoidable even for consumable electronics. This is a bonding process. In the past, such kind of cold diffusion connections was only at home in active component packaging technology, but now, PCB and substrate assemblies are very often made using this process.
The current “peak of iceberg” is a DCB technology (direct copper bonding) which requires not only absolutely clean surface, but also an activated copper surface (and special temporary chemical treatment for time between cleaning and bonding.
Such processes requires different approach to cleaning than before. Machines must be controlled very precisely not only by time and temperature, but for many other parameters.
We are proud, that PBT Works is one of companies, which can offer a machine, capable to fulfil this complex task.
Managing director of PBT Works
MIT trainer IPC 610