Why Clean Solder Residues?
Though solder clean process is pretty important, the cleanliness of a PCB assembly is still contentious in the electronics manufacturing industry. So when choosing your assemblies, how do you decide what to specify?
Firstly, you should know what does clean actually stand for. It is better to regard as lumps of unwanted debris - mysterious white residues and corrosion of any kind are obviously not allowed. Cause such things not only make the assembly look untidy, but also damage the functionality or long term reliability of the product. So, most of the time, when we’re talking about cleanliness, it means flux residues.
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.
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.
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.