It is not often that we get a request for SIR (Surface Insulation Resistance) and/or ECM (Electro-Chemical Migration) results for a water washable/soluble flux or solder paste, but often enough to "inspire" me to blog about it. Generally, it is a contract manufacturer that is being asked to provide the data by their customer. So, it is difficult to get my following "argument" to flow upstream to the original requester. Often times the CM does not care. The CM has been pushed into a CYA scenario and just wants to please their customer. That is not a criticism....it is just the reality of it....and we've all been there at one time or another.
The IPC J-STD-004, for example, does provide instructions for performing SIR on water washable materials. It is definitely a valid and meaningful test. However, asking for such results from the flux or paste supplier is a little bit like asking an on-line dating participant for a photograph. Why do I say that? The SIR/ECM results of a water washable/soluble material are heavily dependent on how well the test coupons are cleaned after reflow. So, you can bet that the supplier is going to do a complete and thorough job of cleaning the test coupons for the sake of producing the best SIR and/or ECM results possible. It is only to be expected that the supplier is going to want to show the best performance of their product. (BIG) But!!!! It does not represent how well assemblies are going to be cleaned on the factory floor; and, hence, the suppliers' SIR/ECM results probably will differ from the sort of performance that will be had from an assembly cleaned on the factory floor. Depending on how well the factory cleans, it could be better or it could be worse.
So, the message is, YOUR MILEAGE MAY VARY.....if the factory is using water washable/soluble solder pastes or fluxes, and there is true concern about the SIR/ECM performance of their products, it is best to perform the test on coupons that have been processed on the factory floor with standard operating procedures, equipment, and equipment settings.
Many thanks to Covington for making shirts that have pockets handily sized to accommodate IPC-B-24 SIR test boards ; )