When electronic assemblies have questionable levels of cleanliness, the end result can vary from no-trouble-found, intermittent performance issues, all the way up to hard failure, customer returns. At times, contamination is discovered at ICT or burn-in testing; however, suspect product frequently already exists in inventory or the field.
There are only a few options at this point:
- Scrap the inventories and rebuild,
- Continue to ship and hope for the best, or
- Perform rescue cleaning on as many assemblies as possible to acceptable cleanliness levels – levels that do not increase the risk of electrical leakage or electrochemical migration in the field service environment.
Rescue Cleaning Considerations
When choosing the rescue cleaning method, there are considerations to keep in mind. If the original flux choice was water-soluble, the performance problem might be corrosion of the base metals - which can occur even without any electrical bias being applied to the assembly. If corrosion is such that the integrity of the solder joints is in question, a rescue cleaning operation will not help.
However, in the case of a no-clean flux not being fully activated, therefore still active and hygroscopic, the choice of rescue cleaning is a good one. Cleaning a processed no-clean flux can be a tough task and will very likely need the addition of a saponifier to help break down the outer shell. When dealing with low standoff and/or tight pitch components, the use of steam to provide additional energy, may also be necessary. Our studies have shown that gaps of 12 mils or less are very difficult to penetrate with only de-ionized (DI) water, even when it is of good quality and sufficient temperature.
Normally a batch type wash system will not be effective in removing all of the flux residues and can actually accelerate current leakage issues. When a no-clean flux is partially cleaned, the outer protective shell is removed, exposing the oxides and any contaminants to available atmospheric moisture. An applied bias differential will then cause electrical leakage. A standard inline washer is preferred, since the process is far more repeatable and measureable - necessary attributes when tackling a tough cleaning job. Consistent fluid flow, under proper process parameters, is the best option for fully removing flux and other process residues.
An ExampleA client came to Foresite with an issue with an assembly process, wherein no-clean flux had not been properly, thermally exposed. The activator was still active and was causing current leakage issues during burn-in testing.
We performed visual inspection and ion chromatography (IC) under the QFN components (after mechanical removal). The IC data showed highly-elevated weak organic acids (WOAs) – a result of the active flux. After cleaning the assemblies with high quality DI water and saponifier in our inline wash system, final IC testing showed a 93% reduction in the WOA concentration, to well within our cleanliness limits.
For more details on Foresite’s rescue cleaning services, please go to http://foresiteinc.com/services/cleaning-recovery/rescue-cleaning/.