Recently, a client experienced a high rate of product returns that were identified as no trouble found (NTF) when bench tested. The assembly included a hand-soldered connector on the wave solder side of the assembly. A low-solids no-clean paste, wave flux, and cored solder were used on the assembly and qualified with standard SIR testing. Product built during the product validation process also performed well during ESS testing. Now, eight months into the production cycle, boards with less than sixty days of field service are experiencing a high percentage of intermittent problems in the field.

Upon initial investigation, nothing had changed. The board vendor and their processes, the materials, thermal profiles, and equipment were the same as at the time of qualification. The assemblies showed no differences that could be identified by trained visual inspectors and instructors and cleanliness testing on the production floor consistently showed low levels of NaCl equivalents (< 6.0 µg/sq. in.). So what was the cause?

Finding the Root Cause

SEM and EDX analysis were inconclusive as to the root cause of the failure. Our investigation of the failures centered on a direct comparison of field returns and current production samples. We also checked the cleanliness of the incoming bare boards and the components that were common to the typical failed circuits (0805 resistors and connector). Ion chromatography results were as follows: The data indicates that the failed assemblies and current production samples were similarly high in chloride and weak organic acid (WOA). The high chloride likely came from bare board HASL fluxes and cleaning processes. The connector and resistors exhibited low levels of ionic contamination as received and were not contributing to the issue. Boards without hand soldering of the J1 connector showed a much lower level of WOA.

The hand soldering process used additional, manually applied wave solder flux for soldering the connector. The extra flux between leads on the connector was not heat activated, and is moisture absorbing and conductive. The partially reacted flux residue and high chloride levels from the bare board fabrication process create a product that will continue to have intermittent performance problems until enough moisture is absorbed and hard electro migration failures occur.

Now that we know the root cause of the NTF problems, the question is “can we clean a no-clean?” When no-clean fluxes are cleaned with water only, a thicker white haze is formed between the connector leads and heavy white residue is seen between leads on the board surface. Cleaning with water only showed no reduction in the residues and a heavy visible flux residue. Cleaning was only effective when a saponifier was used at low pressure at 140 °F wash temperature, then rinsed with steam and DI water. The data show that the board fabrication and WOA residues have been greatly reduced (below our recommended limits) and functional testing of the effectively cleaned assemblies indicated great product performance (100% of cleaned boards passed) even after 16 hours of 40 °C / 90 %RH exposure. Control non-cleaned assemblies were also exposed to identical environmental conditions and 90% failed due to stray voltage and leakage. 

Conclusions

Heavy, partially reacted low-solids no-clean fluxes can cause electrical performance problems by absorbing environmental moisture and creating a conductive path on sensitive circuitry. Cleaning with a saponifier and steam with low pressure on the entire assembly can remove the moisture absorbing flux and even clean fabrication residues that may cause long term reliability risks. So, YES, you can clean a no-clean!

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