Issue link: https://iconnect007.uberflip.com/i/1415897
80 SMT007 MAGAZINE I OCTOBER 2021 be two SIR patterns on the board (on produc- tion boards for products meant for critical ap- plications). One of those patterns will be cov- ered with the components with lowest stand- off and the other pattern should have no com- ponent on it. is will serve as a control since you should always get very high SIR value as there should be no flux at that spot. Please re- fer to SIR pattern guidelines as discussed in the referenced chapter 1 . Finally, it must be noted that there is no such thing as the best flux, the best cleaning method, or the best method for determining cleanliness. ese variables depend on the application. us, using the guidelines discussed in this column, the user must establish requirements for flux, clean- ing, and cleanliness testing based on empirical data for a particular application. is means that the cleanliness tests (SIR, solvent extraction, and visual) should be performed on cleaned ran- domly-selected assemblies as a check on the pro- cess. ere is no substitute for good process con- trol because, if a bad board passes the cleanliness test, the failed assembly lot cannot be recalled, recleaned, or retested. SMT007 References 1. Ray Prasad, Surface Mount Technology, Prin- ciples and Practice, second edition, Figures 13.13, 13.14 and 13.15. Ray Prasad is the president of Ray Prasad Consultancy Group and author of the textbook Surface Mount Technology: Principles and Practice. Prasad is also an inductee to the IPC Hall of Fame—the highest hon- or in the electronics industry— and has decades of experience in all areas of SMT, including his leadership roles implementing SMT at Boeing and Intel; helping OEM and EMS clients across the globe set up strong, internal, self-sustaining SMT infrastructure; and teaching on-site, in-depth SMT classes. He can be reached at smtsolver@rayprasasd.com and regularly offers in-depth SMT classes. Details about classes can be found at rayprasad.com. To read past columns or contact Prasad, click here. ly for rework, leaving behind lots of flux that was never heat activated to become benign. Recommendations for Cleanliness Requirements Based on data over multiple decades now, my sense is that there are three simple require- ments that should be considered (and may be given the force of an industry standard such as J-STD-001 and IPC 610 in future revisions). 1. ere should be no visible flux residue except for some no-clean flux residue. But there should be no white or corroded- look appearance no matter what kind of flux residue is on the board. 2. Because solvent extract (ROSE) is commonly used, the 10.06 µg/in 2 that has served the industry for so long should be used for all fluxes. However, if users and suppliers agree, they can use some other test, such as ion chromatography (IC) or other mutually acceptable tests, and the level of NaCl equivalent from 2.5 to 4.5 µg/in being used by some companies for IC. 3. e most important criteria, at least to qualify the flux before use, the surface insulation resistance value taken in a humidity chamber at 100 VDC should be 500 MΩ/square to detect any trapped flux under components with practically no stand-off. Of the three tests, given the fact that there is widespread use of low stand-off components such as BTC, LGAs and fine pitch QFPs with practically no stand-off (almost no gap be- tween the bottom of the components and the top of the board), SIR test is the most reliable test. If there is any flux trapped under the com- ponents, SIR value will not meet the 500 meg- ohm (10 to the power 8) requirements. Most boards, when properly cleaned, will show cleanliness as high as 10 to the power 12 or more, no matter what flux is used. ere should