SMT007 Magazine

SMT007-July2021

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52 SMT007 MAGAZINE I JULY 2021 • Allow for better inspection/better cosmetic appearance around the solder joint • Promote the surface for better coating adhesion • Allow for testing to be done where required • Prevent electrical problems (noise or capacitive coupling) • Eliminate a longer-term reliability or corrosion problem As PCB density increases, an understanding of the impact of cleaning becomes more chal- lenging. Premature failure or improper com- ponent functionality is site specific and, there- fore, a localized testing means of cleanliness is crucial. A localized ionic contamination mea- suring method using an extraction technique in the area of interest has become more accept- able to measuring the contaminants le behind aer PCB rework. Gone are the days of the ROSE testing which measures the ionic con- taminants on the entire assembly. ing solution for four square inches of affected area can be used to clean the reworked area. A clean, so-bristled brush is then used to scrub the area followed by an isopropyl alcohol rinse. is is an important step as soiled brushes may bring unwanted debris and flux residue to the to-be-cleaned area. Any excess and leover IPA from the rinsing process should then be cleaned with a lint-free cloth. en the clean- ing process inspection occurs per the agreed upon inspection criteria. Other cleaning pro- cesses can be used if agreed upon between the builder and the end customer. For PCB rework, even for a "no clean" flux formulation, the area around a solder joint area post reflow should be cleaned. e idea of a no residue or no-clean flux is a misnomer. Instead, the flux should be described as low residue. ese residues, along with other con- taminants from the rework area such as air- borne contaminants, human skin, tool debris, etc. need to be removed. ere are numerous reasons to clean an electronics assembly post rework, namely to: Figure 2: White residue around solder joint.

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