Reliability experts contend that roughly 30 percent of PCBA failures are due toย contamination of the bare PCB, and only an estimated 2 percentย ofย contaminated PCBsย are sent back toย the fabricatorย for re-cleaning. This isย largely becauseย of aย basic misunderstanding of the correlation between PCB contaminants and product failure.
When received in-house, bareboards can carry contaminants or โsoilsโโplastic,ย metal, and fiberglass particulate residues, as well as salts, oils,ย and dustโfrom either the fabrication process or environmental exposure. If residues are left on the board, major problemsโelectrochemical migration, corroded open traces, parasitic leakages, andย dendritic growthโcanย potentiallyย lead to in-process nonconformance or shorts during theย productโsย lifecycle.ย To prevent rework and lateย lifecycle failures, as well asย ensureย the boards’ overall reliability,ย theย newly received bareboardsย mustย be tested for soils.ย In general,ย this is doneย usingย an ionic tester.
An ionic tester uses a combination of Isopropyl Alcohol (IPA) and deionized water to rinse the board and collect a sample of possible residues. The sample is then tested for resistance (conductivity), which indicates the boardโs soil levels. High resistance (low conductivity) is the goal. This is indicated by the pass/fail limit (PF), which is expressed in ยตg (Meg-Ohms of resistance) per square inch. If theย testedย PCBย exceeds a 10 PF (as designated byTย IPC-M-650 standards and military specifications 2000A and 208809), residue is too high. A lot should be inspected per your Acceptable Quality Limit (AQL) standards. This willย indicate a PCBย lotโs conformance; non-conformingย lotsย should then be rejected and sent back to the fabricator for re-cleaning.
Tip: Make sure to calibrate the ionic tester before each test according your needs. Pay special attention to your solution resistance (expressed as ยตg) and the IPA and deionized water ratio (75% IPA to 25% deionized water, at ODA) for accurate testing. ย ย ย
Post-Assembly Ionic Testing
Ionic testing on fully assembled boards is generally not conducted unless specifically requested by a customer. Some components run a higher risk of damage from the full liquid submersion ionic testing requires.
This post originally appeared on theย DigiSource Blog
