One-sentence answer: The smartphone midframe and back cover are bonded by dispensing adhesive or a seal strip. Button openings, the SIM tray, the speaker and the USB port are all water ingress paths, and a broken adhesive bead can cause hidden water seepage. Using the CTS Sentinel C28WE (C28) leak detector to perform a vacuum decay air tightness test on the complete phone or the midframe assembly can intercept poor sealing before shipment.
The adhesive bead is affected by adhesive volume, trajectory speed and buildup at corners, making broken beads, bubbles and locally thin areas likely. These are difficult to judge visually and can only be exposed by air tightness testing.
The midframe and back cover are large in size and thin in wall thickness. Internal pressurization causes elastic deformation of the cover, which makes the pressure curve deviate from the true leak, so the pressurization method and differential pressure must be chosen carefully.
After after-sales teardown, the original adhesive layer is destroyed. The sealing consistency of re-dispensed adhesive is usually lower than that of the production line, so repaired units require separate control and re-test.
Sealing locations: the adhesive dispensing ring between the midframe and back cover, the bonding surface between the screen and the midframe, button and SIM tray openings, USB and speaker openings, and the camera decorative part seal.Consequences of leakage: ingress of moisture and liquid causes mainboard corrosion, short circuits and camera fogging; failure to meet the water resistance rating directly triggers customer rejection at acceptance; if repaired units after teardown are not re-tested, they may become the source of subsequent batch complaints.
The midframe and back cover are large thin-wall parts. Internal pressurization causes the cover to bulge elastically, and the pressure curve changes accordingly, which may mask a real leak or produce a misjudgment. Switching to vacuum decay, drawing vacuum from the outside, provides a stable criterion at a smaller differential pressure.
A broken bead forms a continuous channel along the adhesive line, which appears as a continuous pressure drop during vacuum draw. The C28's leak rate resolution of 0.0005 scc/min (a resolution specification, not equal to the minimum leak rate the instrument can detect), together with automatic calibration using the built-in calibrated leak, can reliably judge this type of channel leak.
Yes. After repair, the original adhesive layer has been destroyed and sealing consistency drops noticeably. They should be re-tested using a separate program and stricter judgment limits, and the re-test results should be linked to the technical file for retention.
The two have different criteria and cannot replace each other. The air tightness test gives a quantitative leak rate under a controlled differential pressure, reflecting the overall integrity of the sealing structure, and allows non-destructive 100% inspection; rain and immersion testing simulate the combined effect of external water pressure and surface tension, and are usually performed as sampling or destructive tests. Use a small number of complete-unit tests to confirm the correspondence between the air tightness limit and actual water resistance performance.
The key to air tightness for smartphone midframes and back covers is to avoid deformation from pressurization, intercept adhesive dispensing defects, and control repaired units separately. The CTS C28WE uses vacuum decay, ambient drift correction and 32 programs to cover multi-model on-line testing, and together with custom-formed plugging fixtures forms a complete line of defense for water resistance at shipment.
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