Controlling particles in camera module assembly

Choosing low-particulate materials combined with strict cleanroom procedures helps camera module assembly plants sharply reduce black spot defects in images, one of the biggest causes of scrap in the industry.

Image sensors have pixels only about 1 micron in size. A single 5-micron dust particle landing on the sensor surface or filter is enough to create a visible spot in the image. Dust control in camera assembly is therefore not just a matter of factory housekeeping but a materials and process challenge.

Spot defects are usually only detected at the final image test, after the module has had its lens mounted and adhesive cured, so rework costs are very high or the entire module must be scrapped.

Commonly overlooked dust sources

• Cut edges of tapes and foams that shed particles when the liner is peeled.
• Paper liners that shed fibers during handling.
• Incompletely cured adhesive that produces sticky particles.
• Static electricity on plastics that attracts dust from the air.
• Low-quality gloves, trays and consumables.

Choosing materials for cleanroom environments

Low-particulate tapes and foams

Prefer PET liners over paper, high-density closed-cell foams and edges cut with sharp blades or lasers to minimize loose particles. Suppliers can be asked to perform particle counts on cut samples.

Non-outgassing adhesives

Choose adhesives with a low outgassing index to avoid condensation on the glass and sensor during curing.

Anti-static materials

Trays, protective films and tapes with surface resistance of 10^6–10^9 ohm help reduce dust attraction.

When investigating spot defects, we often find that the culprit is not the cleanroom air but debris from materials that comes in with the parts. Controlling incoming materials is far more effective than upgrading the cleanroom class. — Camera Application Engineer, AMS Vietnam

Process measures to reduce spot defects

1. Maintain a Class 100 or Class 1000 cleanroom at the sensor unpacking station.
2. Use ionized air blow-off before the lens mounting step.
3. Pack materials in double bags, removing the outer bag in the gowning room.
4. Map spot locations in images to trace dust sources.

Setting cleanliness standards for suppliers

One step many plants have yet to take is including cleanliness requirements in the quality agreement with material suppliers. These can specify the maximum particle count per part after vibration, the packaging type, tray material and the die-cutting environment. With clear standards, incoming inspection and complaints also become easier and more transparent.

AMS supplies tapes, foams and protective films packaged for cleanroom use. Contact AMS Vietnam for advice on solutions to reduce spot defects.