Treatment Scheme for Notches with Foreign Particles on Silver Paste Traces of Touch Screen

 Source Control: Eliminate the Generation and Adhesion of Foreign Matter

1. Substrate Cleaning (Copper Surface Treatment):

Prior to dry film lamination, copper-clad substrates shall undergo thorough cleaning and drying. The standard combined process consists of chemical cleaning (acid pickling & alkaline cleaning), mechanical brushing (or sandblasting), high-pressure water rinsing, followed by hot air/infrared drying.

Regular monitoring shall be performed on chemical concentration, solution conductivity of the cleaning line and abrasion status of brushing rollers to guarantee consistent cleaning performance. Material transfer from post-cleaning to dry film lamination shall be completed in a clean environment to avoid secondary contamination.

2. Control of Workshop Environment & Cleanliness:

The core areas (dry film lamination and exposure sections) shall be maintained at a high cleanliness level (Class 10,000 or Class 1,000 as applicable). Control ambient temperature and humidity and restrict unnecessary frequent personnel movement. Operators must wear anti-static cleanroom garments, gloves and hairnets and comply with cleanroom regulations. All materials shall be cleaned prior to being fed into the clean zone.

3.Material Management:

Dry Film: Ensure intact packaging during storage and transportation; inspect shelf life and surface appearance before usage. Conduct proper surface cleaning with tack rollers before dry film lamination.

Silver Paste & Other Inks: Guarantee raw material purity. Fully agitate before application while preventing air bubble entrapment; adopt filter screens with appropriate mesh size (≥400 mesh). All auxiliary materials in contact with panel surfaces, including backing sheets and fixtures, shall be cleaned regularly and kept free from damage and debris shedding.

4.Equipment Maintenance & Management:

Laminator: regularly clean laminating hot rollers to keep their surfaces smooth, free of residual adhesive and damage. Inspect and clean the film feeding path to prevent dust from the dry film’s protective liner from transferring onto the dry film.

Exposure Machine: Critical Control Point!

Photomask (Mask Film): Clean on daily and per-shift basis.

Use dedicated photomask cleaner and lint-free wipes, and inspect the mask for scratches and contaminants. The storage environment for photomasks shall meet specified requirements.

Exposure Stage Glass: Clean at a higher frequency (every 2–4 hours or as required by actual production conditions); ensure both top and bottom surfaces are thoroughly clean and scratch-free.

Vacuum System: Ensure stable vacuum pumping during exposure to achieve tight lamination between dry film and photomask, minimizing light diffraction caused by clearance gaps. This also helps reveal foreign particles via distinct defective shadows during development.

Carry out regular preventive maintenance on all equipment to avoid particle generation caused by worn mechanical parts.

Process Optimization: Improve Process Fault Tolerance

1.Dry Film Selection and Lamination Process:

Select dry films with superior conformability and ductility, which can fully conform and cover substrates with minor surface irregularities or tiny foreign contaminants. Optimize lamination parameters including temperature, pressure and line speed to secure firm adhesion between dry film and copper surface free of air bubbles, meanwhile preventing excessive stretching or film weakness induced by inappropriate process settings.

2.Exposure Process Optimization:

Conduct sufficient exposure energy testing via step wedge to define the optimal exposure energy range. Insufficient exposure energy may result in incomplete photopolymerization, while excessive energy can lead to pattern distortion; properly set energy enables partial polymerization even in slightly light-shielded areas.

Adopt high-resolution LDI (Laser Direct Imaging) technology to replace conventional photomask-based exposure. LDI forms patterns directly from digital data without physical photomasks, completely eliminating foreign contaminant issues and recurring cleaning work related to photomasks. It is the state-of-the-art solution for this defect, albeit with higher equipment investment cost.

3.Pre-Development & Pre-Etching Inspection:

Add a post-development inspection step between development and etching processes. At this stage, unpolymerized dry film has been stripped away by developer to expose bare copper surface, while foreign contaminants may remain attached to panels or fall off. Inspectors or AOI equipment can readily detect circuit notches or excess copper spots formed where contaminants blocked exposure. Defective panels can be reworked via stripping and relamination to avoid irreversible circuit gaps generated after etching.

Detection and Feedback: Establish Rapid Response Mechanism

1.Adoption of Automated Optical Inspection (AOI):

Install AOI inspection after etching to achieve 100% detection of circuit defects including open circuits, notches and shorts, serving as the final quality checkpoint.

A more advanced solution is to deploy AOI inspection after development. It shares the same objective as the aforementioned post-development check yet delivers higher automation and inspection stability.

Defects detected by AOI shall be promptly fed back to upstream processes including exposure, lamination and cleaning for quick root cause analysis and corrective actions.

2.Data Analysis & Traceability:

Record the location, morphology and occurrence frequency of defects, and analyze their correlation with equipment, production shifts and material batch numbers. Apply Statistical Process Control (SPC) to monitor the stability of critical process parameters.

When abnormal surges of notch defects induced by foreign contaminants are identified, suspicious process links can be rapidly pinpointed.

FAQ

1.Q: What mainly causes circuit notch defects?

A: Mostly foreign particles falling from substrate, photomask or equipment chamber block exposure, leaving residual copper after development and forming notches post etching, originating from environment, consumables and equipment maintenance.

2.Q: Can post-etch AOI fundamentally eliminate foreign-matter notches alone?

A: No, defects after etching are irreversible and only screened. Pre-AOI after development is recommended to rework before etching and cut scrap.

3.Q: What’s LDI’s advantage over traditional film exposure on foreign particle defects?

A: LDI needs no physical photomask, completely avoiding notches from dirty or dusty masks.

4.Q: Why clean exposure glass every 2–4 hours?

A: Dust on glass blocks light and forms notch shadows, making it a high-risk defect source requiring frequent cleaning.

5.Q: How does SPC locate sudden rising defects quickly?

A: Collect defect position & frequency data linked with shift, machine and batch number; abnormal parameter fluctuation pinpoints problematic process.