Table of Contents
To achieve the ideal coating for PCBs, it is necessary to have knowledge of wet film thickness (WFT). Precision-measured WFT technology is crucial in ensuring the quality and performance of protective layers, including solder masks and conformal coatings. We’ll examine the thickness of wet film, its significance, and suggest simple methods for measuring it while producing PCBs.
What is Wet Film Thickness?
The wet film thickness is the level at which the liquid settles and stops drying. Wet film thickness is also important in the manufacture of PCBs, as it can affect the way a liquid coat should be done and how he should make the final layer. So, solder masks require the right WFT to ensure uniform coating throughout the entire PCB.
Why Wet Film Thickness Matters
When PCBs are coated, the WFT has an impact on several crucial components of the final product:
- The absence of WFT may lead to inadequate coverage of circuits.
- The strength of the PCBs electrical flow and ability to resist high voltage are determined by its thickness, which is a crucial aspect.
- WFT measurements can enhance the durability of a dry coating by making it resistant to heat, water, and chemicals.
- WFT can assist you in reducing your waste by preventing the use of unnecessary materials.
Features of Wet Film
Ensure Excellent Resolution
Wet film, enough coverage, and a reduced light-scattering distance can all help minimize light loss and errors. While it is challenging to achieve a dry film resolution of 50 microns, wet film resolutions are typically 25 microns or less to enhance the quality of the printed images.
Ensure to Remove Board Edges
The wet film layer does not print on the board’s edges. This leads to issues because films break easily during production and the paste dries out quickly at the borders. It will have an impact on the amount of individuals passing.
Provide Good Coverage & Adhesion
A unique kind of plastic called wet film is affected by light, colors, fillers, light-sensitive ingredients, and a thick blue liquid that is used to mix them.
Because it fills in the surface pits, the wet film forms a strong bond and sticks firmly to the scratched region. This helps achieve high coverage during screen printing, which is crucial for creating intricate patterns required for PCB processing, because it creates a strong adhesive between the wet copper foil and the surface.
Cost Effective & Budget Friendly
Wet film is significantly thinner than dry film. So, it’s cost-effective due to its low packaging costs. For superior inner process results in fine details, wet film can save costs by 20% compared to dry materials.
In addition to increasing the etch rate by 10% to 20% and speeding up faded film to save energy, wet development is 30% more efficient and helps reduce equipment utilization, which lowers expenses.
Key Operating Points for
Applying Wet Film in PCB Manufacturing
Point 1:
Brush Plate Preparation
Point 2:
Exposure Process
To ensure a strong bond between the wet film and the copper surface, the production board must meet specific criteria.
- Surface Condition: The board should be free from corrosion, grease, or wrinkles; it must be clean and smooth.
- Cleaning and Brushing: After removing dirt or other contaminants with a 5% sulfuric acid spray, a special nylon roller with fine bristles is utilized to roughen the copper surface.
- Desired Outcome: The copper surface must be smooth, clean, and devoid of water spots to enhance the adhesion of the wet film.
The procedure of exposure uses UV light to ensure the wet film dries properly in all the correct areas.
- Optimal Exposure Settings: For optimal exposure, use a powerful UV lamp and adjust the exposure time to avoid overheating.
- Clean Environment: Keep the area clean to avoid issues like holes or cracks in the design caused by debris or mess.
- Balanced Exposure: Proper exposure is crucial; too little exposure leads to slow development and weak rust resistance, while too much can cause the lines to either thin or thicken.
Point 3:
Using Wet Ink
Point 4:
Pre-Bake Process
Employing the correct screen printing technique is crucial for wet film thickness.
- Choosing the Right Mesh: When deciding on the amount of ink to be applied, select a mesh that has the appropriate thickness and number of lines per given length.
- Consistent Application: The speed at which you move, the pressure applied on the squeegee, and the viscosity of the ink all influence the final thickness. To avoid issues such as being overly thin, insufficient insulation, and inadequate exposure, the wet film thickness after printing needs to be within 15 to 25 micrometers.
- Environmental Control: The screen printing area should be maintained at approximately 20°C with a humidity level of 50%.
By eliminating the liquid from the wet layer, the drying process firms up the coating.
- Temperature and Duration: Set your oven to a temperature between 80°C and 100°C. Bake for 7–10 minutes on one side, then 10–20 minutes on the other side.
- Avoid over or under-baking: Not baking enough or overbaking can cause uneven drying and development. Insufficient baking time can lead to adhesion issues, as well as sticky edges and surfaces.
- Quality Checks: To ensure the best results, check that the air circulation system is working properly and monitor the oven’s conditions.
Point 5:
Developing
Point 6:
Etching and Stripping
During development, the parts of the wet film for PCB that are not covered are taken away to expose the circuit design.
- Controlled Parameters: Keep the developer concentration between 10 and 12 grams per liter and maintain the temperature between 30 and 34 degrees Celsius.
- Nozzle Maintenance: Regularly clean the nozzles to ensure consistent functioning and even pressure.
- Avoid Over-Development: Too much heat or extended development can harm the wet layer and lessen the accuracy of the etched pattern.
To get the desired circuit design, etching eliminates the exposed copper sections.
- Etching Solutions: Choose ammonia, ferric chloride, or acidic copper as your etching solutions, based on the thickness of the copper foil and your requirements.
- Regular Use: While ejecting, keep a steady liquid pressure and distribute it evenly to prevent copper loss at the edges.
- Wet Film Removal: Utilize a 4–7% sodium hydroxide solution at 50–60°C to eliminate the wet film.
PCB Coating Process:
Wet Film Vs Dry Film in Manufacturing
Conversion Between Wet and Dry Film Thickness
Wet Film Thickness Measurement in PCB
-Using Wet Film Thickness Gauge
One instrument used to test the wet film thickness of coatings during production is a wet film thickness gauge.
(1) Comb Gauge
The comb gauge is the most widely used instruments for determining the thickness of wet films.It is usually made from corrosion-resistant stainless steel and has teeth of different lengths.
To measure using a comb gauge, place the test plate horizontally.
- Apply the comb-type wet film thickness gauge to the wet film whose thickness needs to be measured.
- At the top, display the brief reading.
- Position the eccentric wheel of the instrument slightly above the wet film by adjusting the broad space between them.
- Then, rotate forward half a turn (180°), roll back half a turn (180°), and move.
- Observe the point at which the instrument’s central rim initially comes into contact with the wet film surface.
- Take the reading and apply the average value as a single reading.
(2) Roller Wet Film Thickness Gauge
The roller wet film thickness gauge reads the wet film thickness by rolling on it according to the roller’s scale. This type of thickness gauge can be utilized on flat substrates in the laboratory or for production control.
To measure the paint film using a roller gauge, place the test plate on a stable horizontal surface to prevent movement or jumping.
- Make a large distance between the eccentric wheel and the wet film, just above it, and then move on the wet film to measure the thickness with the little reading on top.
- After that, roll it forward half a turn (180°), back half a turn (180°), and move.
- Check the point at which the instrument’s central rim first makes contact with the wet film surface, take a reading, and determine the average value of that reading.
(3) Non-Contact Wet Film Thickness Gauge
- Non-contact wet film thickness gauges employ optical or laser technology to measure.
- It can measure wet film without contact and is suitable for high-precision, non-destructive measurements.
Coating Methods
Wet Film: A liquid coat. Applied by screen print or spray. Several thicknesses can be obtained-needs to be applied, carefully wet and drying for uniformities.
Dry Film: A solid, already coated film. Applied via heat and pressure for control of layer thickness on printed circuit board. Better effectiveness regarding pattern/ tiny pattern and complex graphics are allowed.
Effectiveness and Suitability
Dry film is the preferred choice for advanced PCBs due to its outstanding capability to create precise, intricate patterns. Moreover, it provides enhanced stability during processing.
On the other hand, wet film is typically more cost-effective and easier to use for thicker coatings or standard circuit boards. While dry film, being applied in a solid state, reduces waste and has a less harmful environmental impact, wet film allows for the use of different materials for various applications.
Coating Process Economy
Material Cost:
- Dry film material is more expensive per area compared with wet film chemicals.
Labor and Throughput:
- Dry film has fewer coating steps, which reduces labor costs and increases throughput.
Equipment Cost:
- Dry film equipment, such as laminators, is very expensive; wet film tools have low capital investment.
Waste Disposal Cost:
- Disposal of wet film liquid waste is more expensive than dry film solid waste.
Cost in Different Production Volumes:
- In very high volume production, dry film may be slightly less expensive.
- Wet film is generally cheaper for small batches and short runs.
Conclusion
The process of applying wet film involves several important steps that are important for creating high-quality PCBs. During manufacturing, it is essential to properly prepare the surface, adhere to detailed instructions, and maintain a clean work area during the etching process in order to get the best results.
FAQs
A1: A wet film thickness gauge or a dry film thickness gauge, such as an eddy current or ultrasonic instrument, can be used to measure the thickness of soft coatings once the coating has dried.
A2: We consider the solid content of the film along with the targeted dry film thickness to calculate the suitable wet film thickness. The equation is WFT = DFT / Solids Content (%). This ensures optimal performance and coverage.
A3: A wet film thickness gauge, such as a comb gauge or notched gauge, should be employed. Position it on the newly applied coating and measure it at the point where it touches the first dry area and the last wet area to determine the thickness range.
A4: Substances like dry or wet film resists and photoresists, which react to UV light during image creation, are used to apply patterns onto PCBs. These specialized films or coatings facilitate the transfer of the circuit design onto the surface of the PCB.
A5: Dry film is ideal for intricate and detailed PCB designs due to its durability and transparency. Wet film is suitable for simpler tasks or when cost efficiency is a priority.
CONTACT US
English 
Italian 
Hungarian