Glue overflow is a common quality issue in flexible PCB manufacturing, typically occurring during the lamination process. As the temperature increases, the adhesive in the coverlay film begins to flow and accumulates at the circuit PAD positions, forming a substance similar to glue residue. This phenomenon directly affects the appearance and performance of the FPC and can lead to circuit malfunction. In severe cases, it may impact the overall product’s lifespan. Therefore, accurately analyzing the causes of glue overflow and implementing effective solutions is key to ensuring FPC product quality.
Main Causes of Glue Overflow
Improper Parameter Settings
The coverlay film is typically coated with adhesive and dried during processing. If parameters such as temperature and time are not properly controlled, excessive adhesive flow may occur during the semi-curing stage, resulting in glue overflow. This is because the flowability of the adhesive increases when heated. If the heating time is too long or the temperature is too high, the adhesive will become overly fluid within the layer, leading to glue overflow. Additionally, uneven distribution of adhesive during the coating process can make it difficult to control the adhesive flow during lamination.
Storage Environment Impact
The storage conditions of the coverlay film significantly affect its performance. The coverlay film should be stored in a low-temperature (below 10°C) and dry environment. The optimal storage temperature is between 0°C and 5°C, and the storage time should not exceed 90 days. If the coverlay film is stored beyond the specified time or in a humid environment, the components of the adhesive may absorb moisture, reducing the stability of the adhesive and making glue overflow more likely.
Improper Design
During the FPC product design phase, improper matching of the flexible circuit substrate and coverlay film can easily lead to glue overflow. For example, if the coverlay film is too thick or the copper foil is too thin, the adhesive may overflow during the lamination process, forming unwanted glue stains.
Additionally, due to the special nature of the product, certain special designs may be implemented, such as additional small contact points to meet specific design requirements. However, if these contact points lack adequate spacing or have insufficient space, the adhesive may concentrate in these areas during lamination, making glue overflow more likely.
Solutions to Glue Overflow
Strengthen Incoming Material Inspection
First, FPC manufacturers should conduct strict incoming inspections of raw materials such as coverlay film. By regularly sampling the quality of the adhesive in the coverlay film, manufacturers can ensure the adhesive’s flowability and stability meet standards. If glue overflow exceeds acceptable levels, the supplier should be contacted immediately for returns or exchanges to prevent defective materials from entering the production process and causing further issues in subsequent stages.
Improve Storage Conditions
To prevent moisture or deterioration of the coverlay film, FPC manufacturers should ensure materials are stored in dedicated refrigerated environments. The temperature of the cold storage should be maintained between 0°C and 5°C, and storage time should be strictly controlled. If moisture is detected in the adhesive due to improper storage, the manufacturer can use a low-temperature pre-baking method to treat the coverlay film. Baking the coverlay film at a temperature of 60°C to 80°C for 2 to 4 hours can effectively improve the adhesive’s condition and reduce the likelihood of glue overflow.
Rational Product Structure Design
To avoid glue overflow caused by improper structure, FPC manufacturers should consider the proper matching of the flexible circuit substrate and coverlay film during the design phase. Ensuring the adhesive thickness does not significantly differ from the substrate thickness can reduce the chance of overflow. Additionally, for small contact points in the design that may cause localized glue overflow, the design should be adjusted to increase the surrounding space to avoid excessive concentration of adhesive.
Operational Standards and Employee Training
Improper operation is another major cause of glue overflow. During lamination, employees must strictly align the coverlay film with the substrate according to process requirements. Particular attention should be paid to the calibration of positioning fixtures and operational precision. Additionally, before lamination, the coverlay film should be carefully checked for contamination or burrs, which should be removed in a timely manner. To improve operational accuracy and reduce mistakes, manufacturers should regularly train employees to ensure they follow correct operational procedures.
Adjusting Process Parameters
To address glue overflow, FPC manufacturers can adjust process parameters to reduce the amount of overflow. For example, extending the pre-pressing time, appropriately lowering the lamination temperature, reducing pressure, and shortening the lamination time can all help control glue overflow. If the pressure of the press is uneven, it should be tested using sensing paper, and the equipment supplier should be contacted to adjust the machine settings. Additionally, selecting release films and auxiliary materials with good adhesive absorption properties, such as PTFE release films and silicone gaskets, can also effectively reduce glue overflow.
Ending
Glue overflow is a common quality issue in the FPC lamination process, which not only affects the appearance of the product but also impacts the performance and stability of the circuit. Through in-depth analysis of the causes of glue overflow, effective preventive and control measures can be taken from raw material inspection, storage conditions, product design, process parameters, and employee operations to reduce the occurrence of glue overflow. For FPC manufacturers, strictly controlling every stage of the production process to ensure product quality is essential for improving product stability and reliability, thereby meeting customer requirements.
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