First of all, let’s understand what is overflow glue

Bubble and glue overflow are common quality anomalies in the FPC lamination process. Glue overflow refers to the fact that during the lamination process, the temperature rises and the glue system in the COVERLAY flows, resulting in glue stains similar to the EXPORY series on the PAD position of the FPC circuit.

Let’s discuss the reasons for the overflow of glue

There are many reasons for the overflow of glue, which are related to the processing process of the protective film (COVERLAY); it is related to the process parameters of the FPC factory process, the storage environment, and the operation methods of the employees. Below, we discuss specific factors:

1. One of the specific factors that produce glue overflow: it is determined by the parameters in the manufacturing process of COVERLAY.

When CL enters the drying stage after coating (COATING), if the parameters such as temperature and time are not properly controlled, the flow rate of the adhesive system during the semi-curing process will be too large. In addition, if the CL glue system is not evenly distributed during coating, it is difficult to control the amount of glue overflow during the lamination process.

When such products are shipped to customers, the amount of glue spilled during incoming inspection will be significantly higher than the indicated value on the product specification.

2. The second specific factor that produces glue overflow: Coverlay glue overflow is related to the storage environment.

At present, the storage conditions of Taihong Coverlay are below 10°C, the best storage temperature is 0°C-5°C, and the storage time is 90 days.

If the storage time exceeds the storage time or the storage conditions do not meet the requirements, the COVERLAY will easily absorb moisture in the air and cause the glue system to become unstable, and it is easy to produce glue overflow.

3. The third specific factor that produces glue overflow: Whether the customer’s product structure is reasonable is an important reason for the phenomenon of glue overflow.

In the product design process, the collocation of FCCL and CL should be as reasonable as possible. If the thickness of COVERLAY glue system is far away from the thickness of the base copper foil, it is very likely that glue overflow will occur. The mistakes of FPC structure matching should be avoided from the source.

4. The fourth specific factor that produces glue overflow: the special design of the customer’s FPC finished product will also lead to local glue overflow.

With the emergence of high-precision products, in some FPC products, independent PAD bits are designed. In the process of pressing and heating, because there are no gaps around, the smaller the PAD position, the more obvious the glue overflow phenomenon.

When pressing and splicing, the operation method of employees has a direct impact on the overflow of glue.

During lamination and false connection, the alignment of the protective film CL and the substrate FCCL is not accurate, which will cause the PAD to be attached to the glue during the lamination process. In addition, the protective film CL has burrs after punching. If the employee does not remove it, it will also cause glue overflow after pressing.

5. The fifth specific factor that produces glue overflow: The generation of glue overflow is related to the process parameter setting of the FPC factory.

In the setting of process parameters, if the pressure is too large, the time is too long, and the pressure of the press is not uniform, it may lead to the overflow of glue. In addition, the control of glue overflow is also related to the glue absorption performance of the auxiliary materials for hot pressing.

Discuss solutions for spillage

We have already known the reasons for the overflow of glue, so we can prescribe the right medicine, and propose different solutions according to the specific situation.

Solutions for overflowing glue:

Glue spills are caused by the manufacturing process of Coverlay.

Then, FPC manufacturers should strictly inspect incoming materials. If the excess glue exceeds the standard in the sampling inspection of incoming materials, contact the supplier to return the goods, otherwise it will be difficult to control the overflow of glue in the production process.

The glue overflow is caused by the storage environment.

Due to the short shelf life of the protective film (CL) (the shelf life of the FPC manufacturer is generally less than two months), customers need to do a good job of evaluation when purchasing, and try not to use expired products.

It is best for FPC manufacturers to build a special freezer to preserve the protective film. If the CL glue system is damp due to the unsatisfactory storage conditions, the CL can be pre-baked at low temperature (60-80℃; 2-4 hours), which can greatly improve the amount of CL glue overflow. In addition, CL that is not used up on the day needs to be put back in the freezer in time for storage.

Local glue spillage caused by independent small PAD bits

This phenomenon is the most common quality abnormality encountered by most domestic FPC manufacturers. If the process parameters are changed simply to solve the overflow, it will bring new problems such as air bubbles or insufficient peel strength. The process parameters can only be adjusted reasonably.

The smaller the independent PAD bit is, the more difficult it is to control the amount of glue overflow. At present, some domestic methods are to use a special sassafrass pen to smear the glue stains on the PAD position, and then use an eraser to clean it.

If the glue overflow is not properly controlled, when there is a large area of ​​glue overflow, it can be soaked in a 2% NaOH solution for 3-5 minutes, and then the glue can be removed by over-grinding. (Note: PI is not resistant to strong alkali and cannot be soaked for too long, otherwise the FPC finished product will be deformed greatly)

Glue overflow caused by operation method

In the false connection, it is necessary to ask the staff to accurately align the alignment, correct the alignment fixture, and at the same time increase the alignment check. Avoid glue overflow due to inaccurate alignment.

At the same time, do a good job of “5S” when pressing the false connection. Before the alignment, it is necessary to check whether the protective film CL is polluted and whether there is burr. If there is, it is necessary to remove the burr of the protective film. Cultivating employees to develop good operating habits is conducive to improving product yield.

Glue overflow caused by FPC factory process.

If a fast press is used for pressing, then appropriately extending the pre-pressing time, reducing the pressure, reducing the temperature, and reducing the pressing time will help reduce the amount of glue overflowing. If the pressure of the press is not uniform, you can use the induction paper to test whether the pressure of the press is uniform, and you can contact the supplier of the fast press to debug the machine. Selecting Neflon release film, glass fiber cloth and adding silicone gasket with better glue absorption performance is an important way to improve the excessive glue overflow.

Glue overflow improvement measures for traditional presses:

(1) At present, Taihong rheological test is mainly based on rising temperature and fixed temperature increasing time. (The pressure part should not be able to test at present)

(2) It can be seen from the above picture that the maximum flow temperature of the glue of Taihong CL (protective film) is 115℃. If the pressure is applied immediately, it may cause the maximum amount of glue overflow, and relatively the best filling point can be achieved.

(3) When the elasticity and viscosity reach 108~123℃, it can be seen from the figure that the flow gradually deteriorates. Therefore, if the amount of glue overflowing during the pressing is too large, the upper pressing point can be extended to 123 °C and then the second stage can be pressed. Improve the problem of glue overflow, or prolong the preloading time of retransmission.