1. First, let's understand what is overflow glue

Bubbles and glue overflow are a common quality abnormality in the process of flexible circuit board pressing. Glue overflow refers to the fact that the temperature rise during the pressing process causes the glue system to flow in the COVERLAY, which results in a glue stain problem similar to the EXPORY series on the PAD position of the FPC circuit.

2. 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, discuss the specific factors:

2.1. Determined by the parameters in the COVERLAY manufacturing process.

When CL is coated (COATING) and then enters the drying stage, if the temperature, time and other parameters are not properly controlled, it will cause the glue system to flow too much during the semi-curing process. In addition, if the CL glue is not uniformly distributed during coating, it is difficult to control the amount of glue overflow during the pressing process.

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

2.2. COVERLAY spilled glue is related to the storage environment.

At present, the storage condition of Taihong COVERLAY is below 10℃, the best storage temperature is 0℃-5℃, and the storage time is 90 days.

If the storage time is exceeded or the storage conditions are not up to the requirements, the COVERLAY will easily absorb moisture in the air and cause the glue system to be unstable, and it is easy to cause glue overflow.

2.3. Whether the customer's product structure is reasonable or not is an important reason for the overflow of glue.

In the product design process, the combination of FCCL and CL should be as reasonable as possible. If the thickness of the COVERLAY glue system is larger than the thickness of the base copper foil, glue overflow is very likely. Mistakes in FPC structure matching should be avoided from the source.

2.4. The special design of the customer's FPC finished product will also cause partial glue overflow.

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

When the false connection is pressed, the operation method of the staff has a direct impact on the overflow of glue.

When the false joint is pressed, the alignment of the protective film CL and the substrate FCCL is not accurate, which will lead to excessive pressing

2.5. The production of spilled glue is related to the process parameter settings 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 cause glue overflow. In addition, the control of glue overflow is also related to the glue absorption performance of auxiliary materials for hot pressing.

3. Discuss the solution of overflow glue

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

Solutions for spilled glue:

3.1. The glue overflow is caused by the COVERLAY manufacturing process.

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

3.2. The spilled glue is caused by the storage environment.

Due to the short shelf life of the protective film (CL) (the storage period by the FPC manufacturer is generally less than two months), the customer needs to make an evaluation when purchasing the goods, and try not to use expired products.

It is best for FPC manufacturers to build a special freezer to store the protective film. If the CL glue system becomes damp due to storage conditions that cannot meet the requirements, the CL can be pre-baked at a low temperature (60-80°C; 2-4 hours) to a large extent to improve the amount of CL glue overflow. In addition, the CL that has not been used up that day needs to be returned to the freezer for storage in time.

3.3. Local glue overflow caused by independent small PAD positions

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

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

If the glue overflow is improperly controlled, it can be soaked in 2% NaOH solution for 3-5 minutes when there is a large area of glue overflow. (Note: PI is not resistant to strong alkali, so it should not be soaked for too long, otherwise the FPC finished product will deform greatly)

3.4. Glue overflow caused by operation

In the false connection, employees need to be required to accurately align the alignment, correct the alignment fixture, and increase the alignment inspection effort. Avoid glue overflow due to misalignment.

At the same time, do the "5S" work during the pressing and false connection. Before alignment, check whether the protective film CL is contaminated and whether there are burrs. If necessary, remove the protective film burrs. Cultivating employees to develop good operating habits will help improve product yield.

3.5. Glue overflow caused by the process of the FPC factory.

If a fast press is used for pressing, then appropriately extending the pre-pressing time, reducing the pressure, lowering the temperature, and reducing the pressing time will help reduce the amount of glue overflow. If the pressure of the press is uneven, you can use 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 and equipment. Choosing Neflon release film, glass fiber cloth, and adding silicone gaskets with good glue absorption performance is an important way to improve the excessive glue overflow.

4. Measures to improve glue overflow of traditional presses:

4.1. At present, Taihong's rheological test is mainly based on temperature increase and fixed temperature increase time. (The pressure part should not be able to test at present)

4.2. The maximum flow temperature of the glue of Taihong CL (protective film) is 115℃. If it is pressed immediately, it may cause the maximum amount of glue overflow, and the best filling point can also be achieved.

4.3. When the elasticity and viscosity are 108~123℃, the flow will gradually deteriorate from the figure. Therefore, if the amount of glue overflow is too large, the upper pressing point can be extended to 123℃ and then the second stage of pressure can be improved. Glue overflow, or the pre-pressing time is prolonged.