PCB Tech - Causes and solutions of circuit board pressing and overflowing glue

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

Bubbles and glue overflow are a relatively common quality abnormal phenomenon in the FPC pressing process. 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 the occurrence of glue stains on the PAD position of the FPC circuit similar to the EXPORY series.

2. Let's discuss the causes of overflow

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 preservation environment, and the operation methods of the employees. Below, discuss the specific factors:

1. One of the specific factors that cause glue overflow: it is 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 controlled properly, 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 the incoming inspection will be significantly higher than the indicated value on the product specification.

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

At present, the storage condition of Taihong COVERLAY is below 10 degree Celsius, the best storage temperature is 0 degree Celsius-5 degree Celsius, 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.

3. The third specific factor causing the glue overflow: whether the customer's product structure is reasonable or not is an important reason for the glue overflow phenomenon.

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 at the source.

4. The fourth specific factor causing the glue overflow: the special design of the customer's FPC product will also cause local 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.

In the case of pressing and false connection, the operation method of employees 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 cause the PAD to be attached to the glue during the pressing process. In addition, the protective film CL has burrs after punching. If employees do not remove them, it will cause glue overflow after pressing.

5. The fifth of the specific factors that cause glue overflow: The production of glue overflow is related to the process parameter settings of the FPC factory.

In the setting of process parameters, if the pressure is too high, 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 the auxiliary materials used for hot pressing.

Three, 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.

The solution corresponding to overflow glue:

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

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 and exchange the goods, otherwise it is difficult to control the overflow in the production process.

Glue overflow 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 assessment when purchasing the goods and try not to use expired products as much as possible.

FPC manufacturers had better build a special freezer to store the protective film. If the CL glue system becomes damp due to insufficient storage conditions, the CL can be pre-baked at a low temperature (60-80°C; 2-4 hours), which can greatly 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.

Local glue overflow caused by independent small PAD position

This phenomenon is one of the most common quality abnormalities 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 stains on the PAD position, and then use an eraser to clean it.

If the glue overflow is not properly controlled, when a large area of glue overflow occurs, it can be soaked in a 2% NaOH solution for 3-5 minutes, and then the glue can be rubbed off by brushing. (Note: PI is not resistant to strong alkalis, so it should not be soaked for too long, otherwise the FPC finished product will deform greatly)

Glue overflow caused by operation method

In the false connection, the employees need to be required to accurately align the alignment, correct the alignment fixture, and increase the alignment check. 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 is conducive to improving product yield.

Glue overflow caused by the process of FPC factory.

If a fast press is used for the pressing, then appropriately extending the pre-pressing time, reducing the pressure, lowering the temperature, and reducing the pressing time are all conducive to reducing the amount of glue overflow. If the pressure of the press is not uniform, 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.

The measures to improve the glue overflow of the traditional press:

(1) At present, the Taihong rheological test is mainly based on temperature rise and fixed temperature increase time. (The pressure part should not be tested at this time)

(2) From the above picture, we can see that the maximum flow temperature of Taihong CL (protective film) glue is 115 degree Celsius. If it is pressed immediately, it may cause the maximum amount of glue overflow, and the best filling point can also be achieved.

(3) After the elasticity and viscosity are 108~123 degree Celsius, the flow will gradually become worse. Therefore, if the amount of glue overflow is too large, the upper pressing point can be extended to 123 degree Celsius and then the second stage of pressure can be improved to improve the problem of glue overflow., Or re-transmit the pre-compression time to lengthen.