Why Does Overflow Glue Happen During The Fpc Pressing Process?
First, let's understand what is overflow glue
Bubbles and glue overflow are a common quality anomaly in the flexible circuit board lamination process. Glue overflow refers to the temperature increase in the lamination process that causes the glue system to flow in the COVERLAY, resulting in a glue stain problem similar to the EXPORY series at the PAD position of the FPC line.
Second, let's discuss the causes of overflow glue
There are many reasons for the overflow of glue, and it is 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 mode of the employees. Below, from specific factors to discuss:
1. One of the specific factors that produce overflow glue: 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 properly controlled, it will lead to excessive flow of the glue during the semi-curing process. In addition, if the distribution of the CL glue system is uneven, it is difficult to control the amount of glue spill 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 in the product specification.
2. Two of the specific factors that cause overflow glue: COVERLAY overflow glue is related to the storage environment.
At present, the storage condition of Taihong Coverover 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 do not meet the requirements, COVERLAY is easy to absorb moisture in the air and cause the glue system to become unstable, and it is easy to produce overflow glue.
3. The third factor that causes the spilled glue: Whether the customer product structure is reasonable is an important reason for the spilled glue 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 and the thickness of the copper foil of the base material are far apart, then there is a high possibility of glue overflow. The mistake of FPC structure matching should be avoided from the source.
4. The fourth specific factor for overflowing glue: the special design of the customer's FPC finished product will also cause local overflowing glue.
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 is no gap around it, the phenomenon of glue overflow is more obvious when the PAD position is smaller.
When pressing the false joint, the employee's operation method has a direct impact on the overflow glue.
When the false connection is pressed, the alignment of the protective film CL and the substrate FCCL is inaccurate, which will result in over pressing
5. The five specific factors for producing overflow glue: The generation of overflow glue is related to the process parameter setting of 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 glue overflow. In addition, the control of the overflow glue is also related to the suction performance of the auxiliary materials for hot pressing.
Third, discuss the solution of overflowing glue
We have already known the causes of spilled glue, so we can treat the disease according to the specific situation and propose different solutions according to the specific situation.
Corresponding solution for overflow glue:
Spilled glue is caused by COVERLAY manufacturing process.
Then, FPC manufacturers should strictly inspect the incoming materials. If the spilled glue exceeds the standard during the incoming sample inspection, contact the supplier to return or exchange the goods. Otherwise, it is difficult to control the spilled glue during the production process.
Spilled glue 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), the customer needs to make an evaluation when purchasing, and avoid using expired products as much as possible.
FPC manufacturers are better to establish a special freezer to save the protective film. If the CL glue system is damp due to the lack of storage conditions, you can pre-bake the CL at a low temperature (60-80°C; 2-4 hours) to a large extent can improve the amount of CL glue overflow. In addition, CLs that have not been used up on the same day need to be put back in the freezer for storage.
Local spillage caused by independent small PAD bits
This phenomenon is the most common quality anomaly encountered by most domestic FPC manufacturers. If the process parameters are changed simply to solve the problem of overflowing, 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 spilled glue. At present, some domestic methods are to smear the glue on the PAD position with a special sassafras pen, and then clean it with an eraser.
If the overflow glue is not properly controlled, a large area of overflow glue can be soaked with about 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 and should not be immersed for too long, otherwise the finished FPC will deform greatly)
Spilled glue brought by operation mode
In the case of false connection, it is necessary to require employees to accurately align, correct the alignment fixture, and increase the inspection strength of alignment. Avoid spillage due to misalignment.
At the same time, do the "5S" work when pressing the false joint. Before the alignment, check whether the protective film CL is contaminated and has burrs. If necessary, remove the protective film burrs. Cultivating employees to develop good operating habits is conducive to improving product yield.
Spilled glue caused by FPC factory process.
If a fast pressing machine is used for pressing, properly extending the pre-pressing time, reducing the pressure, lowering the temperature, and reducing the pressing time will help to reduce 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 even. You can contact the supplier of the fast press to debug the machine equipment. Choosing Neflon release film, glass fiber cloth, and adding silicone gaskets with better adhesive performance is an important way to improve the excessive amount of glue.
Measures to improve the overflow of traditional press:
(1) At present, the rheological test of Taiwan Rainbow is mainly based on the temperature rise and the fixed temperature increase time. (The stress part should not be tested at present)
(2) It can be seen from the above figure that the maximum flow temperature of Taihong CL (protective film) glue is 115°C. If the pressure is applied immediately, the maximum amount of glue may be caused, and the optimal glue filling point can be reached relatively.
(3) When the elasticity and viscosity are 108~123℃, the flow can be seen to deteriorate slowly. Therefore, if the amount of glue overflow is too large, the upper pressure point can be extended to 123℃ and then the second stage pressure can be applied. Improve the problem of overflowing glue, or lengthen the time of pre-pressure transmission.