Analysis and improvement of flexible circuit board lamination overflow glue
1. First, let's understand what is overflow glue
Bubbles and overflow are a common quality anomaly in the FPC lamination process. Overfilling refers to the fact that during the lamination process, the temperature rises and causes the glue system in the COVERLAY to flow, which causes the same problem as the EXPORY series of glue stains on the PAD position of the FPC line.
2 Let's discuss the causes of overflowing 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 plant process process, the preservation environment, and the operation methods of employees. In the following, it will be discussed from the specific factors:
1. One of the specific factors that cause overflow glue: it is determined by the parameters in the manufacturing process of COVERLAY.
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 rubber system to flow too much during the semi-curing process. In addition, if the distribution of CL glue is not uniform, 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 overflow during incoming inspection will be significantly higher than the value indicated in the product specification.
2. The second specific factor of overflowing glue: COVERLAY overflowing 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 cannot meet the requirements, COVERLAY easily absorbs moisture in the air and causes the rubber system to be unstable, and it is easy to cause overflow.
3. Part three of the specific factors that cause spillage: Whether the customer's product mix is reasonable is an important reason for the phenomenon of spillage.
FCC 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 relatively large, then the overflow of glue is very likely. The mistake of FPC structure matching should be avoided from the source.
4. The fourth specific factor that causes spillage: The special design of the finished product of the customer's FPC will also lead to local spillage.
With the emergence of high-precision products, in some FPC products, independent PAD bits have been designed. In the process of pressing and heating, because there is no gap around it, the smaller the PAD position is, the more obvious the overflow of glue is.
When crimping false joints, the operation method of employees has a direct impact on the overflow of glue.
During the false bonding, the alignment of the protective film CL and the base material FCCL is inaccurate, which will cause the PAD to be glued during the bonding 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 lamination.
5. Fifth specific factor that causes overflow glue: The occurrence 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 uneven, it may cause the phenomenon of glue overflow. In addition, the overflow control is also related to the adhesive performance of the auxiliary materials used for hot pressing.
3 explore the solution of overflow glue
We already know the cause of overflowing glue, so we can prescribe the right medicine and propose different solutions according to the specific situation.
Overflow is caused by COVERLAY manufacturing process.
Then, FPC manufacturers should strictly inspect the incoming materials. If the overflow of glue exceeds the standard in the incoming sampling, contact the supplier to return or exchange, otherwise it is difficult to control the overflow of glue during the production process.
Spills are caused by the storage environment.
Since the shelf life of the protective film (CL) is short (storage period to FPC manufacturers is generally less than two months), customers need to make a good evaluation when purchasing, and do not 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 gets wet due to the storage conditions not meeting the requirements, the CL can be pre-baked at low temperature (60-80 ° C; 2-4 hours) to a large extent, which can improve the CL overflow. In addition, CLs that have not been used on the day need to be returned to the refrigerator in time for storage.
Local spillage caused by independent small PAD bits
This phenomenon is one of the most common quality abnormalities encountered by most domestic FPC manufacturers. If the process parameters are simply changed in order to solve the glue overflow, new problems such as bubbles or insufficient peel strength will be brought. You can only reasonably adjust the process parameters.
The smaller the independent PAD bit, the more difficult it is to control the amount of spilled glue. At present, some domestic methods use special pens to smear the glue stains on the PAD bits, and then clean them with an eraser.
If the spillage is improperly controlled, when a large area of spillage occurs, it can be immersed in a NaOH solution of about 2% for 3-5 minutes, and then it can be rubbed off by brushing. (Note: PI is not resistant to strong alkali, and should not be soaked for too long, otherwise the FPC finished product will deform greatly)
Spilled by operation
When making false connections, employees are required to accurately align, correct the alignment fixture, and increase the inspection of alignment. Avoid spillage due to misalignment.
At the same time, do the 5S work when pressing the false joints. Before the alignment, check whether the protective film CL is contaminated and has burrs. If necessary, remove the burrs of the protective film. Training employees to develop good operating habits is conducive to improving product yield.
Overfill caused by FPC plant process.
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 will help reduce the amount of glue overflow. 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. You can contact the fast press supplier to debug the machine and equipment. Choosing a good Neflon release film, glass fiber cloth, and adding a silicone gasket is an important way to improve the excessive glue overflow.
Measures for improving the overflow of traditional presses:
(1) At present, Taihong's rheological test is mainly based on temperature rise and fixed temperature increase time. (The stress part should not be tested at this time)
(2) From the figure above, we can know that the maximum flow temperature of Taihong CL (protective film) glue is 115 ℃. If it is pressed immediately, it will cause the maximum glue overflow, which can also reach the optimal filling point.
(3) After the elasticity and viscosity are reduced to 108 ~ 123 ° C, the flow can be understood to gradually deteriorate. Therefore, if the amount of glue overflow is too large, the upper compression point can be extended to 123 ° C and then the second stage pressure can be increased Improve the glue overflow problem, or prolong the time of pre-pressing.