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Bubbles and glue spills we know are common quality abnormalities in the crimping process of flexible circuit board.

Ⅰ、First, let’s understand what is glue overflow?

Bubble and glue overflow are common quality abnormalities in the crimping process of flexible circuit board. Glue overflow refers to the glue system flowing in coverlay due to the increase of temperature in the pressing process, resulting in glue stains similar to export series on the pad of FPC line.

Ⅱ、 Let’s discuss the causes of glue overflow?

There are many reasons for glue overflow, which are related to the processing process of protective film (coverlay); It is related to the process parameters of FPC plant, storage environment, operation mode of employees, etc. Next, we will discuss the specific factors:

1. One of the specific factors causing glue overflow: it is determined by the parameters in coverlay manufacturing process.

When CL enters the dry drying stage after coating, if the temperature, time and other parameters are not controlled properly, it will lead to excessive flow in the semi curing process. In addition, if the CL adhesive system is unevenly distributed during coating, it is difficult to control the glue overflow in the pressing process.

When such products are delivered to customers, the glue overflow will be significantly higher than the indicated value in the product specification during incoming inspection

2. The second specific factor causing glue overflow: coverlay glue overflow is related to the storage environment.

At present, the storage conditions of Taihong coverlay are 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 fail to meet the requirements, coverlay is easy to absorb moisture in the air, resulting in the instability of the rubber system, which is easy to overflow.

3. The third specific factor causing glue overflow: whether the customer’s product structure is reasonable is an important reason for glue overflow.

In the process of product design, the combination of FCCL and Cl should be as reasonable as possible. If the thickness of coverlay adhesive system is far from the thickness of base copper foil, it is very likely to overflow. Mistakes in FPC structure matching should be avoided from the source.

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

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

During crimping and false connection, the operation mode of employees has a direct impact on the glue overflow.

In case of false crimping, the alignment between the protective film Cl and the substrate FCCL is not accurate, which will lead to too much crimping.

5. The fifth specific factor causing glue overflow: the generation of glue overflow is related to the process parameter setting of FPC plant.

In the setting of process parameters, excessive pressure, long time and uneven press pressure may lead to glue overflow. In addition, the control of glue overflow is also related to the glue absorption performance of hot pressing by-products.

Ⅲ、Discuss the solution of glue overflow.

We have known the causes of glue overflow, so we can suit the remedy to the case and put forward different solutions according to the specific situation.

Solution for glue overflow:

1. Glue overflow is caused by coverlav manufacturing process.

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

The glue overflow is caused by the storage environment.

Since the shelf life of protective film (CL) is short (generally less than two months when it is stored in FPC manufacturer), customers need to make evaluation when purchasing and try not to use expired products.

FPC manufacturers had better establish a special freezer to preserve the protective film. If the CL glue system is damped due to the storage conditions not meeting the requirements, the CL can be pre dried at low temperature (60-80 ℃; 2-4 hours), which can greatly improve the glue overflow of CL. In addition, CL not used up on the dry day shall be put back into the freezer for storage in time.

2. Glue overflow is caused by coverlav manufacturing process.

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

The glue overflow is caused by the storage environment.

Since the shelf life of protective film (CL) is short (generally less than two months when it is stored in FPC manufacturer), customers need to make evaluation when purchasing and try not to use expired products.

FPC manufacturers had better establish a special freezer to preserve the protective film. If the CL glue system is damped due to the storage conditions not meeting the requirements, the CL can be pre dried at low temperature (60-80 ℃; 2-4 hours), which can greatly improve the glue overflow of CL. In addition, CL not used up on the dry day shall be put back into the freezer for storage in time.

3. Local glue overflow caused by independent small pad position

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

The smaller the independent pad position, the more difficult it is to control the glue overflow. At present, some domestic methods are to use a special eraser to smear the glue stains on the pad, and then wipe them with an eraser.

If the glue overflow is not controlled properly, in case of large-area glue overflow, soak it in about 2% NaOH solution for 3-5 minutes, and then grind off the glue residue by grinding and brushing. (Note: Pi is not resistant to strong alkali and cannot be soaked for too long, otherwise the FPC finished product will be deformed greatly)

4. Glue overflow caused by operation mode

In case of false connection, the staff shall be required to accurately align, correct the alignment fixture, and increase the inspection strength of alignment at the same time. Avoid glue overflow due to inaccurate alignment.

At the same time, do a good job of “5S” during crimping and false connection. Before alignment, check whether the protective film CL is polluted and whether there are burrs. If so, remove the burrs of the protective film. Cultivating employees to develop good operating habits is conducive to improving the yield of products.

5. Glue overflow caused by FPC process.

If the fast press is used for pressing, properly extending the preloading time, reducing the pressure, reducing the temperature and reducing the pressing time are beneficial to reduce the 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. You can contact the fast press supplier to debug the machine and equipment. Selecting nifluorone release film, glass fiber cloth and adding silica gel gasket with good glue absorption performance is an important way to improve the excessive glue overflow.

Ⅳ、 Improvement measures for glue overflow of traditional press:

(1) At present, the rheological test of Taihong mainly focuses on temperature rise and fixed temperature increase time. (the pressure part cannot be tested at present)

(2) It can be seen from the above figure that the maximum flow temperature of Taihong Cl (protective film) is 115 ℃. If it is pressed immediately, it may cause the maximum glue overflow, and the best glue filling point can also be achieved.

(3) After the elasticity and viscosity reach 108 ~ 123 ℃, it can be seen from the figure that the flow slowly deteriorates. Therefore, if the amount of glue overflow is too large, the upper pressing point can be extended to 123 ℃ and then two-stage pressure can be applied, which can improve the problem of glue overflow or prolong the time of pressure transmission and preloading.