Causes and solutions for the glue overflow on the pads of the printed circuit board

一，First, let's take a look at what is overflowing glue Bubbles and spills are a common quality anomaly in the FPC press-fit process. Overfill refers to the temperature rise in the press-fit process that causes the glue to flow in the COVERLAY, resulting in a glue problem in the APPD series on the FPC line.

二，Second, let's discuss the cause of the overflowing glue. There are many reasons for the overflow glue, which is related to the processing process of the protective film (COVERLAY); it is related to the process parameters of the FPC plant process, the storage environment, and the operation mode of the employees. Below, discuss the specific factors:

1. One of the specific factors that produce the spill: determined by the parameters in the COVERLAY manufacturing process.

When CL is applied to the drying stage after COATING, if the parameters such as temperature and time are improperly controlled, the flow rate of the glue system during the semi-curing process will be too large. In addition, if the CL glue is unevenly distributed during coating, it is difficult to control the amount of overflow during the pressing process.

When such products are shipped to the customer, the amount of glue spilled during the incoming inspection will be significantly higher than the indicated value in the product specification.
2. The second factor that produces spilled glue: COVERLAY is related to the storage environment. At present, the storage condition of Taiflex COVERLAY is below 10 °C, the best storage temperature is 0 °C-5 °C, and the storage time is 90 days. If the storage time or storage conditions are not met, COVERLAY tends to absorb moisture in the air and cause the rubber system to be unstable, which is easy to produce glue.

3. The third factor that produces the overflow glue: whether the customer product structure is reasonable or not is an important reason for the phenomenon of overflow.

In the product design process, the combination of FCCL and CL should be as reasonable as possible. If the thickness of the COVERLAY tape is larger than the thickness of the base copper foil, it is highly probable that the glue will overflow. The mistake of FPC structure matching should be avoided from the source.

4. The fourth factor that produces the overflow glue: the special design of the customer's FPC finished product will also lead to local overflow.

With the advent of high-precision products, independent PAD bits have been designed in some FPC products. In the process of pressing and heating, there is no gap around it, and the smaller the PAD position, the more the gel overflow phenomenon is more obvious.

When pressing the dummy connection, the employee's operation mode has a direct impact on the glue overflow.

In the case of press-fitting, the alignment of the protective film CL and the substrate FCCL is inaccurate, resulting in a PAD on the gel during the press-fitting process. In addition, the protective film CL has burrs after the punching type, and if the employee does not remove it, it will cause the glue to overflow after pressing.

5. The specific factor of generating glue overflow: the generation of overflow glue is related to the process parameter setting of the FPC factory.

In the setting of the 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 the glue to overflow. In addition, the control of the overflow rubber is also related to the suction performance of the sub-materials for hot pressing.

Third, explore the solution of overflow glue

We already know the cause of the spillage, so we can prescribe the right medicine and propose different solutions according to the specific situation.

Solution corresponding to overflow glue:

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

Then, FPC manufacturers should strictly check the material incoming materials. If the overflowing rubber exceeds the standard in the incoming sampling inspection, contact the supplier for return, otherwise it is difficult to control the overflow in the production process.

The overflow glue is caused by the storage environment.

Since the protective film (CL) has a short shelf life (less than two months until the FPC manufacturer's shelf life), the customer needs to make an evaluation when purchasing the product, and try not to use the expired product.

It is best for FPC manufacturers to set up special refrigerators to preserve the protective film. If the CL glue is wet due to the failure of the storage conditions, the CL can be pre-baked at a low temperature (60-80 ° C; 2-4 hours) to greatly improve the amount of CL overflow. In addition, the CL that has not been used that day needs to be put back into the freezer in time.

Local overflow caused by independent small PAD positions

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

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

If the overflow glue is not properly controlled, it can be soaked for about 3-5 minutes with a 2% NaOH solution when a large area overflows, and then the rubber can be worn away by a brush. (Note: PI is not resistant to strong alkali, can not be soaked for too long, otherwise the FPC finished product is larger)

Excess glue caused by the operation mode

In the case of a dummy connection, the employee is required to accurately align the position, correct the alignment fixture, and increase the inspection strength of the alignment. Avoid spilling glue due to misalignment.

At the same time, do a good job of “5S” when pressing the dummy joint. Before the alignment, check whether the protective film CL is contaminated, whether there is any burrs, and if necessary, remove the burr of the protective film. Cultivating employees to develop good operating habits is conducive to improving product yield.

Overflow caused by the FPC plant process.

If the press is fast press, then prolonging the preload time, reducing the pressure, lowering the temperature, and reducing the press time are all beneficial to 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 quick press supplier to debug the machine. Selecting Niflon release film, glass fiber cloth and silicone gasket with better glue absorption performance is an important way to improve the excessive amount of glue.

Improvement measures for the overflow of traditional presses:

(1) At present, the Taiflex rheological test is based on the increase in temperature and fixed temperature. (The pressure part should not be tested at present)

(2) As can be seen from the above figure, the maximum flow temperature of  Taiflex CL (protective film) glue is 115 °C. If the pressure is applied immediately, it may cause the maximum amount of glue overflow, and the best glue filling point can be achieved.

(3) When the elasticity and viscosity are further 108~123°C, the flow will slowly deteriorate. Therefore, if the amount of pressure-filled glue is too large, the upper press-fit point can be extended to 123°C and then the second-stage pressure can improve the overflow problem. Or re-pressurization preloading time is elongated.