3.2 Product performance requirements

    Epoxy resin potting materials should meet the following basic requirements: good performance, long pot life, suitable for mass automatic production line operations; low viscosity, strong impregnation, and can fill the components and lines; fillers during potting and curing The powder components have low sedimentation and no delamination; the curing exothermic peak is low, and the curing shrinkage is small; the cured product has excellent electrical and mechanical properties, good heat resistance, good adhesion to a variety of materials, water absorption and The coefficient of thermal expansion is small; in some occasions, the potting compound is also required to have properties such as flame retardancy, weather resistance, heat conduction, and high and low temperature resistance.

 

 

3.3 Storage conditions

    If the bottom potting glue of BGA/CSP can be stored at ±5°C for three months, only a low-cost refrigerator can meet the requirement of ±5°C. The bottom potting glue can also meet the minimum storage conditions of ±5℃.

 

3.4 The service life of bottom potting glue

    The service life refers to the time that the underfill can be used effectively after being taken out of the freezing condition. Effective use refers to the continuity and consistency of the dispensing volume that can be guaranteed at a certain dispensing speed, so its viscosity must be stable throughout its service life.

 

3.5 Performance of bottom potting glue

    Whether the potting method of the bottom potting compound is simple and easy to operate will have an impact on the output and cost of the product. Low-viscosity and low-density glue can be glued from the needle under a small pressure, and the speed is also very fast. The underfill of flip chip has a viscosity of 15 000 cps or higher. Because this type of filler contains siliceous fillers with a specific gravity of 1.8, it is necessary to use pressure system equipment under the requirements of high-speed and high-precision dispensing. Such as screw pumps and piston pumps.

Many CSPs and all BGAs are larger in size than flip-chips, so a higher flow rate of glue is required to meet the requirements of yield. The CSP/BGA bottom potting glue 3113 produced by 3513 and Beijing United Titanium is specially developed for CSP/BGA. Its viscosity is lower than 4 000 cps. Because the modified glue has no fillers, the specific gravity is usually 1.17, so it can be realized by using a more economical and simple dispensing machine.

 

3.6 Flow speed

    As mentioned above, CSP/BGA bottom potting compound has lower viscosity and density compared to flip chip underfill. This makes the CSP/BGA bottom potting glue flow faster, which is more ideal for potting larger areas.

During the experiment, we analyzed the flow of 3513 glue, as shown in the figure below.

 

    The flow rate of the bottom potting glue is an important performance parameter index for whether the glue can completely fill the bottom of the BGA chip. The following is a performance parameter index that we passed on ZYMET's flip chip underfill X6-82-5LV and LOCTITE An example of the X-ray comparison of the produced CSP/BGA bottom potting glue 3513 to the chip after pouring glue shows that the fluidity of the bottom potting glue 3513 is better than that of flip chip underfill.

Flip chip underfill X6-82-5LV bottom potting glue 3513

 

    In addition, the bottom of the CSP/BGA chip must be filled with glue. It must be ensured that there are no bubbles at the bottom to achieve high reliability of the device. How to avoid bubbles during the filling process, mainly to avoid: circuit board or component moisture, Whether the flow pattern and flux of the potting glue affect the potting glue.

The following figure shows the X-RAY view after using 3513 bottom potting compound to encapsulate BGA devices.

The glue at the bottom of the dried device and the unbaked device.