| Molded underfill is a process that completes the overmolding and underfill of the chip at one time.Compared with the traditional capillary underfill process with the highest application rate,it can effectively save time and cost.However,due to the complexity of the structure of the bump area under the flip chip,the imbalance of its flow is very easy to lead to air trapped in the bump area,which affects the reliability of the chip,Therefore,the molded underfill process can not be widely used in flip chip package.This paper mainly studies the molded underfill in flip chip process.Through the combination of experiment,simulation and analytical model,this paper studies the molded underfill process and flow mechanism,puts forward the prediction model of bump area flow,and designs the appropriate cavity and chip structure.(1)A visual experimental platform equivalent to the molded underfill is built to study the influence of geometric parameters on the flow in the bump area.For sample bump size ≥ 150μm,in the flow process of high viscosity,the denser the bumps are arranged,the more serious the trapped air is formed.When the gap height of the chip is less than or equal to 150 um,the area of the formed cavitation will increase significantly.When the ratio of the pitch to diameter increased from 0.66 to 0.8,the time for the complete filling reduced by 42%;In addition,it is found that the square of the flow front position is linear with time in the process of flow filling in the bump area,which conforms to the characteristics of porous model;For sample bump size <100μm.In the process of low viscosity flow,the filling pressure is small,the flow front position is linear with time,and the flow velocity is also affected by the geometric parameters of the bump;(2)The numerical model calculated by COMSOL multiphysics and the analytical model are combined to theoretically analyze the molded underfill process.Firstly,verify the reliability of the numerical model for the prediction of cavitation formation area.It is concluded that the wetting wall and wall slip are the boundary conditions that must be considered in complex area.Then,the pressure difference between the inlet and outlet of the bump area is obtained according to the simplified numerical model,which is brought into the analytical model as a boundary condition.It is in good agreement with the experimental results of the flow front position in the bump area,and the error is between 0.17～ 0.88 mm.It is verified that the model can be used to predict the bump area flow when the bump size ≥150μm.Through the prediction model,the effects of chip geometric parameters,material properties and inlet speed on the results are studied.(3)The Moldflow Insight software is used to optimize and study the process parameters and structural parameters of the molded underfill.Firstly,the structure of the chip is analyzed.It is found that the side arrangement of the bump is 1.7 times larger than the center arrangement,and the chip stacking or flat placement has no effect on the final void area;The effects of different gate positions are analyzed.It is found that the flow is better when the gate is located in the middle or inclined upper corner of the chip,and the cavitation area ratio is controlled at about 17%;By changing the length of the cavity,prolonging the flow time,the void area ratio can be effectively reduced to less than 10%.If added resistance structure in the end of the cavity,void area ratio can be reduced to less than 1%;Then the experimental design of the process parameters is carried out,and the mold temperature is the main factor affecting the void formation.The main work of this paper focuses on the optimization of the process parameters and geometric structure,explores the flow mechanism in the bump area,and establishes a model to provide support and reference for the optimization of the molded underfill process. |
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