Investigation Of Moisture Absorption And Baking Bahaviors Of PBGA And PQFP Plastic Packages

The miniaturization of electronic packaging is a key factor with the trend of developments of electronic products.Plastic integrated circuit(IC)is widely used in the low cost,low-weight consumer electronics consumer market.However,due to the polymer used in the packaging is sensitive to moisture and temperature,the components are widely known as the humidity sensitive device(MSDs),and the classification of plastic package IC has became one of the most important question to plastic IC reliability.In general,the moisture of MSDs mainly reflects in: moisture content and temperature significantly affect the heat and mechanical properties of moulded composite to a large degree,and result in the decrease of the elastic modulus and strength;The expansion of sealant causes changes of residual stress in IC encapsulation,which may lead to the formation of microcracks.Water penetrates the molding compound material/device reducing the performance and strength of the interface.This research is based on two typical encapsulation device of PBGA and PQFP for moisture absorption and baking test to observe the production of the defects in practical using.Combining with molecular dynamics and finite element simulation,research on factors influencing the moisture absorption of encapsulation devices,make analysis on the stress distribution of the failure of the reflux process,in order to provide effective suggestions for preventing the failure of moisture sensitive plastic.Firstly the moisture absorption and baking experience were researched,samples of different degree of moisture content were detected after the reflow for three times.The results show that the PAFP devices which is larger in volume can get a higer quantity of water.But the PBGA devices whitch is thinner got a higher percentage of water absorption and whose time of saturation is shorter.The hygroscopic property curve shows the prewetting phase conforms to the fick diffusion law,and later diffused in non-fick's Law.The absorption heat of the PBGA sample epoxy resin is 28.69KJ/mol.Under the condiong of high temperature when reflow,the devices ended in failure with the effect of damp and hot stressof sample moisture absorption and vapor pressure.Cracks of PBGA specimen mainly appear in the wall of hole,and would content to the internal spread with the increase of moisture content.Defect parts of PQFP sample focus on the diagonal of the devices,and easily appear between the leadframe and the support plan of the chip,with the increase of moisture content the crack get extent to internal.With the aging of the humid test,the pores in the spherocrystal and the crack in the conductive silver glue were found.For baking,studies have shown that under the condition of 125°C and nitrogen airflow can obtain the highest rate of baking,but the difference between effects on baking rate under air and nitrogen airflow is not obvious.A long time baking can effectively reduce the defect of the absorbed PQFP specimen after reflow.But has little effects on PBGA sample,baking can not completely remove defects.More attention shoulf be paid to prevent moisture absorption of thinner encapsulation devices.Molecular dynamics(MD)simulations were conducted to investigate the moisture diffusion coefficient of moisture into the epoxy molding compound(EMC)and the EMC/Cu interface at different temperature and with different amount of moisture absorbed in the system.The results showed that the diffusion coefficient increases at high temperature and with a low amount of moisture contain in the EMC.At the same condition,the diffusion coefficient of moisture through the interface of EMC/Cu is higher than in the EMC.The hydrogen bonding was also found to play a significant role at high temperature in increasing the diffusion coefficient.The finite element simulation model of PBGA is used to explore the stress distribution of different moisture absorption sample reflux.After absorption,the die plastic and other polymer materials will have a certain degree of damp heat expansion.Materials contact with the polymer materials such an lead frame,welding plate and the chip basic don't absorb the moisture.They also have smaller thermal expansion and the expandion coefficient mismatch.As a result the expansion coefficient mismatch and produce larger damp heat stress.Corresponding to the test resule,obviously,for moisture sensitive device moistureproof and pre baking is very important.