Research on Micro Crack Induced during the Process of Assembling BGA

Because of higher assembling density, better electrical and mechanical performances and higher reliability than other package styles, more and more BGA package styles are widely and successfully used. Micro crack induced by process stress becomes one of the critical factors which affect yield and long term reliability of BGA solder joints. It becomes more serious with the application of lead free soldering because lead free solder alloy have higher stiffness and lower ductibility compared with the traditional SnPb solder alloy. So Pb-free boards are more susceptible to pad crater micro crack due to flexure for shocking and manufacturing compared with SnPb boards. The long-term reliability of BGA joints may be greatly decreased due to the crack in the BGA joints. Up to now, crack mechanism and how to reduce crack have not be fully understood. The possible reasons which may introduce the crack are investigated in this paper. The various process stresses during the process of assembly or handling PCB are focused on in this paper. A practical computer main board with a CPU BGA often fails due to micro crack. To aim at this problem, three experiments are designed and tested under different operation manners and processes situation with the application of strain gage. First, the strains of CPU socket result from in-circuit tests are measured under different test times. The results show the micro crack is the potential risk according to Intel company's acceptance criterion. Second, the strains generated from process stress of handing PCB are measured and compared with various handling manners. Result shows improper operation manner introduce micro crack directly. Third, the process stress results from installing battery and PC card are also reviewed. To deal with these problems, a novel integrative solution with optimal test fixture clamp and setup, proper handing manner, reasonable integration method and setup is planned and verified. Experimental data shows no strains exceed the acceptance criterion. From the above experimental results, ICT process, mechanical assembly process, functional test, handing PCB are the main source of excess process stresses which induce micro crack. Adhesive application as an additional method is investigated. This method proves to be an effective way to reduce the strain; as a result, crack in the BGA joints may be largely reduced or even eliminated effectively. These experimental results can provide useful guidance to the practical manufacturing process. More researches on crack should be conducted to reduce crack and improve the long-term reliability of BGA