In　the　plastic　ball　grid　array　package　(PBGA)　assembly,　there　are　many　solder　balls　and　copper　pads.　The　whole　packaging　varies　a　lot　in　size.　This　multi-scale　structure　brings　difficulties　in　building　the　finite　element　model.　The　paper　refers　a　method　to　avoid　difficulties　by　replacing　the　unconcern　copper　pad/solder　mask　layer　with　a　homogenous　material　layer.　The　finite　element　method　(FEM)　is　used　to　prove　simplification　model＇s　accuracy.