20 by adhesive a first bock 24 and a second block 25 are formed on the inside plate 30 for being received in the first clearance 15 and the second clearance 16 the first block 24 and the second block 25 each have a main face 24 a 25 a and are made of a flexible insulating material such as rubber thus the first block 24 and the second block 25 can be deformedly pushed to be easily received in the first clearance 15 and the second clearance 16 additionally the insulating characteristic of the first block 24 and the second block 25 may prevent the electronic components from short circuiting the shape and the arrangement of the first block 24 and the second block 25 are similar to that of the first clearance 15 and the second clearance 16 in this exemplary embodiment the first block 24 and the second block 25 are rectangular strips to assemble the first block 24 and the second block 25 to the chassis 10 as shown in fig3 the inside plate 30 is firstly mounted on the cover 20 by adhesive as shown in fig1 5 6 the cover 20 with the inside plate 30 is positioned on the chassis 10 the first block 24 is received in the first clearance 15 and its main face 24 a is spaced apart from the motherboard 11 thus a first airflow passage 18 is defined between the first electronic component 12 and the second electronic component 13 the second block 25 is received in the second clearance 16 and its main face 25 a is also spaced apart from the motherboard 11 thus a second air flow passage 19 is defined between.