AN533

Through-hole packages

13/22

With screws: The following precautionary measures should be taken:

●

To avoid tab distortion, a rectangular washer must be put between the screw head and

the tab, and a compression washer must be put between the tab and the nut.

Figure 12.

Correct assembly

●

Take care to avoid mechanical shock during screwing.

●

Keep the screw straight.

●

Appropriate screwing torque should be used, excessive screwing torque may cause the

distortion of the tab and induce bad thermal contact. In addition it can generate cracks

in the die.

Figure 13.

Incorrect assembly

The thermal contact resistance depends on the force generated by the applied torque on the

screw:

Where:

●

T is applied torque on the screw in N.m

●

P is pitch in m

●

D is screw diameter in m

●

r is rubbing factor: # 0.12 for steel-steel with grease and # 0.2 for steel-aluminium

torque for an M3 screw used for the TO220AB is given in Figure 14.

Washer

M3 screw

Washer

Lock washer

Nut

Contact

grease

Heatsink

Component

Washer

M3 screw

Washer

Lock washer

Nut

Bad thermal

contact

Heatsink

Component

F =

2 . T .

π

P + r . D .

π