Mounting and Handling Guidelines for TO220 TO220F TO247 Cases / Packages
The TO220, TO220F and TO247 are the popular packages for power devices because of their versatility and ability to dissipate moderate amounts of heat. This application note describes the basic guidelines for handling power MOSFETs in TO220, TO220 and TO247 packages shown in Figure 1. Please note that only mechanical and soldering guidelines are covered here. Additional precautions are required for isolating high voltage rated devices to meet safety regulations.
Securing a good thermal interface between the package and heat sink is essential to achieve better heat dissipation and improve the device reliability. Two popular methods of attaching the TO-220 and TO247 devices to a heat sink use either a clip or a small screw with associated hardware.
Several types of clip mountings are possible where a clip holds the TO-220 and TO247 devices against the heat sink and a screw tightens the clip. A single clip can be designed to hold multiple devices. It is recommended that the clip pressure be applied right on the plastic body of the package for the lowest thermal resistance.
Individual screw mounting methods are shown in Figure 2. Figure 2.(a) shows the hardware sequence when the heat sink is thick and tapped to hold the screw and in Figure 2.(b) screw is held by a nut on the back of the heat sink. The nylon washer isolates the screw from the device tab. The insulator is thermally conductive and transfers the heat away from the device to the heat sink. Even if the device is not required to be isolated from the heat sink, it is necessary to use a filler between the device tab and the heat sink to ensure good thermal contact. This may be a mica or a silicone pad or the old fashioned heat sink compound. Note that in both cases the metallic washer between the screw head and the device tab is rectangular in shape and no spring washers are used on the tab side.
If the device is soldered on PCB and attached to the heat sink, make sure that the heat sink attachment is made prior to soldering the leads on PCB. Otherwise the shearing stress between the soldered leads and the tab can easily damage the die during tightening.
There is not exposed metal for TO220F with full plastic package so it does not need insulted components and can be fixed by the screw directly.
Mounting on Heat sink During tightening, the device has a tendency to rotate in the direction of the screw. In order to prevent the rotation, the device is usually restrained sideways, which can subject the die inside to shearing stress as shown in Figure 3 and cause it to delaminate. This may show up as higher Rds in case of power MOSFETs, causing greater heat dissipation and eventual failure. The rectangular washer above the tab is recommended to minimize this stress.
Typical screw sizes are ISO M3.0, ISO M3.5 or 6-32 UNF. Care should be taken not to over-tighten the screw, especially when the operation is done manually. Studies have shown that beyond a certain limit, higher torque on the screw does not improve the thermal resistance any further(Figure 4). Applying higher torques only increases shear stress without any thermal benefits. At excessive torques over 20 in-lb, the screw itself will be stressed, eventually cracking up above 24 in-lb. The following table gives recommended torque ranges with metric and imperial units. A torque controlled screwdriver is strongly recommended for achieving optimum performance.
The TO220, TO220F and TO247 are the popular packages for power devices because of their versatility and ability to dissipate moderate amounts of heat. This application note describes the basic guidelines for handling power MOSFETs in TO220, TO220 and TO247 packages shown in Figure 1. Please note that only mechanical and soldering guidelines are covered here. Additional precautions are required for isolating high voltage rated devices to meet safety regulations.
Securing a good thermal interface between the package and heat sink is essential to achieve better heat dissipation and improve the device reliability. Two popular methods of attaching the TO-220 and TO247 devices to a heat sink use either a clip or a small screw with associated hardware.
Several types of clip mountings are possible where a clip holds the TO-220 and TO247 devices against the heat sink and a screw tightens the clip. A single clip can be designed to hold multiple devices. It is recommended that the clip pressure be applied right on the plastic body of the package for the lowest thermal resistance.
Individual screw mounting methods are shown in Figure 2. Figure 2.(a) shows the hardware sequence when the heat sink is thick and tapped to hold the screw and in Figure 2.(b) screw is held by a nut on the back of the heat sink. The nylon washer isolates the screw from the device tab. The insulator is thermally conductive and transfers the heat away from the device to the heat sink. Even if the device is not required to be isolated from the heat sink, it is necessary to use a filler between the device tab and the heat sink to ensure good thermal contact. This may be a mica or a silicone pad or the old fashioned heat sink compound. Note that in both cases the metallic washer between the screw head and the device tab is rectangular in shape and no spring washers are used on the tab side.
If the device is soldered on PCB and attached to the heat sink, make sure that the heat sink attachment is made prior to soldering the leads on PCB. Otherwise the shearing stress between the soldered leads and the tab can easily damage the die during tightening.
There is not exposed metal for TO220F with full plastic package so it does not need insulted components and can be fixed by the screw directly.
Mounting on Heat sink During tightening, the device has a tendency to rotate in the direction of the screw. In order to prevent the rotation, the device is usually restrained sideways, which can subject the die inside to shearing stress as shown in Figure 3 and cause it to delaminate. This may show up as higher Rds in case of power MOSFETs, causing greater heat dissipation and eventual failure. The rectangular washer above the tab is recommended to minimize this stress.
Typical screw sizes are ISO M3.0, ISO M3.5 or 6-32 UNF. Care should be taken not to over-tighten the screw, especially when the operation is done manually. Studies have shown that beyond a certain limit, higher torque on the screw does not improve the thermal resistance any further(Figure 4). Applying higher torques only increases shear stress without any thermal benefits. At excessive torques over 20 in-lb, the screw itself will be stressed, eventually cracking up above 24 in-lb. The following table gives recommended torque ranges with metric and imperial units. A torque controlled screwdriver is strongly recommended for achieving optimum performance.