TO-220 mounting quality is the thermal design: the tab carries the heat, and the interface between tab and heatsink—pressure, interface material, insulation—decides how much of the datasheet RθJC you actually keep. This guide covers screw torque, thermal interface material, electrical insulation, airflow, and the installation defects that make a good part run hot.
How Mounting Quality Becomes Thermal Performance
The junction temperature equation for a tab package is TJ = TC + (PD × RθJC), where the case temperature TC is set by everything downstream of the tab: the interface resistance, the heatsink, and the airflow. The datasheet RθJC describes the die-to-case drop only; the tab-to-ambient side is built by the assembly. A part with an excellent 2.0 °C/W junction-to-case figure—the class of the AMBR40250S—can still run hot if the interface adds several °C/W through trapped air or a loose screw.
The practical consequence: two identical units, one mounted correctly and one dry-mounted, can differ by 20–40 °C in case temperature at the same load. Mounting is not an assembly afterthought; it is the dominant variable in the thermal design.
The effect is easy to quantify in the design phase: if the interface resistance is 1 °C/W instead of 0.5 °C/W, a 20 W loss costs an extra 10 °C of case temperature—before the heatsink is even considered. That 10 °C is the difference between a comfortable margin and a warranty claim.
Screw Torque and Pressure Distribution
The screw sets the pressure that seats the tab against the interface material, and pressure distribution is what transfers heat evenly across the contact area. Too little torque leaves microscopic air gaps and a high interface resistance; too much torque can crack the plastic body or deform the tab, lifting the contact on one side and worsening the very gap the screw was supposed to close.
For M3 hardware on a standard TO-220, a common torque range is roughly 0.4–0.6 N·m, applied with a torque driver rather than feel. The exact number belongs to the mechanical drawing and the screw manufacturer’s recommendation—always follow the package outline and hardware specification for the selected part, and re-check torque on prototype builds before locking the process.
| Screw size | Common torque range (guidance) |
|---|---|
| M2.5 | ~0.3–0.45 N·m |
| M3 | ~0.4–0.6 N·m |
| M3.5 | ~0.6–0.8 N·m |
The ranges are common engineering starting points for TO-220-class packages, not universal values—the package drawing and the screw supplier’s data are authoritative for the selected hardware.
Thermal Interface Material: Filling the Air Gap
Bare metal-to-metal contact is never truly flat: microscopic roughness traps air pockets, and air is a poor heat conductor. The thermal interface material (TIM) fills those gaps and establishes the actual contact conductance. A thin, correctly sized TIM layer—matching the tab area, not smaller—drops the interface resistance dramatically; an undersized or damaged pad leaves part of the tab uncovered and the heat path incomplete.
Two application rules keep the TIM honest. First, choose the pad for the voltage class and thermal conductivity of the application, and confirm it covers the full tab. Second, do not compensate for a bad interface with extra torque—the TIM is compressed by the rated torque, and overtightening beyond it defeats the purpose.
TIM selection also involves thickness and compressibility: a pad that is too thick adds resistance, one that is too thin cannot fill the surface roughness, and a grease that pumps out under thermal cycling needs a retention mechanism. Match the material to the thermal budget and the operating environment rather than defaulting to the cheapest pad.
The pad’s thermal conductivity matters more than its thickness alone: a 0.5 mm pad with high conductivity can outperform a thinner pad with poor conductivity, so compare pads by their effective resistance at the rated torque rather than by material name.
Electrical Insulation When the Tab Is Live
In many rectifier circuits the tab is at a live potential—the cathode of a common-cathode pair, for example, or a bridge leg. If the heatsink is grounded or touchable, the assembly needs an electrically insulating solution: an insulating pad between tab and heatsink, plus an insulating bushing on the mounting screw. The insulation adds thermal resistance, and the pad’s thermal conductivity and thickness set how much; a thick, low-conductivity pad can add several °C/W.
The alternative is a common-tab design: if the system can accept all tabs at the same potential, direct metal-to-metal contact with TIM gives the lowest thermal resistance. The isolation decision is therefore electrical first, thermal second—choose the pad rated for the working voltage, then size the heatsink for the added resistance.
Isolation also has a creepage dimension: the pad must be sized and positioned so the tab cannot creep toward the heatsink or the mounting hardware over temperature and vibration. For higher working voltages, the pad’s rated voltage class and the screw bushing are part of the same safety decision, not separate items.
Heatsink, Airflow, and Common Installation Defects
The heatsink and its airflow complete the chain, and both are part of the mounting. Fin orientation should follow the airflow path; a heatsink in a dead air pocket, or a fan blowing across the fins instead of through them, produces the same symptom as a missing TIM. Conformal coating applied over the tab acts as an unintended insulator, and a loose screw after thermal cycling is a field failure waiting to happen.
| Defect | Symptom | Corrective action |
|---|---|---|
| Dry or missing TIM | Case temperature 20–40 °C high | Apply rated TIM, full tab coverage |
| Loose screw / low torque | Case hot, torque reads low | Re-torque to drawing value |
| Overtorqued screw | Cracked body, uneven contact | Replace part, use torque driver |
| Wrong insulation pad | High thermal resistance | Use pad rated for voltage and conductivity |
| Dead airflow pocket | Heatsink hot, case hotter | Reroute airflow or resize heatsink |
| Conformal coat on tab | Case hot with coat present | Mask the tab area before coating |
Heatsink sizing is the last step: work backward from the junction target, subtract the junction-to-case and interface resistances, and the remainder is the heatsink-to-ambient resistance the airflow must deliver. A 20 W loss with a target 40 °C rise above ambient needs roughly 2 °C/W of total system resistance—an assembly target the mounting quality either meets or misses. Re-torque after thermal cycling is a real checklist item: screws relax as the assembly heats and cools, and a torque check on the first production units catches the drift before it reaches the field.
A torque driver with the correct bit and a calibration check is the difference between repeatable mounting and unit-to-unit variation; the same driver should be used across the production line.
Engineering note. The mounting guidance above follows the mechanical practice for TO-220 packages and the thermal chain used in Good-Ark datasheets—RθJC 2.0 °C/W for the TO-220AB AMBR40250S, with the interface and heatsink completing the junction-to-ambient path. The 0.4–0.6 N·m torque range is a common M3 starting point, not a universal value; always confirm the torque, TIM, and insulation against the part’s mechanical drawing and the hardware specification, and verify the result with a case-temperature measurement.
Frequently Asked Questions
Why does my TO-220 run hot even with a heatsink?
The interface is the usual suspect: dry or missing TIM, loose screw torque, an undersized insulation pad, or a dead airflow pocket all add resistance between the tab and ambient. Fix the interface before changing the part.
What torque should I use for a TO-220 screw?
A common range for M3 hardware is 0.4–0.6 N·m, applied with a torque driver. Confirm the exact value against the package mechanical drawing and the screw manufacturer’s recommendation for the selected part.
Do I need a thermal interface material?
Yes, for reliable contact: bare metal traps air in surface roughness, and air is a poor conductor. A thin TIM layer sized to the tab area fills the gaps and establishes consistent contact conductance.
Is the TO-220 tab electrically live?
It can be. In common-cathode and bridge circuits the tab is at a circuit potential. If the heatsink must be grounded or touched, use an insulating pad and bushing rated for the working voltage.
Can over-tightening damage a TO-220?
Yes. Excess torque can crack the plastic body or deform the tab, lifting contact on one side and increasing the interface resistance. Use a torque driver and stay within the drawing range.
Conclusion
TO-220 thermal performance is built at the assembly line: rated torque, a correctly sized TIM, a properly selected insulation solution, and a heatsink with real airflow. Confirm the torque against the mechanical drawing, and verify the result with a case-temperature measurement at maximum load before freezing the process.
Review the AMBR40250S product page on the Good-Ark site for the TO-220AB outline and thermal ratings, compare tab packages in the Schottky rectifier diodes category, and contact Good-Ark with your mounting and thermal conditions for application support.