Understanding TO-5 and TO-39 Packages: Dimensions, Power and Applications

In the world of discrete semiconductors and sensors, TO metal can packages are still very much alive. Despite the rise of plastic and surface-mount packages, TO-5 and TO-39 remain popular wherever engineers need high reliability, good thermal performance, and hermetic sealing.

As a TO-package manufacturer, clearly explaining the differences between TO-5 and TO-39 helps your customers choose the right outline from the start.

This article walks through mechanical dimensions, thermal/power behavior, and typical applications, and also shows how these differences drive real design decisions.

Overview of TO Metal Can Packages

What is a TO (Transistor Outline) Package?

A TO package (Transistor Outline) is a standardized package family originally defined by JEDEC for discrete transistors. Over time, TO metal cans have been used for:

• Bipolar & MOS transistors

• Voltage regulators and precision references

• Operational amplifiers and analog ICs

• Sensors (temperature, pressure, gas, optical)

• Laser diodes and photodiodes

Typical TO metal can structure:

• Header: round metal base with glass-to-metal seals for leads

• Leads: usually radial leads emerging from the bottom

• Cap: metal cap welded or seam-sealed to the header

• Cavity: can be hermetic (dry, sealed cavity) or, in some low-cost variants, non-hermetic

TO-5 and TO-39 in the TO Family

Both TO-5 and TO-39 belong to this classic family:

• TO-5: one of the earliest and most widely recognized metal can packages.

• TO-39: evolved to offer better thermal capability and slightly larger volume, while keeping a similar footprint.

Both can be supplied with:

• Different lead counts (3, 4, 5, 6, 8, etc.)

• Hermetic glass-to-metal seals

• Optional custom caps (flat, domed, windowed, lensed, etc.)

Mechanical Dimensions: TO-5 vs TO-39

Common Mechanical Structure

Both packages share a similar basic structure:

• Round metal header with multiple leads.

• Metal cap welded or seam-sealed to the header.

• Leads arranged on a circular pitch, designed for through-hole PCB mounting or chassis-mount assemblies.

From the outside, they look similar; the key differences are diameter, height, and “thermal mass”.

Header Dimensions and Cap Size

Exact numbers vary by vendor and JEDEC variant, but the general trends are:

TO-5:

• Smaller outline and internal volume

• Lower profile, shorter cap height

• Suited to lower to medium power and compact designs

TO-39:

• Slightly larger cap/header and internal cavity

• More surface area for heat dissipation

• Better suited to higher power and thermally demanding applications

Dimensional Comparison Table (Indicative Values)

Impact of Dimensions on Design

The dimensional differences translate into practical design choices:

PCB layout:

TO-5 is ideal when board space and height are tight. TO-39 needs slightly more clearance but offers better thermal headroom.

Mechanical robustness:

The slightly larger and heavier TO-39 can be more robust for applications with vibration, shock, or frequent thermal cycling.

Optical or sensor cavity space:

TO-39’s larger cavity can accommodate bigger die, optics, or sensor structures, useful for laser diodes, photodiodes, and complex sensor assemblies.

Thermal and Power Handling: TO-5 vs TO-39

Fundamentals of Power Dissipation in Metal Cans

For both TO-5 and TO-39, power handling is largely defined by:

• RθJC – Junction-to-case thermal resistance

• RθJA – Junction-to-ambient thermal resistance

• Header material & thickness – How well heat spreads from the die to the case

• Mounting conditions – Free air, PCB copper area, or chassis/heat sink attachment

A larger metal surface and more mass generally mean better heat spreading and lower thermal resistance.

Typical Power Ratings

In very rough terms (assuming natural convection and no external heat sink):

• TO-5: suitable for low to moderate power, e.g., small-signal devices or regulators with modest dissipation.

• TO-39: better suited when the device must dissipate more power or operate at higher ambient temperatures.

With proper heat sinking (e.g., clamping the metal can to a chassis), both can handle significantly more power than in free air.

Thermal Performance Comparison Table (Indicative Values)

Again, these are indicative engineering-level values only; exact ratings depend on the device design and manufacturer.

How Package Size Affects Power

TO-5 works well for:

• Precision references, small amplifiers, small-signal transistors, low-power sensors.

• Designs where power dissipation is controlled and compact size is a priority.

TO-39 is a better choice when:

• Devices dissipate higher power or operate in high ambient temperatures.

• Additional thermal safety margin is required (aerospace, industrial, defense).

Electrical and Assembly Considerations

Lead Count and Pinout Flexibility

Both TO-5 and TO-39 support a range of lead counts:

• 3 leads: classic transistor or simple sensor.

• 4–5 leads: regulators, references, amplifiers, bridge sensors.

• 6–8 or more: multi-function or multi-channel ICs, complex sensor modules.

TO-39’s slightly larger cavity can offer more comfortable routing for higher lead counts or more complex internal wiring, but for many cases the difference is small.

Insulation and Creepage

Thanks to glass-to-metal seals, both packages offer:

• High insulation resistance

• Good dielectric strength between leads and case.

• Well-defined creepage and clearance distances around the lead glass seals.

For high-voltage or isolation-critical applications, designers should check:

• Maximum working voltage

• Creepage/clearance around the header.

• Any customer-specific insulation or hipot test requirements.

Soldering and Assembly

Assembly guidelines are very similar:

• Wave or hand soldering for through-hole leads.

• Avoid excessive lead forming right at the glass seal to prevent cracking.

• Observe maximum lead temperature and dwell times during soldering.

For hermetic cans, pay attention to:

• Cleaning solvents

• Flux residues

• Mechanical stress during board depaneling or handling

Typical Applications of TO-5 and TO-39 Packages

TO-5 Common Applications

TO-5 is often chosen when compact size and good reliability are required, but power is moderate:

• Small-signal transistors and low-power regulators.

• Precision analog ICs (references, low-noise amplifiers, instrumentation).

• Photodiodes and basic optoelectronic devices.

• Temperature sensors, simple gas or pressure sensors.

• Low-power signal conditioning modules.

TO-39 Common Applications

TO-39 is popular where extra thermal headroom or mechanical robustness is needed:

• Medium power transistors and driver stages.

• Laser diodes and high-output LEDs.

Sensors used in harsh environments:

• Industrial temperature/pressure.

• Automotive, aerospace, or defense sensing modules.

• High-reliability analog circuits with higher dissipation.

Application Mapping Table

How to Choose Between TO-5 and TO-39 as a Designer

Key Selection Criteria

When your customer (or your internal design team) chooses between TO-5 and TO-39, the main questions are:

Power dissipation

• How many watts must the device safely dissipate?

• What is the max ambient temperature?

Space and height

• Is PCB area and enclosure height very limited?

• Is a slightly taller package acceptable?

Lead count and pinout complexity

• How many pins are needed?

• Is there room for comfortable internal routing?

Environment & reliability

• Will the device see vibration, shock, wide temperature swings, or radiation?

• Is long-term hermetic reliability essential?

Cost and availability

• Is the customer open to a larger, slightly higher-cost package for extra margin?

• Or is minimal cost and size the top priority?

Design Trade-offs

Choose TO-5 when:

• Power dissipation is modest

• Compact size and lower profile are important

• The environment is not extremely harsh

Choose TO-39 when:

• Thermal simulations show limited margin in TO-5

• The device must work reliably at higher ambient temps

• The application is mission-critical (aerospace, defense, demanding industrial)

Example Design Scenarios

Example 1: Low-power analog amplifier

A precision low-noise amplifier dissipates 200–300 mW in a lab/industrial environment. TO-5 is typically sufficient, offering compact size and hermeticity without overkill.

Example 2: Industrial driver in a hot cabinet

A transistor dissipates 1 W continuously inside an enclosure at 70°C ambient. TO-39 is more suitable, giving better thermal margin and long-term reliability.

Manufacturer’s Perspective: Customization Options

As a TO package manufacturer, you can differentiate by offering customization on both TO-5 and TO-39:

Header and Cap Customization

Different header materials (Kovar, steel, copper alloys)

Cap styles:

• Flat caps for standard electronics

• Domed caps for extra internal height

• Glass or sapphire windows for optical/laser applications

• Blackened or plated caps for specific optical or thermal behavior

Lead and Plating Options

Lead diameter and lead length adaptable to customer PCB and assembly requirements

Plating systems:

• Tin, tin-lead, Ni/Au, Ag, etc.

• RoHS-compliant options for global markets

Hermeticity and Reliability Testing

You can also highlight:

• Fine and gross leak testing (helium leak methods)

• Thermal cycling, temperature shock, and vibration tests

• 100% testing for aerospace / military-grade parts

This allows you to position TO-5 and TO-39 not just as catalog outlines, but as engineered packaging platforms that you can optimize for each customer’s device.

Conclusion

TO-5 and TO-39 may look similar at first glance, but they serve slightly different roles:

• TO-5: compact, reliable, ideal for low-to-moderate power and space-sensitive designs.

• TO-39: slightly larger, with better thermal performance and mechanical robustness, well-suited to higher power and harsher environments.

For you as a TO-package manufacturer, explaining these differences in terms of dimensions, power, and applications helps customers choose the right outline early, avoid thermal problems, and maximize reliability.